Subversion Repositories zipcpu

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%%
%% Filename: 	spec.tex
%%
%% Project:	Zip CPU -- a small, lightweight, RISC CPU soft core
%%
%% Purpose:	This LaTeX file contains all of the documentation/description
%%		currently provided with this Zip CPU soft core.  It supercedes
%%		any information about the instruction set or CPUs found
%%		elsewhere.  It's not nearly as interesting, though, as the PDF
%%		file it creates, so I'd recommend reading that before diving
%%		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 GPL-3.0 license this file is distributed under.  If not,
%%		just type 'make' in the doc directory and it (should) build
%%		without a problem.
%%		
%%
%% Creator:	Dan Gisselquist
%%		Gisselquist Technology, LLC
%%
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%%
%% Copyright (C) 2015, Gisselquist Technology, LLC
%%
%% 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
%% by the Free Software Foundation, either version 3 of the License, or (at
%% your option) any later version.
%%
%% This program is distributed in the hope that it will be useful, but WITHOUT
%% ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
%% FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
%% for more details.
%%
%% 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
%% target there if the PDF file isn't present.)  If not, see
%% <http://www.gnu.org/licenses/> for a copy.
%%
%% License:	GPL, v3, as defined and found on www.gnu.org,
%%		http://www.gnu.org/licenses/gpl.html
%%
%%
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\documentclass{gqtekspec}
\project{Zip CPU}
\title{Specification}
\author{Dan Gisselquist, Ph.D.}
\email{dgisselq (at) opencores.org}
\revision{Rev.~0.2}
\begin{document}
\pagestyle{gqtekspecplain}
\titlepage
\begin{license}
Copyright (C) \theyear\today, Gisselquist Technology, LLC
 
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
by the Free Software Foundation, either version 3 of the License, or (at
your option) any later version.
 
This program is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.
 
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
copy.
\end{license}
\begin{revisionhistory}
0.2 & 8/19/2015 & Gisselquist & Still Draft, more complete \\\hline
0.1 & 8/17/2015 & Gisselquist & Incomplete First Draft \\\hline
\end{revisionhistory}
% Revision History
% Table of Contents, named Contents
\tableofcontents
\listoffigures
\listoftables
\begin{preface}
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
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?
 
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
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
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
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
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.
 
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
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
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
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
peripherals.
 
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
level. For the first time, I am beginning to understand many of the Computer
Architecture lessons from years ago.
 
To summarize: Because I can, because it is open source, because it is light
weight, and as an exercise in learning. 
 
\end{preface}
 
\chapter{Introduction}
\pagenumbering{arabic}
\setcounter{page}{1}
 
 
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.
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
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
resulted in the choice to drop push and pop instructions, pre-increment and
post-decrement addressing modes, and more.
 
For those who like buzz words, the Zip CPU is:
\begin{itemize}
\item A 32-bit CPU: All registers are 32-bits, addresses are 32-bits,
		instructions are 32-bits wide, etc.
\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
		can access memory.)
\item Wishbone compliant.  All peripherals are accessed just like
		memory across this bus.
\item A Von-Neumann architecture.  (The instructions and data share a 
		common bus.)
\item A pipelined architecture, having stages for {\bf Prefetch},
		{\bf Decode}, {\bf Read-Operand}, the {\bf ALU/Memory}
		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
	need a copy of the Zip CPU licensed under other terms, please 
	contact me.}
\end{itemize}
 
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
capabilities that I was never expecting to need.  These include:
\begin{itemize}
\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
	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
	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.
 
\item {\bf Internal Debug:} Being able to run a debugger from within
	a user process requires an ability to step a user process from
	within a debugger.  It also requires a break instruction that can
	be substituted for any other instruction, and substituted back.
	The break is actually difficult: the break instruction cannot be
	allowed to execute.  That way, upon a break, the debugger should
	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 
	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
	simple prefetch stage.  Any time the PC changed the prefetch would go
	and fetch the new instruction.  While this was perhaps this simplest
	approach, it cost roughly five clocks for every instruction.  This
	was deemed unacceptable, as I wanted a CPU that could execute
	instructions in one cycle.  I therefore have a prefetch cache that
	issues pipelined wishbone accesses to memory and then pushes
	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.
 
 
\item {\bf Operating System:} In order to support an operating system,
	interrupts and so forth, the CPU needs to support supervisor and
	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 
	purpose of the {\bf `trap'} instruction.  This instruction needs to
	place the CPU into supervisor mode (here equivalent to disabling
	interrupts), as well as handing it a parameter such as identifying
	which O/S function was called. 
 
My initial approach to building a trap instruction was to create an external
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
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
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,
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
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}. 
The timer module itself is found in {\tt ziptimer.v}.
 
