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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%%
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%% Filename:    spec.tex
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%%
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%% Project:     Zip CPU -- a small, lightweight, RISC CPU soft core
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%%
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%% Purpose:     This LaTeX file contains all of the documentation/description
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%%              currently provided with this Zip CPU soft core.  It supersedes
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%%              any information about the instruction set or CPUs found
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%%              elsewhere.  It's not nearly as interesting, though, as the PDF
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%%              file it creates, so I'd recommend reading that before diving
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%%              into this file.  You should be able to find the PDF file in
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%%              the SVN distribution together with this PDF file and a copy of
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%%              the GPL-3.0 license this file is distributed under.  If not,
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%%              just type 'make' in the doc directory and it (should) build
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%%              without a problem.
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%%
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%%
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%% Creator:     Dan Gisselquist
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%%              Gisselquist Technology, LLC
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%%
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%%
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%% Copyright (C) 2015, Gisselquist Technology, LLC
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%%
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%% This program is free software (firmware): you can redistribute it and/or
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%% modify it under the terms of  the GNU General Public License as published
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%% by the Free Software Foundation, either version 3 of the License, or (at
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%% your option) any later version.
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%%
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%% This program is distributed in the hope that it will be useful, but WITHOUT
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%% ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
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%% FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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%% for more details.
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%%
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%% You should have received a copy of the GNU General Public License along
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%% with this program.  (It's in the $(ROOT)/doc directory, run make with no
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%% target there if the PDF file isn't present.)  If not, see
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%% <http://www.gnu.org/licenses/> for a copy.
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%%
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%% License:     GPL, v3, as defined and found on www.gnu.org,
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%%              http://www.gnu.org/licenses/gpl.html
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%%
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%%
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\documentclass{gqtekspec}
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\project{Zip CPU}
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\title{Specification}
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\author{Dan Gisselquist, Ph.D.}
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\email{dgisselq (at) opencores.org}
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\revision{Rev.~0.3}
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\begin{document}
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\pagestyle{gqtekspecplain}
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\titlepage
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\begin{license}
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Copyright (C) \theyear\today, Gisselquist Technology, LLC
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58
This project is free software (firmware): you can redistribute it and/or
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modify it under the terms of  the GNU General Public License as published
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by the Free Software Foundation, either version 3 of the License, or (at
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your option) any later version.
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63
This program is distributed in the hope that it will be useful, but WITHOUT
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ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
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FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
66
for more details.
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68
You should have received a copy of the GNU General Public License along
69
with this program.  If not, see \hbox{<http://www.gnu.org/licenses/>} for a
70
copy.
71
\end{license}
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\begin{revisionhistory}
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0.3 & 8/22/2015 & Gisselquist & First completed draft\\\hline
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0.2 & 8/19/2015 & Gisselquist & Still Draft, more complete \\\hline
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0.1 & 8/17/2015 & Gisselquist & Incomplete First Draft \\\hline
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\end{revisionhistory}
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% Revision History
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% Table of Contents, named Contents
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\tableofcontents
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\listoffigures
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\listoftables
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\begin{preface}
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Many people have asked me why I am building the Zip CPU. ARM processors are
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good and effective. Xilinx makes and markets Microblaze, Altera Nios, and both
85
have better toolsets than the Zip CPU will ever have. OpenRISC is also
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available, RISC--V may be replacing it. Why build a new processor?
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88
The easiest, most obvious answer is the simple one: Because I can.
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90
There's more to it, though. There's a lot that I would like to do with a
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processor, and I want to be able to do it in a vendor independent fashion.
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I would like to be able to generate Verilog code that can run equivalently
93
on both Xilinx and Altera chips, and that can be easily ported from one
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manufacturer's chipsets to another. Even more, before purchasing a chip or a
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board, I would like to know that my soft core works. I would like to build a test
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bench to test components with, and Verilator is my chosen test bench. This
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forces me to use all Verilog, and it prevents me from using any proprietary
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cores. For this reason, Microblaze and Nios are out of the question.
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100
Why not OpenRISC? That's a hard question. The OpenRISC team has done some
101
wonderful work on an amazing processor, and I'll have to admit that I am
102
envious of what they've accomplished. I would like to port binutils to the
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Zip CPU, as I would like to port GCC and GDB. They are way ahead of me. The
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OpenRISC processor, however, is complex and hefty at about 4,500 LUTs. It has
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a lot of features of modern CPUs within it that ... well, let's just say it's
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not the little guy on the block. The Zip CPU is lighter weight, costing only
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about 2,300 LUTs with no peripherals, and 3,200 LUTs with some very basic
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peripherals.
109
 
110
My final reason is that I'm building the Zip CPU as a learning experience. The
111
Zip CPU has allowed me to learn a lot about how CPUs work on a very micro
112
level. For the first time, I am beginning to understand many of the Computer
113
Architecture lessons from years ago.
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115
To summarize: Because I can, because it is open source, because it is light
116
weight, and as an exercise in learning.
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118
\end{preface}
119
 
