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%% Filename:    spec.tex
%% Filename:    spec.tex
%%
%%
%% Project:     Zip CPU -- a small, lightweight, RISC CPU soft core
%% Project:     Zip CPU -- a small, lightweight, RISC CPU soft core
%%
%%
%% Purpose:     This LaTeX file contains all of the documentation/description
%% Purpose:     This LaTeX file contains all of the documentation/description
%%              currently provided with this Zip CPU soft core.  It supercedes
%%              currently provided with this Zip CPU soft core.  It supercedes
%%              any information about the instruction set or CPUs found
%%              any information about the instruction set or CPUs found
%%              elsewhere.  It's not nearly as interesting, though, as the PDF
%%              elsewhere.  It's not nearly as interesting, though, as the PDF
%%              file it creates, so I'd recommend reading that before diving
%%              file it creates, so I'd recommend reading that before diving
%%              into this file.  You should be able to find the PDF file in
%%              into this file.  You should be able to find the PDF file in
%%              the SVN distribution together with this PDF file and a copy of
%%              the SVN distribution together with this PDF file and a copy of
%%              the GPL-3.0 license this file is distributed under.  If not,
%%              the GPL-3.0 license this file is distributed under.  If not,
%%              just type 'make' in the doc directory and it (should) build
%%              just type 'make' in the doc directory and it (should) build
%%              without a problem.
%%              without a problem.
%%
%%
%%
%%
%% Creator:     Dan Gisselquist
%% Creator:     Dan Gisselquist
%%              Gisselquist Technology, LLC
%%              Gisselquist Technology, LLC
%%
%%
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%%
%%
%% Copyright (C) 2015, Gisselquist Technology, LLC
%% Copyright (C) 2015, Gisselquist Technology, LLC
%%
%%
%% This program is free software (firmware): you can redistribute it and/or
%% This program is free software (firmware): you can redistribute it and/or
%% modify it under the terms of  the GNU General Public License as published
%% modify it under the terms of  the GNU General Public License as published
%% by the Free Software Foundation, either version 3 of the License, or (at
%% by the Free Software Foundation, either version 3 of the License, or (at
%% your option) any later version.
%% your option) any later version.
%%
%%
%% This program is distributed in the hope that it will be useful, but WITHOUT
%% This program is distributed in the hope that it will be useful, but WITHOUT
%% ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
%% ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
%% FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
%% FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
%% for more details.
%% for more details.
%%
%%
%% You should have received a copy of the GNU General Public License along
%% You should have received a copy of the GNU General Public License along
%% with this program.  (It's in the $(ROOT)/doc directory, run make with no
%% with this program.  (It's in the $(ROOT)/doc directory, run make with no
%% target there if the PDF file isn't present.)  If not, see
%% target there if the PDF file isn't present.)  If not, see
%% <http://www.gnu.org/licenses/> for a copy.
%% <http://www.gnu.org/licenses/> for a copy.
%%
%%
%% License:     GPL, v3, as defined and found on www.gnu.org,
%% License:     GPL, v3, as defined and found on www.gnu.org,
%%              http://www.gnu.org/licenses/gpl.html
%%              http://www.gnu.org/licenses/gpl.html
%%
%%
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\documentclass{gqtekspec}
\documentclass{gqtekspec}
\project{Zip CPU}
\project{Zip CPU}
\title{Specification}
\title{Specification}
\author{Dan Gisselquist, Ph.D.}
\author{Dan Gisselquist, Ph.D.}
\email{dgisselq (at) opencores.org}
\email{dgisselq (at) opencores.org}
\revision{Rev.~0.1}
\revision{Rev.~0.2}
\begin{document}
\begin{document}
\pagestyle{gqtekspecplain}
\pagestyle{gqtekspecplain}
\titlepage
\titlepage
\begin{license}
\begin{license}
Copyright (C) \theyear\today, Gisselquist Technology, LLC
Copyright (C) \theyear\today, Gisselquist Technology, LLC
 
 
This project is free software (firmware): you can redistribute it and/or
This project is free software (firmware): you can redistribute it and/or
modify it under the terms of  the GNU General Public License as published
modify it under the terms of  the GNU General Public License as published
by the Free Software Foundation, either version 3 of the License, or (at
by the Free Software Foundation, either version 3 of the License, or (at
your option) any later version.
your option) any later version.
 
 
This program is distributed in the hope that it will be useful, but WITHOUT
This program is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.
for more details.
 
 
You should have received a copy of the GNU General Public License along
You should have received a copy of the GNU General Public License along
with this program.  If not, see \hbox{<http://www.gnu.org/licenses/>} for a
with this program.  If not, see \hbox{<http://www.gnu.org/licenses/>} for a
copy.
copy.
\end{license}
\end{license}
\begin{revisionhistory}
\begin{revisionhistory}
 
0.2 & 8/19/2015 & Gisselquist & Still Draft, more complete \\\hline
0.1 & 8/17/2015 & Gisselquist & Incomplete First Draft \\\hline
0.1 & 8/17/2015 & Gisselquist & Incomplete First Draft \\\hline
\end{revisionhistory}
\end{revisionhistory}
% Revision History
% Revision History
% Table of Contents, named Contents
% Table of Contents, named Contents
\tableofcontents
\tableofcontents
% \listoffigures
\listoffigures
\listoftables
\listoftables
\begin{preface}
\begin{preface}
Many people have asked me why I am building the Zip CPU. ARM processors are
Many people have asked me why I am building the Zip CPU. ARM processors are
good and effective. Xilinx makes and markets Microblaze, Altera Nios, and both
good and effective. Xilinx makes and markets Microblaze, Altera Nios, and both
have better toolsets than the Zip CPU will ever have. OpenRISC is also
have better toolsets than the Zip CPU will ever have. OpenRISC is also
available. Why build a new processor?
available, RISC--V may be replacing it. Why build a new processor?
 
 
The easiest, most obvious answer is the simple one: Because I can.
The easiest, most obvious answer is the simple one: Because I can.
 
 
There's more to it, though. There's a lot that I would like to do with a
There's more to it, though. There's a lot that I would like to do with a
processor, and I want to be able to do it in a vendor independent fashion.
processor, and I want to be able to do it in a vendor independent fashion.
I would like to be able to generate Verilog code that can run equivalently
I would like to be able to generate Verilog code that can run equivalently
on both Xilinx and Altera chips, and that can be easily ported from one
on both Xilinx and Altera chips, and that can be easily ported from one
manufacturer's chipsets to another. Even more, before purchasing a chip or a
manufacturer's chipsets to another. Even more, before purchasing a chip or a
board, I would like to know that my chip works. I would like to build a test
board, I would like to know that my chip works. I would like to build a test
bench to test components with, and Verilator is my chosen test bench. This
bench to test components with, and Verilator is my chosen test bench. This
forces me to use all Verilog, and it prevents me from using any proprietary
forces me to use all Verilog, and it prevents me from using any proprietary
cores. For this reason, Microblaze and Nios are out of the question.
cores. For this reason, Microblaze and Nios are out of the question.
 
 
Why not OpenRISC? That's a hard question. The OpenRISC team has done some
Why not OpenRISC? That's a hard question. The OpenRISC team has done some
wonderful work on an amazing processor, and I'll have to admit that I am
wonderful work on an amazing processor, and I'll have to admit that I am
envious of what they've accomplished. I would like to port binutils to the
envious of what they've accomplished. I would like to port binutils to the
Zip CPU, as I would like to port GCC and GDB. They are way ahead of me. The
Zip CPU, as I would like to port GCC and GDB. They are way ahead of me. The
OpenRISC processor, however, is complex and hefty at about 4,500 LUTs. It has
OpenRISC processor, however, is complex and hefty at about 4,500 LUTs. It has
a lot of features of modern CPUs within it that ... well, let's just say it's
a lot of features of modern CPUs within it that ... well, let's just say it's
not the little guy on the block. The Zip CPU is lighter weight, costing only
not the little guy on the block. The Zip CPU is lighter weight, costing only
about 2,000 LUTs with no peripherals, and 3,000 LUTs with some very basic
about 2,000 LUTs with no peripherals, and 3,000 LUTs with some very basic
peripherals.
peripherals.
 
 
My final reason is that I'm building the Zip CPU as a learning experience. The
My final reason is that I'm building the Zip CPU as a learning experience. The
Zip CPU has allowed me to learn a lot about how CPUs work on a very micro
Zip CPU has allowed me to learn a lot about how CPUs work on a very micro
level. For the first time, I am beginning to understand many of the Computer
level. For the first time, I am beginning to understand many of the Computer
Architecture lessons from years ago.
Architecture lessons from years ago.
 
 
To summarize: Because I can, because it is open source, because it is light
To summarize: Because I can, because it is open source, because it is light
weight, and as an exercise in learning.
weight, and as an exercise in learning.
 
 
\end{preface}
\end{preface}
 
 
\chapter{Introduction}
\chapter{Introduction}
\pagenumbering{arabic}
\pagenumbering{arabic}
\setcounter{page}{1}
\setcounter{page}{1}
 
 
 
 
The original goal of the ZIP CPU was to be a very simple CPU.   You might
The original goal of the ZIP CPU was to be a very simple CPU.   You might
think of it as a poor man's alternative to the OpenRISC architecture.
think of it as a poor man's alternative to the OpenRISC architecture.
For this reason, all instructions have been designed to be as simple as
For this reason, all instructions have been designed to be as simple as
possible, and are all designed to be executed in one instruction cycle per
possible, and are all designed to be executed in one instruction cycle per
instruction, barring pipeline stalls.  Indeed, even the bus has been simplified
instruction, barring pipeline stalls.  Indeed, even the bus has been simplified
to a constant 32-bit width, with no option for more or less.  This has
to a constant 32-bit width, with no option for more or less.  This has
resulted in the choice to drop push and pop instructions, pre-increment and
resulted in the choice to drop push and pop instructions, pre-increment and
post-decrement addressing modes, and more.
post-decrement addressing modes, and more.
 
 
For those who like buzz words, the Zip CPU is:
For those who like buzz words, the Zip CPU is:
\begin{itemize}
\begin{itemize}
\item A 32-bit CPU: All registers are 32-bits, addresses are 32-bits,
\item A 32-bit CPU: All registers are 32-bits, addresses are 32-bits,
                instructions are 32-bits wide, etc.
                instructions are 32-bits wide, etc.
\item A RISC CPU.  There is no microcode for executing instructions.
\item A RISC CPU.  There is no microcode for executing instructions.  All
 
        instructions are designed to be completed in one clock cycle.
\item A Load/Store architecture.  (Only load and store instructions
\item A Load/Store architecture.  (Only load and store instructions
                can access memory.)
                can access memory.)
\item Wishbone compliant.  All peripherals are accessed just like
\item Wishbone compliant.  All peripherals are accessed just like
                memory across this bus.
                memory across this bus.
\item A Von-Neumann architecture.  (The instructions and data share a
\item A Von-Neumann architecture.  (The instructions and data share a
                common bus.)
                common bus.)
\item A pipelined architecture, having stages for {\bf Prefetch},
\item A pipelined architecture, having stages for {\bf Prefetch},
                {\bf Decode}, {\bf Read-Operand}, the {\bf ALU/Memory}
                {\bf Decode}, {\bf Read-Operand}, the {\bf ALU/Memory}
                unit, and {\bf Write-back}
                unit, and {\bf Write-back}.  See Fig.~\ref{fig:cpu}
 
\begin{figure}\begin{center}
 
\includegraphics[width=3.5in]{../gfx/cpu.eps}
 
\caption{Zip CPU internal pipeline architecture}\label{fig:cpu}
 
\end{center}\end{figure}
 
                for a diagram of this structure.
\item Completely open source, licensed under the GPL.\footnote{Should you
\item Completely open source, licensed under the GPL.\footnote{Should you
        need a copy of the Zip CPU licensed under other terms, please
        need a copy of the Zip CPU licensed under other terms, please
        contact me.}
        contact me.}
\end{itemize}
\end{itemize}
 
 
Now, however, that I've worked on the Zip CPU for a while, it is not nearly
Now, however, that I've worked on the Zip CPU for a while, it is not nearly
as simple as I originally hoped.  Worse, I've had to adjust to create
as simple as I originally hoped.  Worse, I've had to adjust to create
capabilities that I was never expecting to need.  These include:
capabilities that I was never expecting to need.  These include:
\begin{itemize}
\begin{itemize}
\item {\bf Extenal Debug:} Once placed upon an FPGA, some external means is
\item {\bf Extenal Debug:} Once placed upon an FPGA, some external means is
        still necessary to debug this CPU.  That means that there needs to be
        still necessary to debug this CPU.  That means that there needs to be
        an external register that can control the CPU: reset it, halt it, step
        an external register that can control the CPU: reset it, halt it, step
        it, and tell whether it is running or not.  Another register is placed
        it, and tell whether it is running or not.  My chosen interface
        similar to this register, to allow the external controller to examine
        includes a second register similar to this control register.  This
 
        second register allows the external controller or debugger to examine
        registers internal to the CPU.
        registers internal to the CPU.
 