\item {\bf Pipeline Stalls:} My original plan was to not support pipeline
	stalls at all, but rather to require the compiler to properly schedule
	all instructions so that stalls would never be necessary.  After trying
	to build such an architecture, I gave up, having learned some things:
 
	For example, in  order to facilitate interrupt handling and debug
	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
	the pipeline from.  Once restarted, it must act as though it had
	never stopped.  This killed my idea of delayed branching, since what
	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
	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
	enters into either the ALU or memory unit, the instruction is
	guaranteed to complete.  If the logic recognizes a branch or a 
	condition that would render the instruction entering into this stage
	possibly inappropriate (i.e. a conditional branch preceeding a store
	instruction for example), then the pipeline stalls for one cycle
	until the conditional branch completes.  Then, if it generates a new
	PC address, the stages preceeding are all wiped clean.
 
	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
	everything before them.  Likewise, anything that has entered the ALU
	or memory stage when the CPU is placed to sleep continues to completion.	
	To handle this logic, each pipeline stage has three control signals:
	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
	is stalled.  This allows the pipeline to fill any time a later stage
	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
	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
	idea, my own implementation didn't work out quite so nicely.
 
	As an example, the decode module produces a {\em lot} of
	control wires and registers.  Creating a module out of this, with
	only the simplest of logic within it, seemed to be more a lesson
	in passing wires around, rather than encapsulating logic.
 
	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
	such.  However, other modules depend upon writeback results other
	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
	own module.
 
	The result is that the majority of the CPU code can be found in
	the {\tt zipcpu.v} file.
\end{itemize}
 
With that introduction out of the way, let's move on to the instruction
set.
 
\chapter{CPU Architecture}\label{chap:arch}
 
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,
where the first operand (always a register) is the source of the data to be
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}
The Zip CPU supports two sets of sixteen 32-bit registers, a supervisor
and a user set as shown in Fig.~\ref{fig:regset}. 
\begin{figure}\begin{center}
\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
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
supervisor state, whereas the user state can only access the user registers.
 
The status register is special, and bears further mention.  The lower
10 bits of the status register form a set of CPU state and condition codes. 
Writes to other bits of this register are preserved.
 
Of the eight condition codes, the bottom four are the current flags:
		Zero (Z),
		Carry (C),
		Negative (N),
		and Overflow (V).
 
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
	wait for an interrupt (if interrupts are enabled), or to
	completely halt (if interrupts are disabled).
The sixth bit is a global interrupt enable bit (GIE).  When this
	sixth bit is a '1' interrupts will be enabled, else disabled.  When 
	interrupts are disabled, the CPU will be in supervisor mode, otherwise
	it is in user mode.  Thus, to execute a context switch, one only
	need enable or disable interrupts.  (When an interrupt line goes
	high, interrupts will automatically be disabled, as the CPU goes
	and deals with its context switch.)
 
The seventh bit is a step bit.  This bit can be
	set from supervisor mode only.  After setting this bit, should
	the supervisor mode process switch to user mode, it would then 
	accomplish one instruction in user mode before returning to supervisor
	mode.  Then, upon return to supervisor mode, this bit will 
	be automatically cleared.  This bit has no effect on the CPU while in
	supervisor mode.
 
	This functionality was added to enable a userspace debugger
	functionality on a user process, working through supervisor mode
	of course.
 
 
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
(break enabled), or whether the break instruction will simply send send the
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
within supervisor mode.
 
This functionality was added to enable an external debugger to
	set and manage breakpoints.
 
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.
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.
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
interrupt, and cleared on any return to userspace command.  This allows the
supervisor, in supervisor mode, to determine whether it got to supervisor
mode from a trap or from an external interrupt or both.
 
These status register bits are summarized in Tbl.~\ref{tbl:ccbits}.
\begin{table}
\begin{center}
\begin{tabular}{l|l}
Bit & Meaning \\\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
7 & Halt on break, to support an external debugger \\\hline
6 & Step, single step the CPU in user mode\\\hline
5 & GIE, or Global Interrupt Enable \\\hline
4 & Sleep \\\hline
3 & V, or overflow bit.\\\hline
2 & N, or negative bit.\\\hline
1 & C, or carry bit.\\\hline
0 & Z, or zero bit. \\\hline
\end{tabular}
\caption{Condition Code / Status Register Bits}\label{tbl:ccbits}
\end{center}\end{table}
 
