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%% Filename:    spec.tex

%% Filename:    spec.tex

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%% 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 supersedes

%%              currently provided with this Zip CPU soft core.  It supersedes

%%              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

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%%              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.

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%% 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

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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}

\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}

0.3 & 8/22/2015 & Gisselquist & First completed draft\\\hline

0.3 & 8/22/2015 & Gisselquist & First completed draft\\\hline

0.2 & 8/19/2015 & Gisselquist & Still Draft, more complete \\\hline

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, RISC--V may be replacing it. 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.

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 2,300 LUTs with no peripherals, and 3,200 LUTs with some very basic

about 2,300 LUTs with no peripherals, and 3,200 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}

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.  All

\item A RISC CPU.  There is no microcode for executing instructions.  All

        instructions are designed to be completed in one clock cycle.

        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}.  See Fig.~\ref{fig:cpu}

                unit, and {\bf Write-back}.  See Fig.~\ref{fig:cpu}

\begin{figure}\begin{center}

\begin{figure}\begin{center}

\includegraphics[width=3.5in]{../gfx/cpu.eps}

\includegraphics[width=3.5in]{../gfx/cpu.eps}

\caption{Zip CPU internal pipeline architecture}\label{fig:cpu}

\caption{Zip CPU internal pipeline architecture}\label{fig:cpu}

\end{center}\end{figure}

\end{center}\end{figure}

                for a diagram of this structure.

                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 External Debug:} Once placed upon an FPGA, some external means is

\item {\bf External 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.  My chosen interface

        it, and tell whether it is running or not.  My chosen interface

        includes a second register similar to this control register.  This

        includes a second register similar to this control register.  This

        second register allows the external controller or debugger to examine

        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

        Incidentally, this break messes with the prefetch cache and the

        pipeline: if you change an instruction partially through the pipeline,

        pipeline: if you change an instruction partially through the pipeline,

        the whole pipeline needs to be cleansed.  Likewise if you change

        the whole pipeline needs to be cleansed.  Likewise if you change

        an instruction in memory, you need to make sure the cache is reloaded

        an instruction in memory, you need to make sure the cache is reloaded

        with the new instruction.

        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 an external

My initial approach to building a trap instruction was to create an external

peripheral which, when written to, would generate an interrupt and could

peripheral which, when written to, would generate an interrupt and could

return the last value written to it.  In practice, this approach didn't work

return the last value written to it.  In practice, this approach didn't work

at all: the CPU executed two instructions while waiting for the

at all: the CPU executed two instructions while waiting for the

trap interrupt to take place.  Since then, I've decided to keep the rest of

trap interrupt to take place.  Since then, I've decided to keep the rest of

the CC register for that purpose so that a write to the CC register, with the

the CC register for that purpose so that a write to the CC register, with the

GIE bit cleared, could be used to execute a trap.  This has other problems,

GIE bit cleared, could be used to execute a trap.  This has other problems,

though, primarily in the limitation of the uses of the CC register.  In

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

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

to ``announce'' special CPU features (floating point, etc).  So the trap

instruction still switches to interrupt mode, but the CC register is not

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

nearly as useful for telling the supervisor mode processor what trap is being

executed.

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 {\tt zipsystem.v}.

external to the CPU as part of the CPU System, found in {\tt zipsystem.v}.

The timer module itself is found in {\tt ziptimer.v}.

The timer module itself is found in {\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

        all 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 what

        never stopped.  This killed my idea of delayed branching, since what

        would be the appropriate program counter to restart at?  The one the

        would be the appropriate program counter to restart at?  The one the

        CPU was going to branch to, or the ones in the delay slots?  This

        CPU was going to branch to, or the ones in the delay slots?  This

        also makes the idea of compressed instruction codes difficult, since,

        also makes the idea of compressed instruction codes difficult, since,

        again, where do you restart on interrupt?

        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 preceding a store

        possibly inappropriate (i.e. a conditional branch preceding 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 preceding are all wiped clean.

        PC address, the stages preceding 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

        This approach is also different from other pipeline approaches.  Instead

        of keeping the entire pipeline filled, each stage is treated

        of keeping the entire pipeline filled, each stage is treated

        independently.  Therefore, individual stages may move forward as long

        independently.  Therefore, individual stages may move forward as long

        as the subsequent stage is available, regardless of whether the stage

        as the subsequent stage is available, regardless of whether the stage

        behind it is filled.

        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 second 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}

\section{Simplified Bus}

The bus architecture of the Zip CPU is that of a simplified WISHBONE 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.

It has been simplified in this fashion: all operations are 32--bit operations.

are 32--bits.  All instructions are also 32--bits wide.  Everything has been

are 32--bits.  All instructions are also 32--bits wide.  Everything has been

built around the 32--bit word.