 
\item {\bf Internal Debug:} Being able to run a debugger from within
\item {\bf Internal Debug:} Being able to run a debugger from within
        a user process requires an ability to step a user process from
        a user process requires an ability to step a user process from
        within a debugger.  It also requires a break instruction that can
        within a debugger.  It also requires a break instruction that can
        be substituted for any other instruction, and substituted back.
        be substituted for any other instruction, and substituted back.
        The break is actually difficult: the break instruction cannot be
        The break is actually difficult: the break instruction cannot be
        allowed to execute.  That way, upon a break, the debugger should
        allowed to execute.  That way, upon a break, the debugger should
        be able to jump back into the user process to step the instruction
        be able to jump back into the user process to step the instruction
        that would've been at the break point initially, and then to
        that would've been at the break point initially, and then to
        replace the break after passing it.
        replace the break after passing it.
 
 
 
        Incidentally, this break messes with the prefetch cache and the
 
        pipeline: if you change an instruction partially through the pipeline,
 
        the whole pipeline needs to be cleansed.  Likewise if you change
 
        an instruction in memory, you need to make sure the cache is reloaded
 
        with the new instruction.
 
 
\item {\bf Prefetch Cache:} My original implementation had a very
\item {\bf Prefetch Cache:} My original implementation had a very
        simple prefetch stage.  Any time the PC changed the prefetch would go
        simple prefetch stage.  Any time the PC changed the prefetch would go
        and fetch the new instruction.  While this was perhaps this simplest
        and fetch the new instruction.  While this was perhaps this simplest
        approach, it cost roughly five clocks for every instruction.  This
        approach, it cost roughly five clocks for every instruction.  This
        was deemed unacceptable, as I wanted a CPU that could execute
        was deemed unacceptable, as I wanted a CPU that could execute
        instructions in one cycle.  I therefore have a prefetch cache that
        instructions in one cycle.  I therefore have a prefetch cache that
        issues pipelined wishbone accesses to memory and then pushes
        issues pipelined wishbone accesses to memory and then pushes
        instructions at the CPU.  Sadly, this accounts for about 20\% of the
        instructions at the CPU.  Sadly, this accounts for about 20\% of the
        logic in the entire CPU, or 15\% of the logic in the entire system.
        logic in the entire CPU, or 15\% of the logic in the entire system.
 
 
 
 
\item {\bf Operating System:} In order to support an operating system,
\item {\bf Operating System:} In order to support an operating system,
        interrupts and so forth, the CPU needs to support supervisor and
        interrupts and so forth, the CPU needs to support supervisor and
        user modes, as well as a means of switching between them.  For example,
        user modes, as well as a means of switching between them.  For example,
        the user needs a means of executing a system call.  This is the
        the user needs a means of executing a system call.  This is the
        purpose of the {\bf `trap'} instruction.  This instruction needs to
        purpose of the {\bf `trap'} instruction.  This instruction needs to
        place the CPU into supervisor mode (here equivalent to disabling
        place the CPU into supervisor mode (here equivalent to disabling
        interrupts), as well as handing it a parameter such as identifying
        interrupts), as well as handing it a parameter such as identifying
        which O/S function was called.
        which O/S function was called.
 
 
My initial approach to building a trap instruction was to create
My initial approach to building a trap instruction was to create an external
        an external peripheral which, when written to, would generate an
peripheral which, when written to, would generate an interrupt and could
        interrupt and could return the last value written to it.  This failed
return the last value written to it.  In practice, this approach didn't work
        timing requirements, however: the CPU executed two instructions while
at all: the CPU executed two instructions while waiting for the
        waiting for the trap interrupt to take place.  Since then, I've
trap interrupt to take place.  Since then, I've decided to keep the rest of
        decided to keep the rest of the CC register for that purpose so that a
the CC register for that purpose so that a write to the CC register, with the
        write to the CC register, with the GIE bit cleared, could be used to
GIE bit cleared, could be used to execute a trap.  This has other problems,
        execute a trap.
though, primarily in the limitation of the uses of the CC register.  In
 
particular, the CC register is the best place to put CPU state information and
 
to ``announce'' special CPU features (floating point, etc).  So the trap
 
instruction still switches to interrupt mode, but the CC register is not
 
nearly as useful for telling the supervisor mode processor what trap is being
 
executed.
 
 
Modern timesharing systems also depend upon a {\bf Timer} interrupt
Modern timesharing systems also depend upon a {\bf Timer} interrupt
        to handle task swapping.  For the Zip CPU, this interrupt is handled
        to handle task swapping.  For the Zip CPU, this interrupt is handled
        external to the CPU as part of the CPU System, found in
external to the CPU as part of the CPU System, found in {\tt zipsystem.v}.
        {\tt zipsystem.v}.  The timer module itself is found in
The timer module itself is found in {\tt ziptimer.v}.
        {\tt ziptimer.v}.
 
 
 
\item {\bf Pipeline Stalls:} My original plan was to not support pipeline
\item {\bf Pipeline Stalls:} My original plan was to not support pipeline
        stalls at all, but rather to require the compiler to properly schedule
        stalls at all, but rather to require the compiler to properly schedule
        instructions so that stalls would never be necessary.  After trying
        all instructions so that stalls would never be necessary.  After trying
        to build such an architecture, I gave up, having learned some things:
        to build such an architecture, I gave up, having learned some things:
 
 
        For example, in  order to facilitate interrupt handling and debug
        For example, in  order to facilitate interrupt handling and debug
        stepping, the CPU needs to know what instructions have finished, and
        stepping, the CPU needs to know what instructions have finished, and
        which have not.  In other words, it needs to know where it can restart
        which have not.  In other words, it needs to know where it can restart
        the pipeline from.  Once restarted, it must act as though it had
        the pipeline from.  Once restarted, it must act as though it had
                never stopped.  This killed my idea of delayed branching, since
        never stopped.  This killed my idea of delayed branching, since what
                what would be the appropriate program counter to restart at?
        would be the appropriate program counter to restart at?  The one the
                The one the CPU was going to branch to, or the ones in the
        CPU was going to branch to, or the ones in the delay slots?  This
                delay slots?
        also makes the idea of compressed instruction codes difficult, since,
 
        again, where do you restart on interrupt?
 
 
        So I switched to a model of discrete execution: Once an instruction
        So I switched to a model of discrete execution: Once an instruction
        enters into either the ALU or memory unit, the instruction is
        enters into either the ALU or memory unit, the instruction is
        guaranteed to complete.  If the logic recognizes a branch or a
        guaranteed to complete.  If the logic recognizes a branch or a
        condition that would render the instruction entering into this stage
        condition that would render the instruction entering into this stage
        possibly inappropriate (i.e. a conditional branch preceeding a store
        possibly inappropriate (i.e. a conditional branch preceeding a store
        instruction for example), then the pipeline stalls for one cycle
        instruction for example), then the pipeline stalls for one cycle
        until the conditional branch completes.  Then, if it generates a new
        until the conditional branch completes.  Then, if it generates a new
        PC address, the stages preceeding are all wiped clean.
        PC address, the stages preceeding are all wiped clean.
 
 
        The discrete execution model allows such things as sleeping: if the
        The discrete execution model allows such things as sleeping: if the
        CPU is put to "sleep", the ALU and memory stages stall and back up
        CPU is put to ``sleep,'' the ALU and memory stages stall and back up
        everything before them.  Likewise, anything that has entered the ALU
        everything before them.  Likewise, anything that has entered the ALU
        or memory stage when the CPU is placed to sleep continues to completion.
        or memory stage when the CPU is placed to sleep continues to completion.
        To handle this logic, each pipeline stage has three control signals:
        To handle this logic, each pipeline stage has three control signals:
        a valid signal, a stall signal, and a clock enable signal.  In
        a valid signal, a stall signal, and a clock enable signal.  In
        general, a stage stalls if it's contents are valid and the next step
        general, a stage stalls if it's contents are valid and the next step
        is stalled.  This allows the pipeline to fill any time a later stage
        is stalled.  This allows the pipeline to fill any time a later stage
        stalls.
        stalls.
 
 
 
        This approach is also different from other pipeline approaches.  Instead
 
        of keeping the entire pipeline filled, each stage is treated
 
        independently.  Therefore, individual stages may move forward as long
 
        as the subsequent stage is available, regardless of whether the stage
 
        behind it is filled.
 
 
\item {\bf Verilog Modules:} When examining how other processors worked
\item {\bf Verilog Modules:} When examining how other processors worked
        here on open cores, many of them had one separate module per pipeline
        here on open cores, many of them had one separate module per pipeline
        stage.  While this appeared to me to be a fascinating and commendable
        stage.  While this appeared to me to be a fascinating and commendable
        idea, my own implementation didn't work out quite so nicely.
        idea, my own implementation didn't work out quite so nicely.
 
 
        As an example, the decode module produces a {\em lot} of
        As an example, the decode module produces a {\em lot} of
        control wires and registers.  Creating a module out of this, with
        control wires and registers.  Creating a module out of this, with
        only the simplest of logic within it, seemed to be more a lesson
        only the simplest of logic within it, seemed to be more a lesson
        in passing wires around, rather than encapsulating logic.
        in passing wires around, rather than encapsulating logic.
 
 
        Another example was the register writeback section.  I would love
        Another example was the register writeback section.  I would love
        this section to be a module in its own right, and many have made them
        this section to be a module in its own right, and many have made them
        such.  However, other modules depend upon writeback results other
        such.  However, other modules depend upon writeback results other
        than just what's placed in the register (i.e., the control wires).
        than just what's placed in the register (i.e., the control wires).
        For these reasons, I didn't manage to fit this section into it's
        For these reasons, I didn't manage to fit this section into it's
        own module.
        own module.
 
 
        The result is that the majority of the CPU code can be found in
        The result is that the majority of the CPU code can be found in
        the {\tt zipcpu.v} file.
        the {\tt zipcpu.v} file.
\end{itemize}
\end{itemize}
 
 
With that introduction out of the way, let's move on to the instruction
With that introduction out of the way, let's move on to the instruction
set.
set.
 
 
\chapter{CPU Architecture}\label{chap:arch}
\chapter{CPU Architecture}\label{chap:arch}
 
 
The Zip CPU supports a set of two operand instructions, where the first operand
The Zip CPU supports a set of two operand instructions, where the second operand
(always a register) is the result.  The only exception is the store instruction,
(always a register) is the result.  The only exception is the store instruction,
where the first operand (always a register) is the source of the data to be
where the first operand (always a register) is the source of the data to be
stored.
stored.
 
 
 
\section{Simplified Bus}
 
The bus architecture of the Zip CPU is that of a simplified WISHBONE bus.
 
It has been simplified in this fashion: all operations are 32--bit operations.
 
The bus is neither little endian nor bit endian.  For this reason, all words
 
are 32--bits.  All instructions are also 32--bits wide.  Everything has been
 
built around the 32--bit word.
 
 
\section{Register Set}
\section{Register Set}
The Zip CPU supports two sets of sixteen 32-bit registers, a supervisor
The Zip CPU supports two sets of sixteen 32-bit registers, a supervisor
and a user set.  The supervisor set is used in interrupt mode, whereas
and a user set as shown in Fig.~\ref{fig:regset}.
the user set is used otherwise.  Of this register set, the Program Counter (PC)
\begin{figure}\begin{center}
is register 15, whereas the status register (SR) or condition code register
\includegraphics[width=3.5in]{../gfx/regset.eps}
 
\caption{Zip CPU Register File}\label{fig:regset}
 
\end{center}\end{figure}
 
The supervisor set is used in interrupt mode when interrupts are disabled,
 
whereas the user set is used otherwise.  Of this register set, the Program
 
Counter (PC) is register 15, whereas the status register (SR) or condition
 
code register
(CC) is register 14.  By convention, the stack pointer will be register 13 and
(CC) is register 14.  By convention, the stack pointer will be register 13 and
noted as (SP)--although the instruction set allows it to be anything.
noted as (SP)--although there is nothing special about this register other
 
than this convention.
The CPU can access both register sets via move instructions from the
The CPU can access both register sets via move instructions from the
supervisor state, whereas the user state can only access the user registers.
supervisor state, whereas the user state can only access the user registers.
 
 
The status register is special, and bears further mention.  The lower
The status register is special, and bears further mention.  The lower
8 bits of the status register form a set of condition codes.  Writes to other
10 bits of the status register form a set of CPU state and condition codes.
bits are preserved, and can be used as part of the trap architecture--examined
Writes to other bits of this register are preserved.
by the O/S upon any interrupt, cleared before returning.
 
 
 
Of the eight condition codes, the bottom four are the current flags:
Of the eight condition codes, the bottom four are the current flags:
                Zero (Z),
                Zero (Z),
                Carry (C),
                Carry (C),
                Negative (N),
                Negative (N),
                and Overflow (V).
                and Overflow (V).
 
 
The next bit is a clock enable (0 to enable) or sleep bit (1 to put
The next bit is a clock enable (0 to enable) or sleep bit (1 to put
        the CPU to sleep).  Setting this bit will cause the CPU to
        the CPU to sleep).  Setting this bit will cause the CPU to
        wait for an interrupt (if interrupts are enabled), or to
        wait for an interrupt (if interrupts are enabled), or to
        completely halt (if interrupts are disabled).
        completely halt (if interrupts are disabled).
The sixth bit is a global interrupt enable bit (GIE).  When this
The sixth bit is a global interrupt enable bit (GIE).  When this
        sixth bit is a '1' interrupts will be enabled, else disabled.  When
        sixth bit is a '1' interrupts will be enabled, else disabled.  When
        interrupts are disabled, the CPU will be in supervisor mode, otherwise
        interrupts are disabled, the CPU will be in supervisor mode, otherwise
        it is in user mode.  Thus, to execute a context switch, one only
        it is in user mode.  Thus, to execute a context switch, one only
        need enable or disable interrupts.  (When an interrupt line goes
        need enable or disable interrupts.  (When an interrupt line goes
        high, interrupts will automatically be disabled, as the CPU goes
        high, interrupts will automatically be disabled, as the CPU goes
        and deals with its context switch.)
        and deals with its context switch.)
 