\section{Conditional Instructions}
Most, although not quite all, instructions are conditionally executed.  From
the four condition code flags, eight conditions are defined.  These are shown
in Tbl.~\ref{tbl:conditions}.
\begin{table}
\begin{center}
\begin{tabular}{l|l|l}
Code & Mneumonic & Condition \\\hline
3'h0 & None & Always execute the instruction \\
3'h1 & {\tt .Z} & Only execute when 'Z' is 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'h4 & {\tt .GT} & Greater than ('N' not set, 'Z' not set) \\
3'h5 & {\tt .LT} & Less than ('N' set) \\
3'h6 & {\tt .C} & Carry set\\
3'h7 & {\tt .V} & Overflow set\\
\end{tabular}
\caption{Conditions for conditional operand execution}\label{tbl:conditions}
\end{center}
\end{table}
There is no condition code for less than or equal, not C or not V.  Sorry,
I ran out of space in 3--bits.  Using these conditions will take an extra
instruction.  (Ex: \hbox{\tt TST \$4,CC;} \hbox{\tt STO.NZ R0,(R1)})
 
\section{Operand B}
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,
or an immediate offset by itself.  This value is encoded as shown in
Tbl.~\ref{tbl:opb}.
\begin{table}\begin{center}
\begin{tabular}{|l|l|l|}\hline
Bit 20 & 19 \ldots 16 & 15 \ldots 0 \\\hline
1'b0 & \multicolumn{2}{l|}{20--bit Signed Immediate value} \\\hline
1'b1 & 4-bit Register & 16--bit Signed immediate offset \\\hline
\end{tabular}
\caption{Bit allocation for Operand B}\label{tbl:opb}
\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}
The ZIP CPU supports two addressing modes: register plus immediate, and
immediate address.  Addresses are therefore encoded in the same fashion as
Operand B's, shown above.
 
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
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
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}
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. 
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
are ignored in non-supervisory mode (as though they didn't exist), rather than
being mapped to new instructions or additional capabilities.  The bits
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
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
move capability between 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
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
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
will quietly ignore the supervisor bits while in user mode, and anything
marked as a user register will always be valid.  (Did I just say that in the
last paragraph?)
 
\section{Multiply Operations}
The Zip CPU supports two Multiply operations, a
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
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
CC register field is reserved.
 
\section{Floating Point}
The ZIP CPU does not support floating point operations today.  However, the
instruction set reserves a capability for a floating point operation.  To
execute such an operation, simply set the floating point bit in the CC
register and the following instruction will interpret its registers as
a floating point instruction.  Not all instructions, however, have 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
floating point operations.  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 bit in the CC register.  In all
cases, the floating point bit is cleared one instruction after it is set.
 
The architecture does not support a floating point not-implemented interrupt.
Any soft floating point emulation must be done deliberately.
 