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 as shown in Fig.~\ref{fig:regset}.

and a user set as shown in Fig.~\ref{fig:regset}.

\begin{figure}\begin{center}

\begin{figure}\begin{center}

\includegraphics[width=3.5in]{../gfx/regset.eps}

\includegraphics[width=3.5in]{../gfx/regset.eps}

\caption{Zip CPU Register File}\label{fig:regset}

\caption{Zip CPU Register File}\label{fig:regset}

\end{center}\end{figure}

\end{center}\end{figure}

The supervisor set is used in interrupt mode when interrupts are disabled,

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

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

Counter (PC) is register 15, whereas the status register (SR) or condition

code register

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 there is nothing special about this register other

noted as (SP)--although there is nothing special about this register other

than this convention.

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.

Of the condition codes, the bottom four bits are the current flags:

Of the condition codes, the bottom four bits 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.)  Special logic has been added to

        and deals with its context switch.)  Special logic has been added to

        keep the user mode from setting the sleep register and clearing the

        keep the user mode from setting the sleep register and clearing the

        GIE register at the same time, with clearing the GIE register taking

        GIE register at the same time, with clearing the GIE register taking

        precedence.

        precedence.

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 controls whether a break

The eighth bit is a break enable bit.  This controls whether a break

instruction in user mode will halt the processor for an external debugger

instruction in user mode will halt the processor for an external debugger

(break enabled), or whether the break instruction will simply send send the

(break enabled), or whether the break instruction will simply send send the

CPU into interrupt mode.  Encountering a break in supervisor mode will

CPU into interrupt mode.  Encountering a break in supervisor mode will

halt the CPU independent of the break enable bit.  This bit can only be set

halt the CPU independent of the break enable bit.  This bit can only be set

within supervisor mode.

within supervisor mode.

% Should break enable be a supervisor mode bit, while the break enable bit

% Should break enable be a supervisor mode bit, while the break enable bit

% in user mode is a break has taken place bit?

% in user mode is a break has taken place bit?

%

%

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 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.

These status register bits are summarized in Tbl.~\ref{tbl:ccbits}.

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 returning to user mode\\\hline

9 & Soft trap, set on a trap from user mode, cleared when returning 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}

\caption{Condition Code / Status Register Bits}\label{tbl:ccbits}

\caption{Condition Code / Status Register Bits}\label{tbl:ccbits}

\end{center}\end{table}

\end{center}\end{table}

\section{Conditional Instructions}

\section{Conditional Instructions}

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' 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.  Sorry,

There is no condition code for less than or equal, not C or not V.  Sorry,

\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|}{20--bit 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 immediate values don't make sense for all instructions.

Sixteen and twenty bit immediate values don't make sense for all instructions.

For example, what is the point of a 20--bit immediate when executing a 16--bit

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

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

or arithmetic shift?  In these cases, the extra bits are reserved for future

instruction possibilities.

instruction possibilities.

\section{Address Modes}

\section{Address Modes}

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,\footnote{The two clocks figure

taking two or more clocks per instruction,\footnote{The two clocks figure

comes from the design of the register set, allowing only one write per clock.

comes from the design of the register set, allowing only one write per clock.

That write is either from the memory unit or the ALU, but never both.} these

That write is either from the memory unit or the ALU, but never both.} these

addressing modes have been

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

Each of these instructions can be emulated with a set of instructions from the

existing set.

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 mode.

the CPU needs access to non--supervisory registers while in supervisory mode.

Therefore, the MOV instruction is special and offers access to these registers

Therefore, the MOV instruction is special and offers access to these registers

\ldots when in supervisory mode.  To keep the compiler simple, the extra bits

\ldots when in supervisory mode.  To keep the compiler simple, the extra bits

are ignored in non-supervisory mode (as though they didn't exist), rather than

are ignored in non-supervisory mode (as though they didn't exist), rather than

being mapped to new instructions or additional capabilities.  The bits

being mapped to new instructions or additional capabilities.  The bits

indicating which register set each register lies within are the A-Usr and

indicating which register set each register lies within are the A-Usr and

B-Usr bits.  When set to a one, these refer to a user mode register.  When set

B-Usr bits.  When set to a one, these refer to a user mode register.  When set

to a zero, these refer to a register in the current mode, whether user or

to a zero, these refer to a register in the current mode, whether user or

supervisor.  Further, because a load immediate instruction exists, there is no

supervisor.  Further, because a load immediate instruction exists, there is no

move capability between an immediate and a register: all moves come from either

move capability between an immediate and a register: all moves come from either

a register or a register plus an offset.

a register or a register plus an offset.