 
The seventh bit is a step bit.  This bit can be
The seventh bit is a step bit.  This bit can be
        set from supervisor mode only.  After setting this bit, should
        set from supervisor mode only.  After setting this bit, should
        the supervisor mode process switch to user mode, it would then
        the supervisor mode process switch to user mode, it would then
        accomplish one instruction in user mode before returning to supervisor
        accomplish one instruction in user mode before returning to supervisor
        mode.  Then, upon return to supervisor mode, this bit will
        mode.  Then, upon return to supervisor mode, this bit will
        be automatically cleared.  This bit has no effect on the CPU while in
        be automatically cleared.  This bit has no effect on the CPU while in
        supervisor mode.
        supervisor mode.
 
 
        This functionality was added to enable a userspace debugger
        This functionality was added to enable a userspace debugger
        functionality on a user process, working through supervisor mode
        functionality on a user process, working through supervisor mode
        of course.
        of course.
 
 
 
 
The eighth bit is a break enable bit.  This
The eighth bit is a break enable bit.  This controls whether a break
        controls whether a break instruction will halt the processor for an
instruction in user mode will halt the processor for an external debugger
        external debuggerr (break enabled), or whether the break instruction
(break enabled), or whether the break instruction will simply send send the
        will simply set the STEP bit and send the CPU into interrupt mode.
CPU into interrupt mode.  Encountering a break in supervisor mode will
        This bit can only be set within supervisor mode.
halt the CPU independent of the break enable bit.  This bit can only be set
 
within supervisor mode.
 
 
This functionality was added to enable an external debugger to
This functionality was added to enable an external debugger to
        set and manage breakpoints.
        set and manage breakpoints.
 
 
The ninth bit is reserved for a floating point enable bit.  When set, the
The ninth bit is reserved for a floating point enable bit.  When set, the
arithmetic for the next instruction will be sent to a floating point unit.
arithmetic for the next instruction will be sent to a floating point unit.
Such a unit may later be added as an extension to the Zip CPU.  If the
Such a unit may later be added as an extension to the Zip CPU.  If the
CPU does not support floating point instructions, this bit will never be set.
CPU does not support floating point instructions, this bit will never be set.
 
The instruction set could also be simply extended to allow other data types
 
in this fashion, such as two by 16--bit vector operations or four by 8--bit
 
vector operations.
 
 
The tenth bit is a trap bit.  It is set whenever the user requests a soft
The tenth bit is a trap bit.  It is set whenever the user requests a soft
interrupt, and cleared on any return to userspace command.  This allows the
interrupt, and cleared on any return to userspace command.  This allows the
supervisor, in supervisor mode, to determine whether it got to supervisor
supervisor, in supervisor mode, to determine whether it got to supervisor
mode from a trap or from an external interrupt or both.
mode from a trap or from an external interrupt or both.
 
 
The status register bits are shown below:
These status register bits are summarized in Tbl.~\ref{tbl:ccbits}.
\begin{table}
\begin{table}
\begin{center}
\begin{center}
\begin{tabular}{l|l}
\begin{tabular}{l|l}
Bit & Meaning \\\hline
Bit & Meaning \\\hline
9 & Soft trap, set on a trap from user mode, cleared when returing to user mode\\\hline
9 & Soft trap, set on a trap from user mode, cleared when returing to user mode\\\hline
8 & (Reserved for) Floating point enable \\\hline
8 & (Reserved for) Floating point enable \\\hline
7 & Halt on break, to support an external debugger \\\hline
7 & Halt on break, to support an external debugger \\\hline
6 & Step, single step the CPU in user mode\\\hline
6 & Step, single step the CPU in user mode\\\hline
5 & GIE, or Global Interrupt Enable \\\hline
5 & GIE, or Global Interrupt Enable \\\hline
4 & Sleep \\\hline
4 & Sleep \\\hline
3 & V, or overflow bit.\\\hline
3 & V, or overflow bit.\\\hline
2 & N, or negative bit.\\\hline
2 & N, or negative bit.\\\hline
1 & C, or carry bit.\\\hline
1 & C, or carry bit.\\\hline
0 & Z, or zero bit. \\\hline
0 & Z, or zero bit. \\\hline
\end{tabular}
\end{tabular}
\end{center}
\caption{Condition Code / Status Register Bits}\label{tbl:ccbits}
\end{table}
\end{center}\end{table}
 
 
\section{Conditional Instructions}
\section{Conditional Instructions}
Most, although not quite all, instructions are conditionally executed.  From
Most, although not quite all, instructions are conditionally executed.  From
the four condition code flags, eight conditions are defined.  These are shown
the four condition code flags, eight conditions are defined.  These are shown
in Tbl.~\ref{tbl:conditions}.
in Tbl.~\ref{tbl:conditions}.
\begin{table}
\begin{table}
\begin{center}
\begin{center}
\begin{tabular}{l|l|l}
\begin{tabular}{l|l|l}
Code & Mneumonic & Condition \\\hline
Code & Mneumonic & Condition \\\hline
3'h0 & None & Always execute the instruction \\
3'h0 & None & Always execute the instruction \\
3'h1 & {\tt .Z} & Only execute when 'Z' is set \\
3'h1 & {\tt .Z} & Only execute when 'Z' is set \\
3'h2 & {\tt .NE} & Only execute when 'Z' is not set \\
3'h2 & {\tt .NE} & Only execute when 'Z' is not set \\
3'h3 & {\tt .GE} & Greater than or equal ('N' not set, 'Z' irrelevant) \\
3'h3 & {\tt .GE} & Greater than or equal ('N' not set, 'Z' irrelevant) \\
3'h4 & {\tt .GT} & Greater than ('N' not set, 'Z' not set) \\
3'h4 & {\tt .GT} & Greater than ('N' not set, 'Z' not set) \\
3'h5 & {\tt .LT} & Less than ('N' not set) \\
3'h5 & {\tt .LT} & Less than ('N' set) \\
3'h6 & {\tt .C} & Carry set\\
3'h6 & {\tt .C} & Carry set\\
3'h7 & {\tt .V} & Overflow set\\
3'h7 & {\tt .V} & Overflow set\\
\end{tabular}
\end{tabular}
\caption{Conditions for conditional operand execution}\label{tbl:conditions}
\caption{Conditions for conditional operand execution}\label{tbl:conditions}
\end{center}
\end{center}
\end{table}
\end{table}
There is no condition code for less than or equal, not C or not V.  Using
There is no condition code for less than or equal, not C or not V.  Sorry,
these conditions will take an extra instruction.
I ran out of space in 3--bits.  Using these conditions will take an extra
(Ex: \hbox{\tt TST \$4,CC;} \hbox{\tt STO.NZ R0,(R1)})
instruction.  (Ex: \hbox{\tt TST \$4,CC;} \hbox{\tt STO.NZ R0,(R1)})
 
 
\section{Operand B}
\section{Operand B}
Many instruction forms have a 21-bit source "Operand B" associated with them.
Many instruction forms have a 21-bit source ``Operand B'' associated with them.
This Operand B is either equal to a register plus a signed immediate offset,
This Operand B is either equal to a register plus a signed immediate offset,
or an immediate offset by itself.  This value is encoded as shown in
or an immediate offset by itself.  This value is encoded as shown in
Tbl.~\ref{tbl:opb}.
Tbl.~\ref{tbl:opb}.
\begin{table}\begin{center}
\begin{table}\begin{center}
\begin{tabular}{|l|l|l|}\hline
\begin{tabular}{|l|l|l|}\hline
Bit 20 & 19 \ldots 16 & 15 \ldots 0 \\\hline
Bit 20 & 19 \ldots 16 & 15 \ldots 0 \\\hline
1'b0 & \multicolumn{2}{l|}{Signed Immediate value} \\\hline
1'b0 & \multicolumn{2}{l|}{20--bit Signed Immediate value} \\\hline
1'b1 & 4-bit Register & 16-bit Signed immediate offset \\\hline
1'b1 & 4-bit Register & 16--bit Signed immediate offset \\\hline
\end{tabular}
\end{tabular}
\caption{Bit allocation for Operand B}\label{tbl:opb}
\caption{Bit allocation for Operand B}\label{tbl:opb}
\end{center}\end{table}
\end{center}\end{table}
 
 
 
Sixteen and twenty bit immediates don't make sense for all instructions.  For
 
example, what is the point of a 20--bit immediate when executing a 16--bit
 
multiply?  Likewise, why have a 16--bit immediate when adding to a logical
 
or arithmetic shift?  In these cases, the extra bits are reserved for future
 
instruction possibilities.
 
 
\section{Address Modes}
\section{Address Modes}
The ZIP CPU supports two addressing modes: register plus immediate, and
The ZIP CPU supports two addressing modes: register plus immediate, and
immediate address.  Addresses are therefore encoded in the same fashion as
immediate address.  Addresses are therefore encoded in the same fashion as
Operand B's, shown above.
Operand B's, shown above.
 
 
A lot of long hard thought was put into whether to allow pre/post increment
A lot of long hard thought was put into whether to allow pre/post increment
and decrement addressing modes.  Finding no way to use these operators without
and decrement addressing modes.  Finding no way to use these operators without
taking two or more clocks per instruction, these addressing modes have been
taking two or more clocks per instruction, these addressing modes have been
removed from the realm of possibilities.  This means that the Zip CPU has no
removed from the realm of possibilities.  This means that the Zip CPU has no
native way of executing push, pop, return, or jump to subroutine operations.
native way of executing push, pop, return, or jump to subroutine operations.
 
Each of these instructions can be emulated with a set of instructions from the
 
existing set.
 
 
\section{Move Operands}
\section{Move Operands}
The previous set of operands would be perfect and complete, save only that
The previous set of operands would be perfect and complete, save only that
        the CPU needs access to non--supervisory registers while in supervisory
the CPU needs access to non--supervisory registers while in supervisory mode.
        mode.  Therefore, the MOV instruction is special and offers access
Therefore, the MOV instruction is special and offers access to these registers
        to these registers ... when in supervisory mode.  To keep the compiler
\ldots when in supervisory mode.  To keep the compiler simple, the extra bits
        simple, the extra bits are ignored in non-supervisory mode (as though
are ignored in non-supervisory mode (as though they didn't exist), rather than
        they didn't exist), rather than being mapped to new instructions or
being mapped to new instructions or additional capabilities.  The bits
        additional capabilities.  The bits indicating which register set each
indicating which register set each register lies within are the A-Usr and
        register lies within are the A-Usr and B-Usr bits.  When set to a one,
B-Usr bits.  When set to a one, these refer to a user mode register.  When set
        these refer to a user mode register.  When set to a zero, these refer
to a zero, these refer to a register in the current mode, whether user or
        to a register in the current mode, whether user or supervisor.
supervisor.  Further, because a load immediate instruction exists, there is no
        Further, because
move capability between an immediate and a register: all moves come from either
        a load immediate instruction exists, there is no move capability between
a register or a register plus an offset.
        an immediate and a register: all moves come from either a register or
 
        a register plus an offset.
This actually leads to a bit of a problem: since the MOV instruction encodes
 
which register set each register is coming from or moving to, how shall a
This actually leads to a bit of a problem: since the MOV instruction
compiler or assembler know how to compile a MOV instruction without knowing
        encodes which register set each register is coming from or moving to,
the mode of the CPU at the time?  For this reason, the compiler will assume
        how shall a compiler or assembler know how to compile a MOV instruction
all MOV registers are supervisor registers, and display them as normal.
        without knowing the mode of the CPU at the time?  For this reason,
Anything with the user bit set will be treated as a user register.  The CPU
        the compiler will assume all MOV registers are supervisor registers,
will quietly ignore the supervisor bits while in user mode, and anything
        and display them as normal.  Anything with the user bit set will
marked as a user register will always be valid.  (Did I just say that in the
        be treated as a user register.  The CPU will quietly ignore the
last paragraph?)
        supervisor bits while in user mode, and anything marked as a user
 
        register will always be valid.
 
 
 
\section{Multiply Operations}
\section{Multiply Operations}
While the Zip CPU instruction set supports multiply operations, they are not
The Zip CPU supports two Multiply operations, a
yet fully supported by the CPU.  Two Multiply operations are supported, a
 
16x16 bit signed multiply (MPYS) and the same but unsigned (MPYU).  In both
16x16 bit signed multiply (MPYS) and the same but unsigned (MPYU).  In both
cases, the operand is a register plus a 16-bit immediate, subject to the
cases, the operand is a register plus a 16-bit immediate, subject to the
rule that the register cannot be the PC or CC registers.  The PC register
rule that the register cannot be the PC or CC registers.  The PC register
field has been stolen to create a multiply by immediate instruction.  The
field has been stolen to create a multiply by immediate instruction.  The
CC register field is reserved.
CC register field is reserved.
 