\section{Native Instructions}
The instruction set for the Zip CPU is summarized in
Tbl.~\ref{tbl:zip-instructions}.
\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
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}
	& Sets CC? \\\hline
CMP(Sub) & \multicolumn{4}{l|}{4'h0}
		& \multicolumn{4}{l|}{D. Reg}
		& \multicolumn{3}{l|}{Cond.}
		& \multicolumn{21}{l|}{Operand B}
		& Yes \\\hline
TST(And) & \multicolumn{4}{l|}{4'h1}
		& \multicolumn{4}{l|}{D. Reg}
		& \multicolumn{3}{l|}{Cond.}
		& \multicolumn{21}{l|}{Operand B}
	& Yes \\\hline
MOV & \multicolumn{4}{l|}{4'h2}
		& \multicolumn{4}{l|}{D. Reg}
		& \multicolumn{3}{l|}{Cond.}
		& A-Usr
		& \multicolumn{4}{l|}{B-Reg}
		& B-Usr
		& \multicolumn{15}{l|}{15'bit signed offset}
		& \\\hline
LODI & \multicolumn{4}{l|}{4'h3}
		& \multicolumn{4}{l|}{R. Reg}
		& \multicolumn{24}{l|}{24'bit Signed Immediate}
		& \\\hline
NOOP & \multicolumn{4}{l|}{4'h4}
		& \multicolumn{4}{l|}{4'he}
		& \multicolumn{24}{l|}{24'h00}
		& \\\hline
BREAK & \multicolumn{4}{l|}{4'h4}
		& \multicolumn{4}{l|}{4'he}
		& \multicolumn{24}{l|}{24'h01}
		& \\\hline
{\em Rsrd} & \multicolumn{4}{l|}{4'h4}
		& \multicolumn{4}{l|}{4'he}
		& \multicolumn{24}{l|}{24'bits, but not 0 or 1.}
		& \\\hline
LODIHI & \multicolumn{4}{l|}{4'h4}
		& \multicolumn{4}{l|}{4'hf}
		& \multicolumn{3}{l|}{Cond.}
		& 1'b1
		& \multicolumn{4}{l|}{R. Reg}
		& \multicolumn{16}{l|}{16-bit Immediate}
		& \\\hline
LODILO & \multicolumn{4}{l|}{4'h4}
		& \multicolumn{4}{l|}{4'hf}
		& \multicolumn{3}{l|}{Cond.}
		& 1'b0
		& \multicolumn{4}{l|}{R. Reg}
		& \multicolumn{16}{l|}{16-bit Immediate}
		& \\\hline
16-b MPYU & \multicolumn{4}{l|}{4'h4}
		& \multicolumn{4}{l|}{R. Reg}
		& \multicolumn{3}{l|}{Cond.}
		& 1'b0 & \multicolumn{4}{l|}{Reg}
		& \multicolumn{16}{l|}{16-bit Offset}
		& Yes \\\hline
16-b MPYU(I) & \multicolumn{4}{l|}{4'h4}
		& \multicolumn{4}{l|}{R. Reg}
		& \multicolumn{3}{l|}{Cond.}
		& 1'b0 & \multicolumn{4}{l|}{4'hf}
		& \multicolumn{16}{l|}{16-bit Offset}
		& Yes \\\hline
16-b MPYS & \multicolumn{4}{l|}{4'h4}
		& \multicolumn{4}{l|}{R. Reg}
		& \multicolumn{3}{l|}{Cond.}
		& 1'b1 & \multicolumn{4}{l|}{Reg}
		& \multicolumn{16}{l|}{16-bit Offset}
		& Yes \\\hline
16-b MPYS(I) & \multicolumn{4}{l|}{4'h4}
		& \multicolumn{4}{l|}{R. Reg}
		& \multicolumn{3}{l|}{Cond.}
		& 1'b1 & \multicolumn{4}{l|}{4'hf}
		& \multicolumn{16}{l|}{16-bit Offset}
		& Yes \\\hline
ROL & \multicolumn{4}{l|}{4'h5}
		& \multicolumn{4}{l|}{R. Reg}
		& \multicolumn{3}{l|}{Cond.}
		& \multicolumn{21}{l|}{Operand B, truncated to low order 5 bits}
		& \\\hline
LOD & \multicolumn{4}{l|}{4'h6}
		& \multicolumn{4}{l|}{R. Reg}
		& \multicolumn{3}{l|}{Cond.}
		& \multicolumn{21}{l|}{Operand B address}
		& \\\hline
STO & \multicolumn{4}{l|}{4'h7}
		& \multicolumn{4}{l|}{D. Reg}
		& \multicolumn{3}{l|}{Cond.}
		& \multicolumn{21}{l|}{Operand B address}
		& \\\hline
{\em Rsrd} & \multicolumn{4}{l|}{4'h8}
	&	\multicolumn{4}{l|}{R. Reg}
	&	\multicolumn{3}{l|}{Cond.}
	& 1'b0
	&	\multicolumn{20}{l|}{Reserved}
	& Yes \\\hline
SUB & \multicolumn{4}{l|}{4'h8}
	&	\multicolumn{4}{l|}{R. Reg}
	&	\multicolumn{3}{l|}{Cond.}
	& 1'b1
	&	\multicolumn{4}{l|}{Reg}
	&	\multicolumn{16}{l|}{16'bit signed offset}
	& Yes \\\hline
AND & \multicolumn{4}{l|}{4'h9}
	&	\multicolumn{4}{l|}{R. Reg}
	&	\multicolumn{3}{l|}{Cond.}
	&	\multicolumn{21}{l|}{Operand B}
	& Yes \\\hline
ADD & \multicolumn{4}{l|}{4'ha}
	&	\multicolumn{4}{l|}{R. Reg}
	&	\multicolumn{3}{l|}{Cond.}
	&	\multicolumn{21}{l|}{Operand B}
	& Yes \\\hline
OR & \multicolumn{4}{l|}{4'hb}
	&	\multicolumn{4}{l|}{R. Reg}
	&	\multicolumn{3}{l|}{Cond.}
	&	\multicolumn{21}{l|}{Operand B}
	& Yes \\\hline
XOR & \multicolumn{4}{l|}{4'hc}
	&	\multicolumn{4}{l|}{R. Reg}
	&	\multicolumn{3}{l|}{Cond.}
	&	\multicolumn{21}{l|}{Operand B}
	& Yes \\\hline
LSL/ASL & \multicolumn{4}{l|}{4'hd}
	&	\multicolumn{4}{l|}{R. Reg}
	&	\multicolumn{3}{l|}{Cond.}
	&	\multicolumn{21}{l|}{Operand B, imm. trucated to 6 bits}
	& Yes \\\hline
ASR & \multicolumn{4}{l|}{4'he}
	&	\multicolumn{4}{l|}{R. Reg}
	&	\multicolumn{3}{l|}{Cond.}
	&	\multicolumn{21}{l|}{Operand B, imm. trucated to 6 bits}
	& Yes \\\hline
LSR & \multicolumn{4}{l|}{4'hf}
	&	\multicolumn{4}{l|}{R. Reg}
	&	\multicolumn{3}{l|}{Cond.}
	&	\multicolumn{21}{l|}{Operand B, imm. trucated to 6 bits}
	& Yes \\\hline
\end{tabular}
\caption{Zip CPU Instruction Set}\label{tbl:zip-instructions}
\end{center}\end{table}
 
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
24--bit hole.  Likewise, the subtract leaves half of its space open, since a
subtract immediate is the same as an add with a negated immediate.  This
spaces are reserved for future enhancements.
 