This actually leads to a bit of a problem: since the MOV instruction encodes

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

which register set each register is coming from or moving to, how shall a

compiler or assembler know how to compile a MOV instruction without knowing

compiler or assembler know how to compile a MOV instruction without knowing

the mode of the CPU at the time?  For this reason, the compiler will assume

the mode of the CPU at the time?  For this reason, the compiler will assume

all MOV registers are supervisor registers, and display them as normal.

all MOV registers are supervisor registers, and display them as normal.

Anything with the user bit set will be treated as a user register.  The CPU

Anything with the user bit set will be treated as a user register.  The CPU

will quietly ignore the supervisor bits while in user mode, and anything

will quietly ignore the supervisor bits while in user mode, and anything

\section{Multiply Operations}

\section{Multiply Operations}

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}

instruction set reserves two possibilities for future floating point

instruction set reserves two possibilities for future floating point

operations.

operations.

The first floating point operation hole in the instruction set involves

The first floating point operation hole in the instruction set involves

Not all instructions, however, have floating point equivalents.  Further, the

Not all instructions, however, have floating point equivalents.  Further, the

immediate fields do not apply in floating point mode, and must be set to

immediate fields do not apply in floating point mode, and must be set to

zero.  Not all instructions make sense as floating point operations.

zero.  Not all instructions make sense as floating point operations.

Therefore, only the CMP, SUB, ADD, and MPY instructions may be issued as

Therefore, only the CMP, SUB, ADD, and MPY instructions may be issued as

floating point instructions.  Other instructions allow the examining of the

floating point instructions.  Other instructions allow the examining of the

floating point bit in the CC register.  In all cases, the floating point bit

floating point bit in the CC register.  In all cases, the floating point bit

is cleared one instruction after it is set.

is cleared one instruction after it is set.

The other possibility for floating point operations involves exploiting the

The other possibility for floating point operations involves exploiting the

hole in the instruction set that the NOOP and BREAK instructions reside within.

hole in the instruction set that the NOOP and BREAK instructions reside within.

In both cases, the Zip CPU would support 32--bit single precision floats

In both cases, the Zip CPU would support 32--bit single precision floats

The current architecture does not support a floating point not-implemented

The current architecture does not support a floating point not-implemented

interrupt.  Any soft floating point emulation must be done deliberately.

interrupt.  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}

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

TST(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

                & \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

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.}

        &       \multicolumn{21}{l|}{Operand B}

        &       \multicolumn{21}{l|}{Operand B}

        & 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. truncated to 6 bits}

        &       \multicolumn{21}{l|}{Operand B, imm. truncated 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. truncated to 6 bits}

        &       \multicolumn{21}{l|}{Operand B, imm. truncated 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. truncated to 6 bits}

        &       \multicolumn{21}{l|}{Operand B, imm. truncated 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 are the only instructions within one particular

NOOP and BREAK instructions are the only instructions within one particular

\section{Derived Instructions}

\section{Derived Instructions}

are single cycle instructions.  The derived instruction tables,

are single cycle instructions.  The derived instruction tables,

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

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

        & \hbox{MOV.cond \$Addr+PC,PC}

        & \hbox{MOV.cond \$Addr+PC,PC}

        & Branch or jump on condition.  Works for 15--bit

        & Branch or jump on condition.  Works for 15--bit

                signed 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 sets the flags. \\\hline

        and sets the 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 instruction. \\\hline

        wait until interrupt instruction. \\\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. Note the required cleanup instruction after

        & Jump to Subroutine. Note the required cleanup instruction after

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 in this document are 32-bit wordlength addresses.

        all other addresses in this document are 32-bit wordlength addresses.

        For this reason,

        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 other way

        That is, LSL sets the carry flag, so if we did this the other 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 first something then comes along and writes to that

        incremented it first 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,SP \\

        & \parbox[t]{1.5in}{SUB \$1,SP \\

        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

        & Note that this depends upon the calling context to clean up the

        & Note that this depends upon the calling context to clean up the

        stack, as outlined for the JSR instruction.  \\\hline

        stack, as outlined for the JSR instruction.  \\\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

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.

\end{tabular}

\end{tabular}

\end{center}\end{table}

\end{center}\end{table}

\section{Pipeline Stages}

\section{Pipeline Stages}

As mentioned in the introduction, and highlighted in Fig.~\ref{fig:cpu},

As mentioned in the introduction, and highlighted in Fig.~\ref{fig:cpu},

the Zip CPU supports a five stage pipeline.

the Zip CPU supports a five stage pipeline.

\begin{enumerate}

\begin{enumerate}