 
\section{Floating Point}
\section{Floating Point}
The ZIP CPU does not support floating point operations today.  However, the
The ZIP CPU does not support floating point operations today.  However, the
instruction set reserves a capability for a floating point operation.  To
instruction set reserves a capability for a floating point operation.  To
execute such an operation, simply set the floating point bit in the CC
execute such an operation, simply set the floating point bit in the CC
register and the following instruction will interpret its registers as
register and the following instruction will interpret its registers as
a floating point instruction.  Not all instructions, however, have floating
a floating point instruction.  Not all instructions, however, have floating
point equivalents.  Further, the immediate fields do not apply in floating
point equivalents.  Further, the immediate fields do not apply in floating
point mode, and must be set to zero.  Not all instructions make sense as
point mode, and must be set to zero.  Not all instructions make sense as
floating point operations.  Therefore, only the CMP, SUB, ADD, and MPY
floating point operations.  Therefore, only the CMP, SUB, ADD, and MPY
instructions may be issued as floating point instructions.  Other instructions
instructions may be issued as floating point instructions.  Other instructions
allow the examining of the floating point bit in the CC register.  In all
allow the examining of the floating point bit in the CC register.  In all
cases, the floating point bit is cleared one instruction after it is set.
cases, the floating point bit is cleared one instruction after it is set.
 
 
The architecture does not support a floating point not-implemented interrupt.
The architecture does not support a floating point not-implemented interrupt.
Any soft floating point emulation must be done deliberately.
Any soft floating point emulation must be done deliberately.
 
 
\section{Native Instructions}
\section{Native Instructions}
The instruction set for the Zip CPU is summarized in
The instruction set for the Zip CPU is summarized in
Tbl.~\ref{tbl:zip-instructions}.
Tbl.~\ref{tbl:zip-instructions}.
\begin{table}\begin{center}
\begin{table}\begin{center}
\begin{tabular}{|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|c|}\hline
\begin{tabular}{|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|c|}\hline
Op Code & \multicolumn{8}{c|}{31\ldots24} & \multicolumn{8}{c|}{23\ldots 16}
Op Code & \multicolumn{8}{c|}{31\ldots24} & \multicolumn{8}{c|}{23\ldots 16}
        & \multicolumn{8}{c|}{15\ldots 8} & \multicolumn{8}{c|}{7\ldots 0}
        & \multicolumn{8}{c|}{15\ldots 8} & \multicolumn{8}{c|}{7\ldots 0}
        & Sets CC? \\\hline
        & Sets CC? \\\hline
CMP(Sub) & \multicolumn{4}{l|}{4'h0}
CMP(Sub) & \multicolumn{4}{l|}{4'h0}
                & \multicolumn{4}{l|}{D. Reg}
                & \multicolumn{4}{l|}{D. Reg}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{21}{l|}{Operand B}
                & \multicolumn{21}{l|}{Operand B}
                & Yes \\\hline
                & Yes \\\hline
BTST(And) & \multicolumn{4}{l|}{4'h1}
TST(And) & \multicolumn{4}{l|}{4'h1}
                & \multicolumn{4}{l|}{D. Reg}
                & \multicolumn{4}{l|}{D. Reg}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{21}{l|}{Operand B}
                & \multicolumn{21}{l|}{Operand B}
        & Yes \\\hline
        & Yes \\\hline
MOV & \multicolumn{4}{l|}{4'h2}
MOV & \multicolumn{4}{l|}{4'h2}
                & \multicolumn{4}{l|}{D. Reg}
                & \multicolumn{4}{l|}{D. Reg}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{3}{l|}{Cond.}
                & A-Usr
                & A-Usr
                & \multicolumn{4}{l|}{B-Reg}
                & \multicolumn{4}{l|}{B-Reg}
                & B-Usr
                & B-Usr
                & \multicolumn{15}{l|}{15'bit signed offset}
                & \multicolumn{15}{l|}{15'bit signed offset}
                & \\\hline
                & \\\hline
LODI & \multicolumn{4}{l|}{4'h3}
LODI & \multicolumn{4}{l|}{4'h3}
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{24}{l|}{24'bit Signed Immediate}
                & \multicolumn{24}{l|}{24'bit Signed Immediate}
                & \\\hline
                & \\\hline
NOOP & \multicolumn{4}{l|}{4'h4}
NOOP & \multicolumn{4}{l|}{4'h4}
                & \multicolumn{4}{l|}{4'he}
                & \multicolumn{4}{l|}{4'he}
                & \multicolumn{24}{l|}{24'h00}
                & \multicolumn{24}{l|}{24'h00}
                & \\\hline
                & \\\hline
BREAK & \multicolumn{4}{l|}{4'h4}
BREAK & \multicolumn{4}{l|}{4'h4}
                & \multicolumn{4}{l|}{4'he}
                & \multicolumn{4}{l|}{4'he}
                & \multicolumn{24}{l|}{24'h01}
                & \multicolumn{24}{l|}{24'h01}
                & \\\hline
                & \\\hline
{\em Rsrd} & \multicolumn{4}{l|}{4'h4}
{\em Rsrd} & \multicolumn{4}{l|}{4'h4}
                & \multicolumn{4}{l|}{4'he}
                & \multicolumn{4}{l|}{4'he}
                & \multicolumn{24}{l|}{24'bits, but not 0 or 1.}
                & \multicolumn{24}{l|}{24'bits, but not 0 or 1.}
                & \\\hline
                & \\\hline
LODIHI & \multicolumn{4}{l|}{4'h4}
LODIHI & \multicolumn{4}{l|}{4'h4}
                & \multicolumn{4}{l|}{4'hf}
                & \multicolumn{4}{l|}{4'hf}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{3}{l|}{Cond.}
                & 1'b1
                & 1'b1
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{16}{l|}{16-bit Immediate}
                & \multicolumn{16}{l|}{16-bit Immediate}
                & \\\hline
                & \\\hline
LODILO & \multicolumn{4}{l|}{4'h4}
LODILO & \multicolumn{4}{l|}{4'h4}
                & \multicolumn{4}{l|}{4'hf}
                & \multicolumn{4}{l|}{4'hf}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{3}{l|}{Cond.}
                & 1'b0
                & 1'b0
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{16}{l|}{16-bit Immediate}
                & \multicolumn{16}{l|}{16-bit Immediate}
                & \\\hline
                & \\\hline
16-b MPYU & \multicolumn{4}{l|}{4'h4}
16-b MPYU & \multicolumn{4}{l|}{4'h4}
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{3}{l|}{Cond.}
                & 1'b0 & \multicolumn{4}{l|}{Reg}
                & 1'b0 & \multicolumn{4}{l|}{Reg}
                & \multicolumn{16}{l|}{16-bit Offset}
                & \multicolumn{16}{l|}{16-bit Offset}
                & Yes \\\hline
                & Yes \\\hline
16-b MPYU(I) & \multicolumn{4}{l|}{4'h4}
16-b MPYU(I) & \multicolumn{4}{l|}{4'h4}
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{3}{l|}{Cond.}
                & 1'b0 & \multicolumn{4}{l|}{4'hf}
                & 1'b0 & \multicolumn{4}{l|}{4'hf}
                & \multicolumn{16}{l|}{16-bit Offset}
                & \multicolumn{16}{l|}{16-bit Offset}
                & Yes \\\hline
                & Yes \\\hline
16-b MPYS & \multicolumn{4}{l|}{4'h4}
16-b MPYS & \multicolumn{4}{l|}{4'h4}
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{3}{l|}{Cond.}
                & 1'b1 & \multicolumn{4}{l|}{Reg}
                & 1'b1 & \multicolumn{4}{l|}{Reg}
                & \multicolumn{16}{l|}{16-bit Offset}
                & \multicolumn{16}{l|}{16-bit Offset}
                & Yes \\\hline
                & Yes \\\hline
16-b MPYS(I) & \multicolumn{4}{l|}{4'h4}
16-b MPYS(I) & \multicolumn{4}{l|}{4'h4}
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{3}{l|}{Cond.}
                & 1'b1 & \multicolumn{4}{l|}{4'hf}
                & 1'b1 & \multicolumn{4}{l|}{4'hf}
                & \multicolumn{16}{l|}{16-bit Offset}
                & \multicolumn{16}{l|}{16-bit Offset}
                & Yes \\\hline
                & Yes \\\hline
ROL & \multicolumn{4}{l|}{4'h5}
ROL & \multicolumn{4}{l|}{4'h5}
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{21}{l|}{Operand B, truncated to low order 5 bits}
                & \multicolumn{21}{l|}{Operand B, truncated to low order 5 bits}
                & \\\hline
                & \\\hline
LOD & \multicolumn{4}{l|}{4'h6}
LOD & \multicolumn{4}{l|}{4'h6}
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{4}{l|}{R. Reg}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{21}{l|}{Operand B address}
                & \multicolumn{21}{l|}{Operand B address}
                & \\\hline
                & \\\hline
STO & \multicolumn{4}{l|}{4'h7}
STO & \multicolumn{4}{l|}{4'h7}
                & \multicolumn{4}{l|}{D. Reg}
                & \multicolumn{4}{l|}{D. Reg}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{3}{l|}{Cond.}
                & \multicolumn{21}{l|}{Operand B address}
                & \multicolumn{21}{l|}{Operand B address}
                & \\\hline
                & \\\hline
{\em Rsrd} & \multicolumn{4}{l|}{4'h8}
{\em Rsrd} & \multicolumn{4}{l|}{4'h8}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{3}{l|}{Cond.}
        &       \multicolumn{3}{l|}{Cond.}
        & 1'b0
        & 1'b0
        &       \multicolumn{20}{l|}{Reserved}
        &       \multicolumn{20}{l|}{Reserved}
        & Yes \\\hline
        & Yes \\\hline
SUB & \multicolumn{4}{l|}{4'h8}
SUB & \multicolumn{4}{l|}{4'h8}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{3}{l|}{Cond.}
        &       \multicolumn{3}{l|}{Cond.}
        & 1'b1
        & 1'b1
        &       \multicolumn{4}{l|}{Reg}
        &       \multicolumn{4}{l|}{Reg}
        &       \multicolumn{16}{l|}{16'bit signed offset}
        &       \multicolumn{16}{l|}{16'bit signed offset}
        & Yes \\\hline
        & Yes \\\hline
AND & \multicolumn{4}{l|}{4'h9}
AND & \multicolumn{4}{l|}{4'h9}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{3}{l|}{Cond.}
        &       \multicolumn{3}{l|}{Cond.}
        &       \multicolumn{21}{l|}{Operand B}
        &       \multicolumn{21}{l|}{Operand B}
        & Yes \\\hline
        & Yes \\\hline
ADD & \multicolumn{4}{l|}{4'ha}
ADD & \multicolumn{4}{l|}{4'ha}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{3}{l|}{Cond.}
        &       \multicolumn{3}{l|}{Cond.}
        &       \multicolumn{21}{l|}{Operand B}
        &       \multicolumn{21}{l|}{Operand B}
        & Yes \\\hline
        & Yes \\\hline
OR & \multicolumn{4}{l|}{4'hb}
OR & \multicolumn{4}{l|}{4'hb}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{3}{l|}{Cond.}
        &       \multicolumn{3}{l|}{Cond.}
        &       \multicolumn{21}{l|}{Operand B}
        &       \multicolumn{21}{l|}{Operand B}
        & Yes \\\hline
        & Yes \\\hline
XOR & \multicolumn{4}{l|}{4'hc}
XOR & \multicolumn{4}{l|}{4'hc}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{3}{l|}{Cond.}
        &       \multicolumn{3}{l|}{Cond.}
        &       \multicolumn{21}{l|}{Operand B}
        &       \multicolumn{21}{l|}{Operand B}
        & Yes \\\hline
        & Yes \\\hline
LSL/ASL & \multicolumn{4}{l|}{4'hd}
LSL/ASL & \multicolumn{4}{l|}{4'hd}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{3}{l|}{Cond.}
        &       \multicolumn{3}{l|}{Cond.}
        &       \multicolumn{21}{l|}{Operand B, imm. trucated to 6 bits}
        &       \multicolumn{21}{l|}{Operand B, imm. trucated to 6 bits}
        & Yes \\\hline
        & Yes \\\hline
ASR & \multicolumn{4}{l|}{4'he}
ASR & \multicolumn{4}{l|}{4'he}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{3}{l|}{Cond.}
        &       \multicolumn{3}{l|}{Cond.}
        &       \multicolumn{21}{l|}{Operand B, imm. trucated to 6 bits}
        &       \multicolumn{21}{l|}{Operand B, imm. trucated to 6 bits}
        & Yes \\\hline
        & Yes \\\hline
LSR & \multicolumn{4}{l|}{4'hf}
LSR & \multicolumn{4}{l|}{4'hf}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{4}{l|}{R. Reg}
        &       \multicolumn{3}{l|}{Cond.}
        &       \multicolumn{3}{l|}{Cond.}
        &       \multicolumn{21}{l|}{Operand B, imm. trucated to 6 bits}
        &       \multicolumn{21}{l|}{Operand B, imm. trucated to 6 bits}
        & Yes \\\hline
        & Yes \\\hline
\end{tabular}
\end{tabular}
\caption{Zip CPU Instruction Set}\label{tbl:zip-instructions}
\caption{Zip CPU Instruction Set}\label{tbl:zip-instructions}
\end{center}\end{table}
\end{center}\end{table}
 
 
As you can see, there's lots of room for instruction set expansion.  The
As you can see, there's lots of room for instruction set expansion.  The
NOOP and BREAK instructions leave 24~bits of open instruction address
NOOP and BREAK instructions are the only instructions within one particular
space, minus the two instructions NOOP and BREAK.  The Subtract leaves half
24--bit hole.  Likewise, the subtract leaves half of its space open, since a
of its space open, since a subtract immediate is the same as an add with a
subtract immediate is the same as an add with a negated immediate.  This
negated immediate.
spaces are reserved for future enhancements.
 