\section{Derived Instructions}
The ZIP CPU supports many other common instructions, but not all of them
are single cycle instructions.  The derived instruction tables,
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
the ZIP CPU.  Many of these instructions will have assembly equivalents,
such as the branch instructions, to facilitate working with the CPU.
\begin{table}\begin{center}
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
Mapped & Actual  & Notes \\\hline
\parbox[t]{1.4in}{ADD Ra,Rx\\ADDC Rb,Ry}
	& \parbox[t]{1.5in}{Add Ra,Rx\\ADD.C \$1,Ry\\Add Rb,Ry}
	& Add with carry \\\hline
BRA.Cond +/-\$Addr
	& \hbox{Mov.cond \$Addr+PC,PC}
	& Branch or jump on condition.  Works for 15--bit
		signed address offsets.\\\hline
BRA.Cond +/-\$Addr
	& \parbox[t]{1.5in}{LDI \$Addr,Rx \\ ADD.cond Rx,PC}
	& Branch/jump on condition.  Works for
	23 bit address offsets, but costs a register, an extra instruction,
	and setsthe flags. \\\hline
BNC PC+\$Addr
	& \parbox[t]{1.5in}{Test \$Carry,CC \\ MOV.Z PC+\$Addr,PC}
	& Example of a branch on an unsupported
		condition, in this case a branch on not carry \\\hline
BUSY & MOV \$-1(PC),PC & Execute an infinite loop \\\hline
CLRF.NZ Rx
	& XOR.NZ Rx,Rx
	& Clear Rx, and flags, if the Z-bit is not set \\\hline
CLR Rx
	& LDI \$0,Rx
	& Clears Rx, leaves flags untouched.  This instruction cannot be
		conditional. \\\hline
EXCH.W Rx
	& ROL \$16,Rx
	& Exchanges the top and bottom 16'bit words of Rx \\\hline
HALT
	& Or \$SLEEP,CC
	& Executed while in interrupt mode.  In user mode this is simply a
	wait until interrupt instructioon. \\\hline
INT & LDI \$0,CC
	& Since we're using the CC register as a trap vector as well, this
	executes TRAP \#0. \\\hline
IRET
	& OR \$GIE,CC
	& Also an RTU instruction (Return to Userspace) \\\hline
JMP R6+\$Addr
	& MOV \$Addr(R6),PC
	& \\\hline
JSR PC+\$Addr
	& \parbox[t]{1.5in}{SUB \$1,SP \\\
	MOV \$3+PC,R0 \\
	STO R0,1(SP) \\
	MOV \$Addr+PC,PC \\
	ADD \$1,SP}
	& Jump to Subroutine. Note the required cleanup instruction after
	returning. \\\hline
JSR PC+\$Addr 
	& \parbox[t]{1.5in}{MOV \$3+PC,R12 \\ MOV \$addr+PC,PC}
	&This is the high speed
	version of a subroutine call, necessitating a register to hold the
	last PC address.  In its favor, this method doesn't suffer the
	mandatory memory access of the other approach. \\\hline
LDI.l \$val,Rx
	& \parbox[t]{1.5in}{LDIHI (\$val$>>$16)\&0x0ffff, Rx \\
			LDILO (\$val \& 0x0ffff)}
	& Sadly, there's not enough instruction
		space to load a complete immediate value into any register.
		Therefore, fully loading any register takes two cycles.
		The LDIHI (load immediate high) and LDILO (load immediate low)
		instructions have been created to facilitate this. \\\hline
\end{tabular}
\caption{Derived Instructions}\label{tbl:derived-1}
\end{center}\end{table}
\begin{table}\begin{center}
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
Mapped & Actual  & Notes \\\hline
LOD.b \$addr,Rx
	& \parbox[t]{1.5in}{%
	LDI	\$addr,Ra \\
	LDI	\$addr,Rb \\
	LSR	\$2,Ra \\
	AND	\$3,Rb \\
	LOD	(Ra),Rx \\
	LSL	\$3,Rb \\
	SUB	\$32,Rb \\
	ROL	Rb,Rx \\
	AND \$0ffh,Rx}
	& \parbox[t]{3in}{This CPU is designed for 32'bit word
	length instructions.  Byte addressing is not supported by the CPU or
	the bus, so it therefore takes more work to do. 
 