 
\section{Derived Instructions}
\section{Derived Instructions}
The ZIP CPU supports many other common instructions, but not all of them
The ZIP CPU supports many other common instructions, but not all of them
are single instructions.  The derived instruction tables,
are single cycle instructions.  The derived instruction tables,
Tbls.~\ref{tbl:derived-1}, \ref{tbl:derived-2}, and~\ref{tbl:derived-3},
Tbls.~\ref{tbl:derived-1}, \ref{tbl:derived-2}, and~\ref{tbl:derived-3},
help to capture some of how these other instructions may be implemented on
help to capture some of how these other instructions may be implemented on
the ZIP CPU.  Many of these instructions will have assembly equivalents,
the ZIP CPU.  Many of these instructions will have assembly equivalents,
such as the branch instructions, to facilitate working with the CPU.
such as the branch instructions, to facilitate working with the CPU.
\begin{table}\begin{center}
\begin{table}\begin{center}
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
Mapped & Actual  & Notes \\\hline
Mapped & Actual  & Notes \\\hline
\parbox[t]{1.4in}{ADD Ra,Rx\\ADDC Rb,Ry}
\parbox[t]{1.4in}{ADD Ra,Rx\\ADDC Rb,Ry}
        & \parbox[t]{1.5in}{Add Ra,Rx\\ADD.C \$1,Ry\\Add Rb,Ry}
        & \parbox[t]{1.5in}{Add Ra,Rx\\ADD.C \$1,Ry\\Add Rb,Ry}
        & Add with carry \\\hline
        & Add with carry \\\hline
BRA.Cond +/-\$Addr
BRA.Cond +/-\$Addr
        & Mov.cond \$Addr+PC,PC
        & \hbox{Mov.cond \$Addr+PC,PC}
        & Branch or jump on condition.  Works for 14 bit
        & Branch or jump on condition.  Works for 15--bit
                address offsets.\\\hline
                signed address offsets.\\\hline
BRA.Cond +/-\$Addr
BRA.Cond +/-\$Addr
        & \parbox[t]{1.5in}{LDI \$Addr,Rx \\ ADD.cond Rx,PC}
        & \parbox[t]{1.5in}{LDI \$Addr,Rx \\ ADD.cond Rx,PC}
        & Branch/jump on condition.  Works for
        & Branch/jump on condition.  Works for
        23 bit address offsets, but costs a register, an extra instruction,
        23 bit address offsets, but costs a register, an extra instruction,
        and setsthe flags. \\\hline
        and setsthe flags. \\\hline
BNC PC+\$Addr
BNC PC+\$Addr
        & \parbox[t]{1.5in}{Test \$Carry,CC \\ MOV.Z PC+\$Addr,PC}
        & \parbox[t]{1.5in}{Test \$Carry,CC \\ MOV.Z PC+\$Addr,PC}
        & Example of a branch on an unsupported
        & Example of a branch on an unsupported
                condition, in this case a branch on not carry \\\hline
                condition, in this case a branch on not carry \\\hline
BUSY & MOV \$-1(PC),PC & Execute an infinite loop \\\hline
BUSY & MOV \$-1(PC),PC & Execute an infinite loop \\\hline
CLRF.NZ Rx
CLRF.NZ Rx
        & XOR.NZ Rx,Rx
        & XOR.NZ Rx,Rx
        & Clear Rx, and flags, if the Z-bit is not set \\\hline
        & Clear Rx, and flags, if the Z-bit is not set \\\hline
CLR Rx
CLR Rx
        & LDI \$0,Rx
        & LDI \$0,Rx
        & Clears Rx, leaves flags untouched.  This instruction cannot be
        & Clears Rx, leaves flags untouched.  This instruction cannot be
                conditional. \\\hline
                conditional. \\\hline
EXCH.W Rx
EXCH.W Rx
        & ROL \$16,Rx
        & ROL \$16,Rx
        & Exchanges the top and bottom 16'bit words of Rx \\\hline
        & Exchanges the top and bottom 16'bit words of Rx \\\hline
HALT
HALT
        & Or \$SLEEP,CC
        & Or \$SLEEP,CC
        & Executed while in interrupt mode.  In user mode this is simply a
        & Executed while in interrupt mode.  In user mode this is simply a
        wait until interrupt instructioon. \\\hline
        wait until interrupt instructioon. \\\hline
INT & LDI \$0,CC
INT & LDI \$0,CC
        & Since we're using the CC register as a trap vector as well, this
        & Since we're using the CC register as a trap vector as well, this
        executes TRAP \#0. \\\hline
        executes TRAP \#0. \\\hline
IRET
IRET
        & OR \$GIE,CC
        & OR \$GIE,CC
        & Also an RTU instruction (Return to Userspace) \\\hline
        & Also an RTU instruction (Return to Userspace) \\\hline
JMP R6+\$Addr
JMP R6+\$Addr
        & MOV \$Addr(R6),PC
        & MOV \$Addr(R6),PC
        & \\\hline
        & \\\hline
JSR PC+\$Addr
JSR PC+\$Addr
        & \parbox[t]{1.5in}{SUB \$1,SP \\\
        & \parbox[t]{1.5in}{SUB \$1,SP \\\
        MOV \$3+PC,R0 \\
        MOV \$3+PC,R0 \\
        STO R0,1(SP) \\
        STO R0,1(SP) \\
        MOV \$Addr+PC,PC \\
        MOV \$Addr+PC,PC \\
        ADD \$1,SP}
        ADD \$1,SP}
        & Jump to Subroutine. \\\hline
        & Jump to Subroutine. Note the required cleanup instruction after
 
        returning. \\\hline
JSR PC+\$Addr
JSR PC+\$Addr
        & \parbox[t]{1.5in}{MOV \$3+PC,R12 \\ MOV \$addr+PC,PC}
        & \parbox[t]{1.5in}{MOV \$3+PC,R12 \\ MOV \$addr+PC,PC}
        &This is the high speed
        &This is the high speed
        version of a subroutine call, necessitating a register to hold the
        version of a subroutine call, necessitating a register to hold the
        last PC address.  In its favor, this method doesn't suffer the
        last PC address.  In its favor, this method doesn't suffer the
        mandatory memory access of the other approach. \\\hline
        mandatory memory access of the other approach. \\\hline
LDI.l \$val,Rx
LDI.l \$val,Rx
        & \parbox[t]{1.5in}{LDIHI (\$val$>>$16)\&0x0ffff, Rx \\
        & \parbox[t]{1.5in}{LDIHI (\$val$>>$16)\&0x0ffff, Rx \\
                        LDILO (\$val \& 0x0ffff)}
                        LDILO (\$val \& 0x0ffff)}
        & Sadly, there's not enough instruction
        & Sadly, there's not enough instruction
                space to load a complete immediate value into any register.
                space to load a complete immediate value into any register.
                Therefore, fully loading any register takes two cycles.
                Therefore, fully loading any register takes two cycles.
                The LDIHI (load immediate high) and LDILO (load immediate low)
                The LDIHI (load immediate high) and LDILO (load immediate low)
                instructions have been created to facilitate this. \\\hline
                instructions have been created to facilitate this. \\\hline
\end{tabular}
\end{tabular}
\caption{Derived Instructions}\label{tbl:derived-1}
\caption{Derived Instructions}\label{tbl:derived-1}
\end{center}\end{table}
\end{center}\end{table}
\begin{table}\begin{center}
\begin{table}\begin{center}
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
Mapped & Actual  & Notes \\\hline
Mapped & Actual  & Notes \\\hline
LOD.b \$addr,Rx
LOD.b \$addr,Rx
        & \parbox[t]{1.5in}{%
        & \parbox[t]{1.5in}{%
        LDI     \$addr,Ra \\
        LDI     \$addr,Ra \\
        LDI     \$addr,Rb \\
        LDI     \$addr,Rb \\
        LSR     \$2,Ra \\
        LSR     \$2,Ra \\
        AND     \$3,Rb \\
        AND     \$3,Rb \\
        LOD     (Ra),Rx \\
        LOD     (Ra),Rx \\
        LSL     \$3,Rb \\
        LSL     \$3,Rb \\
        SUB     \$32,Rb \\
        SUB     \$32,Rb \\
        ROL     Rb,Rx \\
        ROL     Rb,Rx \\
        AND \$0ffh,Rx}
        AND \$0ffh,Rx}
        & \parbox[t]{3in}{This CPU is designed for 32'bit word
        & \parbox[t]{3in}{This CPU is designed for 32'bit word
        length instructions.  Byte addressing is not supported by the CPU or
        length instructions.  Byte addressing is not supported by the CPU or
        the bus, so it therefore takes more work to do.
        the bus, so it therefore takes more work to do.
 
 
        Note also that in this example, \$Addr is a byte-wise address, where
        Note also that in this example, \$Addr is a byte-wise address, where
        all other addresses are 32-bit wordlength addresses.  For this reason,
        all other addresses in this document are 32-bit wordlength addresses.
 
        For this reason,
        we needed to drop the bottom two bits.  This also limits the address
        we needed to drop the bottom two bits.  This also limits the address
        space of character accesses using this method from 16 MB down to 4MB.}
        space of character accesses using this method from 16 MB down to 4MB.}
                \\\hline
                \\\hline
\parbox[t]{1.5in}{LSL \$1,Rx\\ LSLC \$1,Ry}
\parbox[t]{1.5in}{LSL \$1,Rx\\ LSLC \$1,Ry}
        & \parbox[t]{1.5in}{LSL \$1,Ry \\
        & \parbox[t]{1.5in}{LSL \$1,Ry \\
        LSL \$1,Rx \\
        LSL \$1,Rx \\
        OR.C \$1,Ry}
        OR.C \$1,Ry}
        & Logical shift left with carry.  Note that the
        & Logical shift left with carry.  Note that the
        instruction order is now backwards, to keep the conditions valid.
        instruction order is now backwards, to keep the conditions valid.
        That is, LSL sets the carry flag, so if we did this the othe way
        That is, LSL sets the carry flag, so if we did this the othe way
        with Rx before Ry, then the condition flag wouldn't have been right
        with Rx before Ry, then the condition flag wouldn't have been right
        for an OR correction at the end. \\\hline
        for an OR correction at the end. \\\hline
\parbox[t]{1.5in}{LSR \$1,Rx \\ LSRC \$1,Ry}
\parbox[t]{1.5in}{LSR \$1,Rx \\ LSRC \$1,Ry}
        & \parbox[t]{1.5in}{CLR Rz \\
        & \parbox[t]{1.5in}{CLR Rz \\
        LSR \$1,Ry \\
        LSR \$1,Ry \\
        LDIHI.C \$8000h,Rz \\
        LDIHI.C \$8000h,Rz \\
        LSR \$1,Rx \\
        LSR \$1,Rx \\
        OR Rz,Rx}
        OR Rz,Rx}
        & Logical shift right with carry \\\hline
        & Logical shift right with carry \\\hline
NEG Rx & \parbox[t]{1.5in}{XOR \$-1,Rx \\ ADD \$1,Rx} & \\\hline
NEG Rx & \parbox[t]{1.5in}{XOR \$-1,Rx \\ ADD \$1,Rx} & \\\hline
NOOP & NOOP & While there are many
NOOP & NOOP & While there are many
        operations that do nothing, such as MOV Rx,Rx, or OR \$0,Rx, these
        operations that do nothing, such as MOV Rx,Rx, or OR \$0,Rx, these
        operations have consequences in that they might stall the bus if
        operations have consequences in that they might stall the bus if
        Rx isn't ready yet.  For this reason, we have a dedicated NOOP
        Rx isn't ready yet.  For this reason, we have a dedicated NOOP
        instruction. \\\hline
        instruction. \\\hline
NOT Rx & XOR \$-1,Rx & \\\hline
NOT Rx & XOR \$-1,Rx & \\\hline
POP Rx
POP Rx
        & \parbox[t]{1.5in}{LOD \$-1(SP),Rx \\ ADD \$1,SP}
        & \parbox[t]{1.5in}{LOD \$-1(SP),Rx \\ ADD \$1,SP}
        & Note
        & Note
        that for interrupt purposes, one can never depend upon the value at
        that for interrupt purposes, one can never depend upon the value at
        (SP).  Hence you read from it, then increment it, lest having
        (SP).  Hence you read from it, then increment it, lest having
        incremented it firost something then comes along and writes to that
        incremented it firost something then comes along and writes to that
        value before you can read the result. \\\hline
        value before you can read the result. \\\hline
PUSH Rx
PUSH Rx
        & \parbox[t]{1.5in}{SUB \$1,SPa \\
        & \parbox[t]{1.5in}{SUB \$1,SPa \\
        STO Rx,\$1(SP)}
        STO Rx,\$1(SP)}
        & \\\hline
        & \\\hline
RESET
RESET
        & \parbox[t]{1in}{STO \$1,\$watchdog(R12)\\NOOP\\NOOP}
        & \parbox[t]{1in}{STO \$1,\$watchdog(R12)\\NOOP\\NOOP}
        & \parbox[t]{3in}{This depends upon the peripheral base address being
        & \parbox[t]{3in}{This depends upon the peripheral base address being
        in R12.
        in R12.
 