	Note also that in this example, \$Addr is a byte-wise address, where
	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
	space of character accesses using this method from 16 MB down to 4MB.}
		\\\hline
\parbox[t]{1.5in}{LSL \$1,Rx\\ LSLC \$1,Ry}
	& \parbox[t]{1.5in}{LSL \$1,Ry \\
	LSL \$1,Rx \\
	OR.C \$1,Ry}
	& Logical shift left with carry.  Note that the
	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
	with Rx before Ry, then the condition flag wouldn't have been right
	for an OR correction at the end. \\\hline
\parbox[t]{1.5in}{LSR \$1,Rx \\ LSRC \$1,Ry}
	& \parbox[t]{1.5in}{CLR Rz \\
	LSR \$1,Ry \\
	LDIHI.C \$8000h,Rz \\
	LSR \$1,Rx \\
	OR Rz,Rx}
	& Logical shift right with carry \\\hline
NEG Rx & \parbox[t]{1.5in}{XOR \$-1,Rx \\ ADD \$1,Rx} & \\\hline
NOOP & NOOP & While there are many
	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
	Rx isn't ready yet.  For this reason, we have a dedicated NOOP
	instruction. \\\hline
NOT Rx & XOR \$-1,Rx & \\\hline
POP Rx 
	& \parbox[t]{1.5in}{LOD \$-1(SP),Rx \\ ADD \$1,SP}
	& Note
	that for interrupt purposes, one can never depend upon the value at
	(SP).  Hence you read from it, then increment it, lest having
	incremented it firost something then comes along and writes to that
	value before you can read the result. \\\hline
PUSH Rx
	& \parbox[t]{1.5in}{SUB \$1,SPa \\
	STO Rx,\$1(SP)}
	& \\\hline
RESET
	& \parbox[t]{1in}{STO \$1,\$watchdog(R12)\\NOOP\\NOOP}
	& \parbox[t]{3in}{This depends upon the peripheral base address being
	in R12.
 
	Another opportunity might be to jump to the reset address from within
	supervisor mode.}\\\hline
RET & \parbox[t]{1.5in}{LOD \$-1(SP),PC}
	& Note that this depends upon the calling context to clean up the
	stack, as outlined for the JSR instruction.  \\\hline
\end{tabular}
\caption{Derived Instructions, continued}\label{tbl:derived-2}
\end{center}\end{table}
\begin{table}\begin{center}
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
RET & MOV R12,PC
	& 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.
	\\\hline
STEP Rr,Rt
	& \parbox[t]{1.5in}{LSR \$1,Rr \\ XOR.C Rt,Rr}
	& Step a Galois implementation of a Linear Feedback Shift Register, Rr,
		using taps Rt \\\hline
STO.b Rx,\$addr
	& \parbox[t]{1.5in}{%
	LDI \$addr,Ra \\
	LDI \$addr,Rb \\
	LSR \$2,Ra \\
	AND \$3,Rb \\
	SUB \$32,Rb \\
	LOD (Ra),Ry \\
	AND \$0ffh,Rx \\
	AND \$-0ffh,Ry \\
	ROL Rb,Rx \\
	OR Rx,Ry \\
	STO Ry,(Ra) }
	& \parbox[t]{3in}{This CPU and it's bus are {\em not} optimized
	for byte-wise operations.
 
	Note that in this example, \$addr 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
	of character accesses from 16 MB down to 4MB.F
	Further, this instruction implies a byte ordering,
	such as big or little endian.} \\\hline
SWAP Rx,Ry 
	& \parbox[t]{1.5in}{
	XOR Ry,Rx \\
	XOR Rx,Ry \\
	XOR Ry,Rx} 
	& While no extra registers are needed, this example
	does take 3-clocks. \\\hline
TRAP \#X
	& LDILO \$x,CC
	& This approach uses the unused bits of the CC register as a TRAP
	address.  The user will need to make certain
	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.,
	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
	then MOV it into the CC register. \\\hline
TST Rx
	& TST \$-1,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 
	approaches won't stall future pipeline stages looking for the value
	of Rx. \\\hline
WAIT
	& Or \$SLEEP,CC
	& Wait 'til interrupt.  In an interrupts disabled context, this
	becomes a HALT instruction.
\end{tabular}
\caption{Derived Instructions, continued}\label{tbl:derived-3}
\end{center}\end{table}
\iffalse
\fi
\section{Pipeline Stages}
\begin{enumerate}
\item {\bf Prefetch}: Read instruction from memory (cache if possible).  This
	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
	in the prefetch cache.
\item {\bf Decode}: Decode instruction into op code, register(s) to read, and
	immediate offset.
\item {\bf Read Operands}: Read registers and apply any immediate values to
	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
	between the instruction that produces the result and the instruction
	that uses it.
\item Split into two tracks: An {\bf ALU} which will accomplish a simple
	instruction, and the {\bf MemOps} stage which accomplishes memory
	read/write.
	\begin{itemize}
	\item Loads stall instructions that access the register until it is
		written to the register set.
	\item Condition codes are available upon completion
	\item Issuing an instruction to the memory while the memory is busy will
		stall the bus.  If the bus deadlocks, only a reset will
		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}
\item {\bf Write-Back}: Conditionally write back the result to register set,
	applying the condition.  This routine is bi-re-entrant: either the
	memory or the simple instruction may request a register write. 
\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}
How the CPU handles some instruction combinations can be telling when
determining what happens in the pipeline.  The following lists some examples:
\begin{itemize}
\item {\bf Delayed Branching}
 
	I had originally hoped to implement delayed branching.  However, what
	happens in debug mode?
	That is, what happens when a debugger tries to single step an
	instruction?  While I can easily single step the computer in either
	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
	from within supervisor mode--such as if a process had a debugger
	attached.  As the processor exists, I would have one result stepping
	the CPU from a debugger, and another stepping it externally.
 
	This is unacceptable, and so this CPU does not support delayed
	branching.
 
\item {\bf Register Result:} {\tt MOV R0,R1; MOV R1,R2 }
 
	What value does
	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
	two, as I have the pipeline designed.
 
	The ZIP CPU architecture requires that R2 must equal R0 at the end of
	this operation.  This may stall the pipeline 1-2 cycles.
 
\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
	updates the flags that the MOV instruction depends
	upon.
 
	The Zip CPU architecture requires that condition codes must be updated
	and available immediately for the next instruction without stalling the
	pipeline.
 
\item {\bf Condition Codes Register Result:} {\tt CMP R0,R1; MOV CC,R2}
 
	At issue is the
	fact that the logic supporting the CC register is more complicated than
	the logic supporting any other register.
 
	The ZIP CPU will stall 1--2 cycles on this instruction, until the
	CC register is valid.
 
\item {\bf Delayed Branching: } {\tt ADD \$x,PC; MOV R0,R1}
 
	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
	register handled differently from an ADD to the PC register?
 
	In the Zip architecture, MOV'es and ADD's use the same logic
	(simplifies the logic).
\end{itemize}
 
As I've studied  this, I find several approaches to handling pipeline 
	issues.  These approaches (and their consequences) are listed below.
 
\begin{itemize}
\item {\bf All All issued instructions complete, Stages stall individually}
 
	What about a slow pre-fetch? 
 
	Nominally, this works well: any issued instruction
	just runs to completion.  If there are four issued instructions in the
	pipeline, with the writeback instruction being a write-to-PC
	instruction, the other three instructions naturally finish.
 
	This approach fails when reading instructions from the flash,
	since such reads require N clocks to clocks to complete.  Thus 
	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
	would produce a different response.
 
\item {\bf Issued instructions may be canceled}
 
	Stages stall individually
 
	First problem: 
	Memory operations cannot be canceled, even reads may have side effects
	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
	a \hbox{\tt MOV.C \$x,PC} followed by a \hbox{\tt MOV \$y,PC}
	instruction?  Which get 
	canceled?
 
	Because it isn't clear what would need to be canceled,
	this instruction combination is not recommended.
 
\item {\bf All issued instructions complete.}
 
	All stages are filled, or the entire pipeline
	stalls.
 
	What about debug control?  What about
	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
	after an interrupt?  What address do you use?  The last issued
	instruction?  But the branch delay slots may make that invalid!
 
	Reading from the CPU debug port in this case yields inconsistent
	results: the CPU will halt or step with instructions stuck in the
	pipeline.  Reading registers will give no indication of what is going
	on in the pipeline, just the results of completed operations, not of
	operations that have been started and not yet completed.
	Perhaps we should just report the state of the CPU based upon what
	instructions (PC values) have successfully completed?  Thus the
	debug instruction is the one that will write registers on the next
	clock.
 
	Suggestion: Suppose we load extra information in the two
	CC register(s) for debugging intermediate pipeline stages?
 
	The next problem, though, is how to deal with the read operand
	pipeline stage needing the result from the register pipeline.a
 
\item {\bf Memory instructions must complete}
 
	All instructions that enter into the memory module *must*
	complete.  Issued instructions from the prefetch, decode, or operand
	read stages may or may not complete.  Jumps into code must be valid,
	so that interrupt returns may be valid.  All instructions entering the
	ALU complete.
 
	This looks to be the simplest approach.
	While the logic may be difficult, this appears to be the only
	re-entrant approach.
 
	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
	as well as interrupts and interrupt returns.  Whenever this flag may
	go high, memory operations and ALU operations will stall until the
	result is known.  When the flag does go high, anything in the prefetch,
	decode, and read-op stage will be invalidated.
 
\end{itemize}
 
 
 
\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}
 
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}
 
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
counter just sets the current value, and it starts counting again from that
value.
 