 
        Another opportunity might be to jump to the reset address from within
        Another opportunity might be to jump to the reset address from within
        supervisor mode.}\\\hline
        supervisor mode.}\\\hline
RET & \parbox[t]{1.5in}{LOD \$-1(SP),R0 \\
RET & \parbox[t]{1.5in}{LOD \$-1(SP),PC}
        MOV \$-1+SP,SP \\
        & Note that this depends upon the calling context to clean up the
        MOV R0,PC}
        stack, as outlined for the JSR instruction.  \\\hline
        & An alternative might be to LOD \$-1(SP),PC, followed
 
        by depending upon the calling program to ADD \$1,SP. \\\hline
 
\end{tabular}
\end{tabular}
\caption{Derived Instructions, continued}\label{tbl:derived-2}
\caption{Derived Instructions, continued}\label{tbl:derived-2}
\end{center}\end{table}
\end{center}\end{table}
\begin{table}\begin{center}
\begin{table}\begin{center}
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
RET & MOV R12,PC
RET & MOV R12,PC
        & This is the high(er) speed version, that doesn't touch the stack.
        & This is the high(er) speed version, that doesn't touch the stack.
        As such, it doesn't suffer a stall on memory read/write to the stack.
        As such, it doesn't suffer a stall on memory read/write to the stack.
        \\\hline
        \\\hline
STEP Rr,Rt
STEP Rr,Rt
        & \parbox[t]{1.5in}{LSR \$1,Rr \\ XOR.C Rt,Rr}
        & \parbox[t]{1.5in}{LSR \$1,Rr \\ XOR.C Rt,Rr}
        & Step a Galois implementation of a Linear Feedback Shift Register, Rr,
        & Step a Galois implementation of a Linear Feedback Shift Register, Rr,
                using taps Rt \\\hline
                using taps Rt \\\hline
STO.b Rx,\$addr
STO.b Rx,\$addr
        & \parbox[t]{1.5in}{%
        & \parbox[t]{1.5in}{%
        LDI \$addr,Ra \\
        LDI \$addr,Ra \\
        LDI \$addr,Rb \\
        LDI \$addr,Rb \\
        LSR \$2,Ra \\
        LSR \$2,Ra \\
        AND \$3,Rb \\
        AND \$3,Rb \\
        SUB \$32,Rb \\
        SUB \$32,Rb \\
        LOD (Ra),Ry \\
        LOD (Ra),Ry \\
        AND \$0ffh,Rx \\
        AND \$0ffh,Rx \\
        AND \$-0ffh,Ry \\
        AND \$-0ffh,Ry \\
        ROL Rb,Rx \\
        ROL Rb,Rx \\
        OR Rx,Ry \\
        OR Rx,Ry \\
        STO Ry,(Ra) }
        STO Ry,(Ra) }
        & \parbox[t]{3in}{This CPU and it's bus are {\em not} optimized
        & \parbox[t]{3in}{This CPU and it's bus are {\em not} optimized
        for byte-wise operations.
        for byte-wise operations.
 
 
        Note that in this example, \$addr is a
        Note that in this example, \$addr is a
        byte-wise address, whereas in all of our other examples it is a
        byte-wise address, whereas in all of our other examples it is a
        32-bit word address. This also limits the address space
        32-bit word address. This also limits the address space
        of character accesses from 16 MB down to 4MB.F
        of character accesses from 16 MB down to 4MB.F
        Further, this instruction implies a byte ordering,
        Further, this instruction implies a byte ordering,
        such as big or little endian.} \\\hline
        such as big or little endian.} \\\hline
SWAP Rx,Ry
SWAP Rx,Ry
        & \parbox[t]{1.5in}{
        & \parbox[t]{1.5in}{
        XOR Ry,Rx \\
        XOR Ry,Rx \\
        XOR Rx,Ry \\
        XOR Rx,Ry \\
        XOR Ry,Rx}
        XOR Ry,Rx}
        & While no extra registers are needed, this example
        & While no extra registers are needed, this example
        does take 3-clocks. \\\hline
        does take 3-clocks. \\\hline
TRAP \#X
TRAP \#X
        & LDILO \$x,CC
        & LDILO \$x,CC
        & This approach uses the unused bits of the CC register as a TRAP
        & This approach uses the unused bits of the CC register as a TRAP
        address.  If these bits are zero, no trap has occurred.  Unlike my
        address.  The user will need to make certain
        previous approach, which was to use a trap peripheral, this approach
 
        has no delay associated with it.  To work, the supervisor will need
 
        to clear this register following any trap, and the user will need to
 
        be careful to only set this register prior to a trap condition.
 
        Likewise, when setting this value, the user will need to make certain
 
        that the SLEEP and GIE bits are not set in \$x.  LDI would also work,
        that the SLEEP and GIE bits are not set in \$x.  LDI would also work,
        however using LDILO permits the use of conditional traps.  (i.e.,
        however using LDILO permits the use of conditional traps.  (i.e.,
        trap if the zero flag is set.)  Should you wish to trap off of a
        trap if the zero flag is set.)  Should you wish to trap off of a
        register value, you could equivalently load \$x into the register and
        register value, you could equivalently load \$x into the register and
        then MOV it into the CC register. \\\hline
        then MOV it into the CC register. \\\hline
TST Rx
TST Rx
        & TST \$-1,Rx
        & TST \$-1,Rx
        & Set the condition codes based upon Rx.  Could also do a CMP \$0,Rx,
        & Set the condition codes based upon Rx.  Could also do a CMP \$0,Rx,
        ADD \$0,Rx, SUB \$0,Rx, etc, AND \$-1,Rx, etc.  The TST and CMP
        ADD \$0,Rx, SUB \$0,Rx, etc, AND \$-1,Rx, etc.  The TST and CMP
        approaches won't stall future pipeline stages looking for the value
        approaches won't stall future pipeline stages looking for the value
        of Rx. \\\hline
        of Rx. \\\hline
WAIT
WAIT
        & Or \$SLEEP,CC
        & Or \$SLEEP,CC
        & Wait 'til interrupt.  In an interrupts disabled context, this
        & Wait 'til interrupt.  In an interrupts disabled context, this
        becomes a HALT instruction.
        becomes a HALT instruction.
</TABLE>
 
\end{tabular}
\end{tabular}
\caption{Derived Instructions, continued}\label{tbl:derived-3}
\caption{Derived Instructions, continued}\label{tbl:derived-3}
\end{center}\end{table}
\end{center}\end{table}
\iffalse
\iffalse
\fi
\fi
\section{Pipeline Stages}
\section{Pipeline Stages}
\begin{enumerate}
\begin{enumerate}
\item {\bf Prefetch}: Read instruction from memory (cache if possible).  This
\item {\bf Prefetch}: Read instruction from memory (cache if possible).  This
        stage is actually pipelined itself, and so it will stall if the PC
        stage is actually pipelined itself, and so it will stall if the PC
        ever changes.  Stalls are also created here if the instruction isn't
        ever changes.  Stalls are also created here if the instruction isn't
        in the prefetch cache.
        in the prefetch cache.
\item {\bf Decode}: Decode instruction into op code, register(s) to read, and
\item {\bf Decode}: Decode instruction into op code, register(s) to read, and
        immediate offset.
        immediate offset.
\item {\bf Read Operands}: Read registers and apply any immediate values to
\item {\bf Read Operands}: Read registers and apply any immediate values to
        them.  This stage will stall if any source operand is pending.
        them.  There is no means of detecting or flagging arithmetic overflow
 
        or carry when adding the immediate to the operand.  This stage will
 
        stall if any source operand is pending.
        A proper optimizing compiler, therefore, will schedule an instruction
        A proper optimizing compiler, therefore, will schedule an instruction
        between the instruction that produces the result and the instruction
        between the instruction that produces the result and the instruction
        that uses it.
        that uses it.
\item Split into two tracks: An {\bf ALU} which will accomplish a simple
\item Split into two tracks: An {\bf ALU} which will accomplish a simple
        instruction, and the {\bf MemOps} stage which accomplishes memory
        instruction, and the {\bf MemOps} stage which accomplishes memory
        read/write.
        read/write.
        \begin{itemize}
        \begin{itemize}
        \item Loads stall instructions that access the register until it is
        \item Loads stall instructions that access the register until it is
                written to the register set.
                written to the register set.
        \item Condition codes are available upon completion
        \item Condition codes are available upon completion
        \item Issuing an instruction to the memory while the memory is busy will
        \item Issuing an instruction to the memory while the memory is busy will
                stall the bus.  If the bus deadlocks, only a reset will
                stall the bus.  If the bus deadlocks, only a reset will
                release the CPU.  (Watchdog timer, anyone?)
                release the CPU.  (Watchdog timer, anyone?)
 
        \item The Zip CPU currently has no means of reading and acting on any
 
        error conditions on the bus.
        \end{itemize}
        \end{itemize}
\item {\bf Write-Back}: Conditionally write back the result to register set,
\item {\bf Write-Back}: Conditionally write back the result to register set,
        applying the condition.  This routine is bi-re-entrant: either the
        applying the condition.  This routine is bi-re-entrant: either the
        memory or the simple instruction may request a register write.
        memory or the simple instruction may request a register write.
\end{enumerate}
\end{enumerate}
 
 
 
The Zip CPU does not support out of order execution.  Therefore, if the memory
 
unit stalls, every other instruction stalls.  Memory stores, however, can take
 
place concurrently with ALU operations, although memory writes cannot.
 
 
\section{Pipeline Logic}
\section{Pipeline Logic}
How the CPU handles some instruction combinations can be telling when
How the CPU handles some instruction combinations can be telling when
determining what happens in the pipeline.  The following lists some examples:
determining what happens in the pipeline.  The following lists some examples:
\begin{itemize}
\begin{itemize}
\item {\bf Delayed Branching}
\item {\bf Delayed Branching}
 
 
        I had originally hoped to implement delayed branching.  However, what
        I had originally hoped to implement delayed branching.  However, what
        happens in debug mode?
        happens in debug mode?
        That is, what happens when a debugger tries to single step an
        That is, what happens when a debugger tries to single step an
        instruction?  While I can easily single step the computer in either
        instruction?  While I can easily single step the computer in either
        user or supervisor mode from externally, this processor does not appear
        user or supervisor mode from externally, this processor does not appear
        able to step the CPU in user mode from within user mode--gosh, not even
        able to step the CPU in user mode from within user mode--gosh, not even
        from within supervisor mode--such as if a process had a debugger
        from within supervisor mode--such as if a process had a debugger
        attached.  As the processor exists, I would have one result stepping
        attached.  As the processor exists, I would have one result stepping
        the CPU from a debugger, and another stepping it externally.
        the CPU from a debugger, and another stepping it externally.
 
 
        This is unacceptable, and so this CPU does not support delayed
        This is unacceptable, and so this CPU does not support delayed
        branching.
        branching.
 
 
\item {\bf Register Result:} {\tt MOV R0,R1; MOV R1,R2 }
\item {\bf Register Result:} {\tt MOV R0,R1; MOV R1,R2 }
 
 
        What value does
        What value does
        R2 get, the value of R1 before the first move or the value of R0?
        R2 get, the value of R1 before the first move or the value of R0?
        Placing the value of R0 into R1 requires a pipeline stall, and possibly
        Placing the value of R0 into R1 requires a pipeline stall, and possibly
        two, as I have the pipeline designed.
        two, as I have the pipeline designed.
 
 
        The ZIP CPU architecture requires that R2 must equal R0 at the end of
        The ZIP CPU architecture requires that R2 must equal R0 at the end of
        this operation.  This may stall the pipeline 1-2 cycles.
        this operation.  This may stall the pipeline 1-2 cycles.
 
 
\item {\bf Condition Codes Result:} {\tt CMP R0,R1;Mov.EQ \$x,PC}
\item {\bf Condition Codes Result:} {\tt CMP R0,R1;Mov.EQ \$x,PC}
 
 
 
 
        At issue is the same item as above, save that the CMP instruction
        At issue is the same item as above, save that the CMP instruction
        updates the flags that the MOV instruction depends
        updates the flags that the MOV instruction depends
        upon.
        upon.
 
 
        The Zip CPU architecture requires that condition codes must be updated
        The Zip CPU architecture requires that condition codes must be updated
        and available immediately for the next instruction without stalling the
        and available immediately for the next instruction without stalling the
        pipeline.
        pipeline.
 
 
\item {\bf Condition Codes Register Result:} {\tt CMP R0,R1; MOV CC,R2}
\item {\bf Condition Codes Register Result:} {\tt CMP R0,R1; MOV CC,R2}
 
 
        At issue is the
        At issue is the
        fact that the logic supporting the CC register is more complicated than
        fact that the logic supporting the CC register is more complicated than
        the logic supporting any other register.
        the logic supporting any other register.
 
 
        The ZIP CPU will stall 1--2 cycles on this instruction, until the
        The ZIP CPU will stall 1--2 cycles on this instruction, until the
        CC register is valid.
        CC register is valid.
 
 
\item {\bf Delayed Branching: } {\tt ADD \$x,PC; MOV R0,R1}
\item {\bf Delayed Branching: } {\tt ADD \$x,PC; MOV R0,R1}
 
 
        At issues is whether or not the instruction following the jump will
        At issues is whether or not the instruction following the jump will
        take place before the jump.  In other words, is the MOV to the PC
        take place before the jump.  In other words, is the MOV to the PC
        register handled differently from an ADD to the PC register?
        register handled differently from an ADD to the PC register?
 