Eight counters are implemented in the Zip System for process accounting.
This may change in the future, as nothing as yet uses these counters.
 
\section{Timer}
 
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.
 
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
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
set the high bit.  Ever after, the timer will interrupt the CPU once per
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}
 
The watchdog timer is no different from any of the other timers, save for one
critical difference: the interrupt line from the watchdog
timer is tied to the reset line of the CPU.  Hence writing a `1' to the 
watchdog timer will always reset the CPU.  
To stop the Watchdog timer, write a '0' to it.  To start it,
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
as it did with the other timers.
 
\section{Jiffies}
 
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
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
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
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.
(There is currently no way to disable a jiffie interrupt once set, other
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.
Once the timer expires, it would write the next Jiffy request to the peripheral
and wake up the process whose timer had expired.
 
Indeed, the Jiffies register is nothing more than a glorified counter with
an interrupt.  Unlike the other counters, the Jiffies register cannot be set.
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
and the register then continues counting as though no interrupt had taken
place.
 
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
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,
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
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{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}
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}
\begin{table}[htbp]
\begin{center}
\begin{wishboneds}
Revision level of wishbone & WB B4 spec \\\hline
Type of interface & Slave, Read/Write, single words only \\\hline
Address Width & 1--bit \\\hline
Port size & 32--bit \\\hline
Port granularity & 32--bit \\\hline
Maximum Operand Size & 32--bit \\\hline
Data transfer ordering & (Irrelevant) \\\hline
Clock constraints & Works at 100~MHz on a Basys--3 board\\\hline
Signal Names & \begin{tabular}{ll}
		Signal Name & Wishbone Equivalent \\\hline
		{\tt i\_clk} & {\tt CLK\_I} \\
		{\tt i\_dbg\_cyc} & {\tt CYC\_I} \\
		{\tt i\_dbg\_stb} & {\tt STB\_I} \\
		{\tt i\_dbg\_we} & {\tt WE\_I} \\
		{\tt i\_dbg\_addr} & {\tt ADR\_I} \\
		{\tt i\_dbg\_data} & {\tt DAT\_I} \\
		{\tt o\_dbg\_ack} & {\tt ACK\_O} \\
		{\tt o\_dbg\_stall} & {\tt STALL\_O} \\
		{\tt o\_dbg\_data} & {\tt DAT\_O}
		\end{tabular}\\\hline
\end{wishboneds}
\caption{Wishbone Datasheet for the Debug Interface}\label{tbl:wishbone-slave}
\end{center}\end{table}
and Tbl.~\ref{tbl:wishbone-master} respectively.
\begin{table}[htbp]
\begin{center}
\begin{wishboneds}
Revision level of wishbone & WB B4 spec \\\hline
Type of interface & Master, Read/Write, single cycle or pipelined\\\hline
Address Width & 32--bit bits \\\hline
Port size & 32--bit \\\hline
Port granularity & 32--bit \\\hline
Maximum Operand Size & 32--bit \\\hline
Data transfer ordering & (Irrelevant) \\\hline
Clock constraints & Works at 100~MHz on a Basys--3 board\\\hline
Signal Names & \begin{tabular}{ll}
		Signal Name & Wishbone Equivalent \\\hline
		{\tt i\_clk} & {\tt CLK\_O} \\
		{\tt o\_wb\_cyc} & {\tt CYC\_O} \\
		{\tt o\_wb\_stb} & {\tt STB\_O} \\
		{\tt o\_wb\_we} & {\tt WE\_O} \\
		{\tt o\_wb\_addr} & {\tt ADR\_O} \\
		{\tt o\_wb\_data} & {\tt DAT\_O} \\
		{\tt i\_wb\_ack} & {\tt ACK\_I} \\
		{\tt i\_wb\_stall} & {\tt STALL\_I} \\
		{\tt i\_wb\_data} & {\tt DAT\_I}
		\end{tabular}\\\hline
\end{wishboneds}
\caption{Wishbone Datasheet for the CPU as Master}\label{tbl:wishbone-master}
\end{center}\end{table}
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.
 
You may wish to notice that neither the {\tt ERR} nor the {\tt RETRY} wires
have been implemented.  What this means is that the CPU is currently unable
to detect a bus error condition, and so may stall indefinitely (hang) should
it choose to access a value not on the bus, or a peripheral that is not
yet properly configured.
 
\chapter{Clocks}\label{chap:clocks}
 
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
core.
\begin{table}[htbp]
\begin{center}
\begin{clocklist}
i\_clk & External & 100~MHz & 100~MHz & System clock.\\\hline
\end{clocklist}
\caption{List of Clocks}\label{tbl:clocks}
\end{center}\end{table}
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 
now, I will only state that it can run at 100~MHz.
 
 
\chapter{I/O Ports}\label{chap:ioports}
 
% Appendices
% Index
\end{document}