 
        In the Zip architecture, MOV'es and ADD's use the same logic
        In the Zip architecture, MOV'es and ADD's use the same logic
        (simplifies the logic).
        (simplifies the logic).
\end{itemize}
\end{itemize}
 
 
As I've studied  this, I find several approaches to handling pipeline
As I've studied  this, I find several approaches to handling pipeline
        issues.  These approaches (and their consequences) are listed below.
        issues.  These approaches (and their consequences) are listed below.
 
 
\begin{itemize}
\begin{itemize}
\item {\bf All All issued instructions complete, Stages stall individually}
\item {\bf All All issued instructions complete, Stages stall individually}
 
 
        What about a slow pre-fetch?
        What about a slow pre-fetch?
 
 
        Nominally, this works well: any issued instruction
        Nominally, this works well: any issued instruction
        just runs to completion.  If there are four issued instructions in the
        just runs to completion.  If there are four issued instructions in the
        pipeline, with the writeback instruction being a write-to-PC
        pipeline, with the writeback instruction being a write-to-PC
        instruction, the other three instructions naturally finish.
        instruction, the other three instructions naturally finish.
 
 
        This approach fails when reading instructions from the flash,
        This approach fails when reading instructions from the flash,
        since such reads require N clocks to clocks to complete.  Thus
        since such reads require N clocks to clocks to complete.  Thus
        there may be only one instruction in the pipeline if reading from flash,
        there may be only one instruction in the pipeline if reading from flash,
        or a full pipeline if reading from cache.  Each of these approaches
        or a full pipeline if reading from cache.  Each of these approaches
        would produce a different response.
        would produce a different response.
 
 
\item {\bf Issued instructions may be canceled}
\item {\bf Issued instructions may be canceled}
 
 
        Stages stall individually
        Stages stall individually
 
 
        First problem:
        First problem:
        Memory operations cannot be canceled, even reads may have side effects
        Memory operations cannot be canceled, even reads may have side effects
        on peripherals that cannot be canceled later.  Further, in the case of
        on peripherals that cannot be canceled later.  Further, in the case of
        an interrupt, it's difficult to know what to cancel.  What happens in
        an interrupt, it's difficult to know what to cancel.  What happens in
        a \hbox{\tt MOV.C \$x,PC} followed by a \hbox{\tt MOV \$y,PC}
        a \hbox{\tt MOV.C \$x,PC} followed by a \hbox{\tt MOV \$y,PC}
        instruction?  Which get
        instruction?  Which get
        canceled?
        canceled?
 
 
        Because it isn't clear what would need to be canceled,
        Because it isn't clear what would need to be canceled,
        this instruction combination is not recommended.
        this instruction combination is not recommended.
 
 
\item {\bf All issued instructions complete.}
\item {\bf All issued instructions complete.}
 
 
        All stages are filled, or the entire pipeline
        All stages are filled, or the entire pipeline
        stalls.
        stalls.
 
 
        What about debug control?  What about
        What about debug control?  What about
        register writes taking an extra clock stage?  MOV R0,R1; MOV R1,R2
        register writes taking an extra clock stage?  MOV R0,R1; MOV R1,R2
        should place the value of R0 into R2.  How do you restart the pipeline
        should place the value of R0 into R2.  How do you restart the pipeline
        after an interrupt?  What address do you use?  The last issued
        after an interrupt?  What address do you use?  The last issued
        instruction?  But the branch delay slots may make that invalid!
        instruction?  But the branch delay slots may make that invalid!
 
 
        Reading from the CPU debug port in this case yields inconsistent
        Reading from the CPU debug port in this case yields inconsistent
        results: the CPU will halt or step with instructions stuck in the
        results: the CPU will halt or step with instructions stuck in the
        pipeline.  Reading registers will give no indication of what is going
        pipeline.  Reading registers will give no indication of what is going
        on in the pipeline, just the results of completed operations, not of
        on in the pipeline, just the results of completed operations, not of
        operations that have been started and not yet completed.
        operations that have been started and not yet completed.
        Perhaps we should just report the state of the CPU based upon what
        Perhaps we should just report the state of the CPU based upon what
        instructions (PC values) have successfully completed?  Thus the
        instructions (PC values) have successfully completed?  Thus the
        debug instruction is the one that will write registers on the next
        debug instruction is the one that will write registers on the next
        clock.
        clock.
 
 
        Suggestion: Suppose we load extra information in the two
        Suggestion: Suppose we load extra information in the two
        CC register(s) for debugging intermediate pipeline stages?
        CC register(s) for debugging intermediate pipeline stages?
 
 
        The next problem, though, is how to deal with the read operand
        The next problem, though, is how to deal with the read operand
        pipeline stage needing the result from the register pipeline.a
        pipeline stage needing the result from the register pipeline.a
 
 
\item {\bf Memory instructions must complete}
\item {\bf Memory instructions must complete}
 
 
        All instructions that enter into the memory module *must*
        All instructions that enter into the memory module *must*
        complete.  Issued instructions from the prefetch, decode, or operand
        complete.  Issued instructions from the prefetch, decode, or operand
        read stages may or may not complete.  Jumps into code must be valid,
        read stages may or may not complete.  Jumps into code must be valid,
        so that interrupt returns may be valid.  All instructions entering the
        so that interrupt returns may be valid.  All instructions entering the
        ALU complete.
        ALU complete.
 
 
        This looks to be the simplest approach.
        This looks to be the simplest approach.
        While the logic may be difficult, this appears to be the only
        While the logic may be difficult, this appears to be the only
        re-entrant approach.
        re-entrant approach.
 
 
        A {\tt new\_pc} flag will be high anytime the PC changes in an
        A {\tt new\_pc} flag will be high anytime the PC changes in an
        unpredictable way (i.e., it doesn't increment).  This includes jumps
        unpredictable way (i.e., it doesn't increment).  This includes jumps
        as well as interrupts and interrupt returns.  Whenever this flag may
        as well as interrupts and interrupt returns.  Whenever this flag may
        go high, memory operations and ALU operations will stall until the
        go high, memory operations and ALU operations will stall until the
        result is known.  When the flag does go high, anything in the prefetch,
        result is known.  When the flag does go high, anything in the prefetch,
        decode, and read-op stage will be invalidated.
        decode, and read-op stage will be invalidated.
 
 
\end{itemize}
\end{itemize}
 
 
 
 
 
 
\chapter{Peripherals}\label{chap:periph}
\chapter{Peripherals}\label{chap:periph}
 
 
 
While the previous chapter describes a CPU in isolation, the Zip System
 
includes a minimum set of peripherals as well.  These peripherals are shown
 
in Fig.~\ref{fig:zipsystem}
 
\begin{figure}\begin{center}
 
\includegraphics[width=3.5in]{../gfx/system.eps}
 
\caption{Zip System Peripherals}\label{fig:zipsystem}
 
\end{center}\end{figure}
 
and described here.  They are designed to make
 
the Zip CPU more useful in an Embedded Operating System environment.
 
 
\section{Interrupt Controller}
\section{Interrupt Controller}
 
 
 
Perhaps the most important peripheral within the Zip System is the interrupt
 
controller.  While the Zip CPU itself can only handle one interrupt, and has
 
only the one interrupt state: disabled or enabled, the interrupt controller
 
can make things more interesting.
 
 
 
The Zip System interrupt controller module supports up to 15 interrupts, all
 
controlled from one register.  Bit~31 of the interrupt controller controls
 
overall whether interrupts are enabled (1'b1) or disabled (1'b0).  Bits~16--30
 
control whether individual interrupts are enabled (1'b0) or disabled (1'b0).
 
Bit~15 is an indicator showing whether or not any interrupt is active, and
 
bits~0--15 indicate whether or not an individual interrupt is active.
 
 
 
The interrupt controller has been designed so that bits can be controlled
 
individually without having any knowledge of the rest of the controller
 
setting.  To enable an interrupt, write to the register with the high order
 
global enable bit set and the respective interrupt enable bit set.  No other
 
bits will be affected.  To disable an interrupt, write to the register with
 
the high order global enable bit cleared and the respective interrupt enable
 
bit set.  To clear an interrupt, write a `1' to that interrupts status pin.
 
Zero's written to the register have no affect, save that a zero written to the
 
master enable will disable all interrupts.
 
 
 
As an example, suppose you wished to enable interrupt \#4.  You would then
 
write to the register a {\tt 0x80100010} to enable interrupt \#4 and to clear
 
any past active state.  When you later wish to disable this interrupt, you would
 
write a {\tt 0x00100010} to the register.  As before, this both disables the
 
interrupt and clears the active indicator.  This also has the side effect of
 
disabling all interrupts, so a second write of {\tt 0x80000000} may be necessary
 
to re-enable any other interrupts.
 
 
 
The Zip System currently hosts two interrupt controllers, a primary and a
 
secondary.  The primary interrupt controller has one interrupt line which may
 
come from an external interrupt controller, and one interrupt line from the
 
secondary controller.  Other primary interrupts include the system timers,
 
the jiffies interrupt, and the manual cache interrupt.  The secondary interrupt
 
controller maintains an interrupt state for all of the processor accounting
 
counters.
 
 
\section{Counter}
\section{Counter}
 
 
The Zip Counter is a very simple counter: it just counts.  It cannot be
The Zip Counter is a very simple counter: it just counts.  It cannot be
halted.  When it rolls over, it issues an interrupt.  Writing a value to the
halted.  When it rolls over, it issues an interrupt.  Writing a value to the
counter just sets the current value, and it starts counting again from that
counter just sets the current value, and it starts counting again from that
value.
value.
 
 
Eight counters are implemented in the Zip System for process accounting.
Eight counters are implemented in the Zip System for process accounting.
This may change in the future, as nothing as yet uses these counters.
This may change in the future, as nothing as yet uses these counters.
 
 
\section{Timer}
\section{Timer}
 
 
The Zip Timer is also very simple: it simply counts down to zero.  When it
The Zip Timer is also very simple: it simply counts down to zero.  When it
transitions from a one to a zero it creates an interrupt.
transitions from a one to a zero it creates an interrupt.
 
 
Writing any non-zero value to the timer starts the timer.  If the high order
Writing any non-zero value to the timer starts the timer.  If the high order
bit is set when writing to the timer, the timer becomes an interval timer and
bit is set when writing to the timer, the timer becomes an interval timer and
reloads its last start time on any interrupt.  Hence, to mark seconds, one
reloads its last start time on any interrupt.  Hence, to mark seconds, one
might set the timer to 100~million (the number of clocks per second), and
might set the timer to 100~million (the number of clocks per second), and
set the high bit.  Ever after, the timer will interrupt the CPU once per
set the high bit.  Ever after, the timer will interrupt the CPU once per
second (assuming a 100~MHz clock).
second (assuming a 100~MHz clock).  This reload capability also limits the
 
maximum timer value to $2^{31}-1$, rather than $2^{32}-1$.
 
 
\section{Watchdog Timer}
\section{Watchdog Timer}
 
 
The watchdog timer is no different from any of the other timers, save for one
The watchdog timer is no different from any of the other timers, save for one
critical difference: the interrupt line from the watchdog
critical difference: the interrupt line from the watchdog
timer is tied to the reset line of the CPU.  Hence writing a `1' to the
timer is tied to the reset line of the CPU.  Hence writing a `1' to the
watchdog timer will always reset the CPU.
watchdog timer will always reset the CPU.
To stop the Watchdog timer, write a '0' to it.  To start it,
To stop the Watchdog timer, write a '0' to it.  To start it,
write any other number to it---as with the other timers.
write any other number to it---as with the other timers.
 
 
While the watchdog timer supports interval mode, it doesn't make as much sense
While the watchdog timer supports interval mode, it doesn't make as much sense
as it did with the other timers.
as it did with the other timers.
 
 
\section{Jiffies}
\section{Jiffies}
 
 
This peripheral is motivated by the Linux use of `jiffies' whereby a process
This peripheral is motivated by the Linux use of `jiffies' whereby a process
can request to be put to sleep until a certain number of `jiffies' have
can request to be put to sleep until a certain number of `jiffies' have
elapsed.  Using this interface, the CPU can read the number of `jiffies'
elapsed.  Using this interface, the CPU can read the number of `jiffies'
from the peripheral (it only has the one location in address space), add the
from the peripheral (it only has the one location in address space), add the
sleep length to it, and write teh result back to the peripheral.  The zipjiffies
sleep length to it, and write the result back to the peripheral.  The zipjiffies
peripheral will record the value written to it only if it is nearer the current
peripheral will record the value written to it only if it is nearer the current
counter value than the last current waiting interrupt time.  If no other
counter value than the last current waiting interrupt time.  If no other
interrupts are waiting, and this time is in the future, it will be enabled.
interrupts are waiting, and this time is in the future, it will be enabled.
(There is currently no way to disable a jiffie interrupt once set, other
(There is currently no way to disable a jiffie interrupt once set, other
than to disable the register in the interrupt controller.)  The processor
than to disable the interrupt line in the interrupt controller.)  The processor
may then place this sleep request into a list among other sleep requests.
may then place this sleep request into a list among other sleep requests.
Once the timer expires, it would write the next Jiffy request to the peripheral
Once the timer expires, it would write the next Jiffy request to the peripheral
and wake up the process whose timer had expired.
and wake up the process whose timer had expired.
 
 
Indeed, the Jiffies register is nothing more than a glorified counter with
Indeed, the Jiffies register is nothing more than a glorified counter with
an interrupt.  Unlike the other counters, the Jiffies register cannot be set.
an interrupt.  Unlike the other counters, the Jiffies register cannot be set.
Writes to the jiffies register create an interrupt time.  When the Jiffies
Writes to the jiffies register create an interrupt time.  When the Jiffies
register later equals the value written to it, an interrupt will be asserted
register later equals the value written to it, an interrupt will be asserted
and the register then continues counting as though no interrupt had taken
and the register then continues counting as though no interrupt had taken
place.
place.
 
 
The purpose of this register is to support alarm times within a CPU.  To
The purpose of this register is to support alarm times within a CPU.  To
set an alarm for a particular process $N$ clocks in advance, read the current
set an alarm for a particular process $N$ clocks in advance, read the current
Jiffies value, and $N$, and write it back to the Jiffies register.  The
Jiffies value, and $N$, and write it back to the Jiffies register.  The
O/S must also keep track of values written to the Jiffies register.  Thus,
O/S must also keep track of values written to the Jiffies register.  Thus,
when an `alarm' trips, it should be remoed from the list of alarms, the list
when an `alarm' trips, it should be remoed from the list of alarms, the list
should be sorted, and the next alarm in terms of Jiffies should be written
should be sorted, and the next alarm in terms of Jiffies should be written
to the register.
to the register.
 
 
 
\section{Manual Cache}
 
 
 
The manual cache is an experimental setting that may not remain with the Zip
 
CPU for very long.  It is designed to facilitate running from FLASH or ROM
 
memory, although the pipe cache really makes this need obsolete.  The manual
 
cache works by copying data from a wishbone address (range) into the cache
 
register, and then by making that memory available as memory to the Zip System.
 
It is a {\em manual cache} because the processor must first specify what
 
memory to copy, and then once copied the processor can only access the cache
 
memory by the cache memory location.  There is no transparency.  It is perhaps
 
best described as a combination DMA controller and local memory.
 
 
 
Worse, this cache is likely going to be removed from the ZipSystem.  Having used
 
the ZipSystem now for some time, I have yet to find a need or use for the manual
 
cache.  I will likely replace this peripheral with a proper DMA controller.
 
 
\chapter{Operation}\label{chap:ops}
\chapter{Operation}\label{chap:ops}
 
 
\chapter{Registers}\label{chap:regs}
\chapter{Registers}\label{chap:regs}
 
 
 
The ZipSystem registers fall into two categories, ZipSystem internal registers
 
accessed via the ZipCPU shown in Tbl.~\ref{tbl:zpregs},
 
\begin{table}[htbp]
 
\begin{center}\begin{reglist}
 
PIC   & {\tt 0xc0000000} & 32 & R/W & Primary Interrupt Controller \\\hline
 
WDT   & {\tt 0xc0000001} & 32 & R/W & Watchdog Timer \\\hline
 
CCHE  & {\tt 0xc0000002} & 32 & R/W & Manual Cache Controller \\\hline
 
CTRIC & {\tt 0xc0000003} & 32 & R/W & Secondary Interrupt Controller \\\hline
 
TMRA  & {\tt 0xc0000004} & 32 & R/W & Timer A\\\hline
 
TMRB  & {\tt 0xc0000005} & 32 & R/W & Timer B\\\hline
 
TMRC  & {\tt 0xc0000006} & 32 & R/W & Timer C\\\hline
 
JIFF  & {\tt 0xc0000007} & 32 & R/W & Jiffies \\\hline
 
MTASK  & {\tt 0xc0000008} & 32 & R/W & Master Task Clock Counter \\\hline
 
MMSTL  & {\tt 0xc0000008} & 32 & R/W & Master Stall Counter \\\hline
 
MPSTL  & {\tt 0xc0000008} & 32 & R/W & Master Pre--Fetch Stall Counter \\\hline
 
MICNT  & {\tt 0xc0000008} & 32 & R/W & Master Instruction Counter\\\hline
 
UTASK  & {\tt 0xc0000008} & 32 & R/W & User Task Clock Counter \\\hline
 
UMSTL  & {\tt 0xc0000008} & 32 & R/W & User Stall Counter \\\hline
 
UPSTL  & {\tt 0xc0000008} & 32 & R/W & User Pre--Fetch Stall Counter \\\hline
 
UICNT  & {\tt 0xc0000008} & 32 & R/W & User Instruction Counter\\\hline
 
Cache  & {\tt 0xc0100000} & & & Base address of the Cache memory\\\hline
 
\end{reglist}
 
\caption{Zip System Internal/Peripheral Registers}\label{tbl:zpregs}
 
\end{center}\end{table}
 
and the two debug registers showin in Tbl.~\ref{tbl:dbgregs}.
 
\begin{table}[htbp]
 
\begin{center}\begin{reglist}
 
ZIPCTRL & 0 & 32 & R/W & Debug Control Register \\\hline
 
ZIPDATA & 1 & 32 & R/W & Debug Data Register \\\hline
 
\end{reglist}
 
\caption{Zip System Debug Registers}\label{tbl:dbgregs}
 
\end{center}\end{table}
 
 
 
 
\chapter{Wishbone Datasheet}\label{chap:wishbone}
\chapter{Wishbone Datasheet}\label{chap:wishbone}
The Zip System supports two wishbone accesses, a slave debug port and a master
The Zip System supports two wishbone accesses, a slave debug port and a master
port for the system itself.  These are shown in Tbl.~\ref{tbl:wishbone-slave}
port for the system itself.  These are shown in Tbl.~\ref{tbl:wishbone-slave}
\begin{table}[htbp]
\begin{table}[htbp]
\begin{center}
\begin{center}
\begin{wishboneds}
\begin{wishboneds}
Revision level of wishbone & WB B4 spec \\\hline
Revision level of wishbone & WB B4 spec \\\hline
Type of interface & Slave, Read/Write, single words only \\\hline
Type of interface & Slave, Read/Write, single words only \\\hline
 
Address Width & 1--bit \\\hline
Port size & 32--bit \\\hline
Port size & 32--bit \\\hline
Port granularity & 32--bit \\\hline
Port granularity & 32--bit \\\hline
Maximum Operand Size & 32--bit \\\hline
Maximum Operand Size & 32--bit \\\hline
Data transfer ordering & (Irrelevant) \\\hline
Data transfer ordering & (Irrelevant) \\\hline
Clock constraints & Works at 100~MHz on a Basys--3 board\\\hline
Clock constraints & Works at 100~MHz on a Basys--3 board\\\hline
Signal Names & \begin{tabular}{ll}
Signal Names & \begin{tabular}{ll}
                Signal Name & Wishbone Equivalent \\\hline
                Signal Name & Wishbone Equivalent \\\hline
                {\tt i\_clk} & {\tt CLK\_I} \\
                {\tt i\_clk} & {\tt CLK\_I} \\
                {\tt i\_dbg\_cyc} & {\tt CYC\_I} \\
                {\tt i\_dbg\_cyc} & {\tt CYC\_I} \\
                {\tt i\_dbg\_stb} & {\tt STB\_I} \\
                {\tt i\_dbg\_stb} & {\tt STB\_I} \\
                {\tt i\_dbg\_we} & {\tt WE\_I} \\
                {\tt i\_dbg\_we} & {\tt WE\_I} \\
                {\tt i\_dbg\_addr} & {\tt ADR\_I} \\
                {\tt i\_dbg\_addr} & {\tt ADR\_I} \\
                {\tt i\_dbg\_data} & {\tt DAT\_I} \\
                {\tt i\_dbg\_data} & {\tt DAT\_I} \\
                {\tt o\_dbg\_ack} & {\tt ACK\_O} \\
                {\tt o\_dbg\_ack} & {\tt ACK\_O} \\
                {\tt o\_dbg\_stall} & {\tt STALL\_O} \\
                {\tt o\_dbg\_stall} & {\tt STALL\_O} \\
                {\tt o\_dbg\_data} & {\tt DAT\_O}
                {\tt o\_dbg\_data} & {\tt DAT\_O}
                \end{tabular}\\\hline
                \end{tabular}\\\hline
\end{wishboneds}
\end{wishboneds}
\caption{Wishbone Datasheet for the Debug Interface}\label{tbl:wishbone-slave}
\caption{Wishbone Datasheet for the Debug Interface}\label{tbl:wishbone-slave}
\end{center}\end{table}
\end{center}\end{table}
and Tbl.~\ref{tbl:wishbone-master} respectively.
and Tbl.~\ref{tbl:wishbone-master} respectively.
\begin{table}[htbp]
\begin{table}[htbp]
\begin{center}
\begin{center}
\begin{wishboneds}
\begin{wishboneds}
Revision level of wishbone & WB B4 spec \\\hline
Revision level of wishbone & WB B4 spec \\\hline
Type of interface & Master, Read/Write, sometimes pipelined \\\hline
Type of interface & Master, Read/Write, single cycle or pipelined\\\hline
 
Address Width & 32--bit bits \\\hline
Port size & 32--bit \\\hline
Port size & 32--bit \\\hline
Port granularity & 32--bit \\\hline
Port granularity & 32--bit \\\hline
Maximum Operand Size & 32--bit \\\hline
Maximum Operand Size & 32--bit \\\hline
Data transfer ordering & (Irrelevant) \\\hline
Data transfer ordering & (Irrelevant) \\\hline
Clock constraints & Works at 100~MHz on a Basys--3 board\\\hline
Clock constraints & Works at 100~MHz on a Basys--3 board\\\hline
Signal Names & \begin{tabular}{ll}
Signal Names & \begin{tabular}{ll}
                Signal Name & Wishbone Equivalent \\\hline
                Signal Name & Wishbone Equivalent \\\hline
                {\tt i\_clk} & {\tt CLK\_O} \\
                {\tt i\_clk} & {\tt CLK\_O} \\
                {\tt o\_wb\_cyc} & {\tt CYC\_O} \\
                {\tt o\_wb\_cyc} & {\tt CYC\_O} \\
                {\tt o\_wb\_stb} & {\tt STB\_O} \\
                {\tt o\_wb\_stb} & {\tt STB\_O} \\
                {\tt o\_wb\_we} & {\tt WE\_O} \\
                {\tt o\_wb\_we} & {\tt WE\_O} \\
                {\tt o\_wb\_addr} & {\tt ADR\_O} \\
                {\tt o\_wb\_addr} & {\tt ADR\_O} \\
                {\tt o\_wb\_data} & {\tt DAT\_O} \\
                {\tt o\_wb\_data} & {\tt DAT\_O} \\
                {\tt i\_wb\_ack} & {\tt ACK\_I} \\
                {\tt i\_wb\_ack} & {\tt ACK\_I} \\
                {\tt i\_wb\_stall} & {\tt STALL\_I} \\
                {\tt i\_wb\_stall} & {\tt STALL\_I} \\
                {\tt i\_wb\_data} & {\tt DAT\_I}
                {\tt i\_wb\_data} & {\tt DAT\_I}
                \end{tabular}\\\hline
                \end{tabular}\\\hline
\end{wishboneds}
\end{wishboneds}
\caption{Wishbone Datasheet for the CPU as Master}\label{tbl:wishbone-master}
\caption{Wishbone Datasheet for the CPU as Master}\label{tbl:wishbone-master}
\end{center}\end{table}
\end{center}\end{table}
I do not recommend that you connect these together through the interconnect.
I do not recommend that you connect these together through the interconnect.
 
Rather, the debug port of the CPU should be accessible regardless of the state
 
of the master bus.
 
 
The big thing to notice is that both the real time clock and the real time
You may wish to notice that neither the {\tt ERR} nor the {\tt RETRY} wires
date modules act as wishbone slaves, and that all accesses to the registers of
have been implemented.  What this means is that the CPU is currently unable
either module are 32--bit reads and writes.  The address bus does not offer
to detect a bus error condition, and so may stall indefinitely (hang) should
byte level, but rather 32--bit word level resolution.  Select lines are not
it choose to access a value not on the bus, or a peripheral that is not
implemented.  Bit ordering is the normal ordering where bit~31 is the most
yet properly configured.
significant bit and so forth.
 
 
 
\chapter{Clocks}\label{chap:clocks}
\chapter{Clocks}\label{chap:clocks}
 
 
This core is based upon the Basys--3 design.  The Basys--3 development board
This core is based upon the Basys--3 design.  The Basys--3 development board
contains one external 100~MHz clock, which is sufficient to run the ZIP CPU
contains one external 100~MHz clock, which is sufficient to run the ZIP CPU
core.
core.
\begin{table}[htbp]
\begin{table}[htbp]
\begin{center}
\begin{center}
\begin{clocklist}
\begin{clocklist}
i\_clk & External & 100~MHz & 100~MHz & System clock.\\\hline
i\_clk & External & 100~MHz & 100~MHz & System clock.\\\hline
\end{clocklist}
\end{clocklist}
\caption{List of Clocks}\label{tbl:clocks}
\caption{List of Clocks}\label{tbl:clocks}
\end{center}\end{table}
\end{center}\end{table}
I hesitate to suggest that the core can run faster than 100~MHz, since I have
I hesitate to suggest that the core can run faster than 100~MHz, since I have
had struggled with various timing violations to keep it at 100~MHz.  So, for
had struggled with various timing violations to keep it at 100~MHz.  So, for
now, I will only state that it can run at 100~MHz.
now, I will only state that it can run at 100~MHz.
 
 
 
 
\chapter{I/O Ports}\label{chap:ioports}
\chapter{I/O Ports}\label{chap:ioports}
 
 
% Appendices
% Appendices
% Index
% Index
\end{document}
\end{document}
 
 
 
 
 
 

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