120
\chapter{Introduction}
121
\pagenumbering{arabic}
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\setcounter{page}{1}
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124
 
125
The original goal of the ZIP CPU was to be a very simple CPU.   You might
126
think of it as a poor man's alternative to the OpenRISC architecture.
127
For this reason, all instructions have been designed to be as simple as
128
possible, and are all designed to be executed in one instruction cycle per
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instruction, barring pipeline stalls.  Indeed, even the bus has been simplified
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to a constant 32-bit width, with no option for more or less.  This has
131
resulted in the choice to drop push and pop instructions, pre-increment and
132
post-decrement addressing modes, and more.
133
 
134
For those who like buzz words, the Zip CPU is:
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\begin{itemize}
136
\item A 32-bit CPU: All registers are 32-bits, addresses are 32-bits,
137
                instructions are 32-bits wide, etc.
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\item A RISC CPU.  There is no microcode for executing instructions.  All
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        instructions are designed to be completed in one clock cycle.
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\item A Load/Store architecture.  (Only load and store instructions
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                can access memory.)
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\item Wishbone compliant.  All peripherals are accessed just like
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                memory across this bus.
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\item A Von-Neumann architecture.  (The instructions and data share a
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                common bus.)
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\item A pipelined architecture, having stages for {\bf Prefetch},
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                {\bf Decode}, {\bf Read-Operand}, the {\bf ALU/Memory}
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                unit, and {\bf Write-back}.  See Fig.~\ref{fig:cpu}
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\begin{figure}\begin{center}
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\includegraphics[width=3.5in]{../gfx/cpu.eps}
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\caption{Zip CPU internal pipeline architecture}\label{fig:cpu}
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\end{center}\end{figure}
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                for a diagram of this structure.
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\item Completely open source, licensed under the GPL.\footnote{Should you
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        need a copy of the Zip CPU licensed under other terms, please
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        contact me.}
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\end{itemize}
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159
Now, however, that I've worked on the Zip CPU for a while, it is not nearly
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as simple as I originally hoped.  Worse, I've had to adjust to create
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capabilities that I was never expecting to need.  These include:
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\begin{itemize}
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\item {\bf External Debug:} Once placed upon an FPGA, some external means is
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        still necessary to debug this CPU.  That means that there needs to be
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        an external register that can control the CPU: reset it, halt it, step
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        it, and tell whether it is running or not.  My chosen interface
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        includes a second register similar to this control register.  This
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        second register allows the external controller or debugger to examine
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        registers internal to the CPU.
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171
\item {\bf Internal Debug:} Being able to run a debugger from within
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        a user process requires an ability to step a user process from
173
        within a debugger.  It also requires a break instruction that can
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        be substituted for any other instruction, and substituted back.
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        The break is actually difficult: the break instruction cannot be
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        allowed to execute.  That way, upon a break, the debugger should
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        be able to jump back into the user process to step the instruction
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        that would've been at the break point initially, and then to
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        replace the break after passing it.
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        Incidentally, this break messes with the prefetch cache and the
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        pipeline: if you change an instruction partially through the pipeline,
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        the whole pipeline needs to be cleansed.  Likewise if you change
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        an instruction in memory, you need to make sure the cache is reloaded
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        with the new instruction.
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\item {\bf Prefetch Cache:} My original implementation had a very
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        simple prefetch stage.  Any time the PC changed the prefetch would go
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        and fetch the new instruction.  While this was perhaps this simplest
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        approach, it cost roughly five clocks for every instruction.  This
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        was deemed unacceptable, as I wanted a CPU that could execute
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        instructions in one cycle.  I therefore have a prefetch cache that
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        issues pipelined wishbone accesses to memory and then pushes
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        instructions at the CPU.  Sadly, this accounts for about 20\% of the
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        logic in the entire CPU, or 15\% of the logic in the entire system.
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197
 
198
\item {\bf Operating System:} In order to support an operating system,
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        interrupts and so forth, the CPU needs to support supervisor and
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        user modes, as well as a means of switching between them.  For example,
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        the user needs a means of executing a system call.  This is the
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        purpose of the {\bf `trap'} instruction.  This instruction needs to
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        place the CPU into supervisor mode (here equivalent to disabling
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        interrupts), as well as handing it a parameter such as identifying
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        which O/S function was called.
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My initial approach to building a trap instruction was to create an external
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peripheral which, when written to, would generate an interrupt and could
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return the last value written to it.  In practice, this approach didn't work
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at all: the CPU executed two instructions while waiting for the
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trap interrupt to take place.  Since then, I've decided to keep the rest of
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the CC register for that purpose so that a write to the CC register, with the
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GIE bit cleared, could be used to execute a trap.  This has other problems,
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though, primarily in the limitation of the uses of the CC register.  In
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particular, the CC register is the best place to put CPU state information and
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to ``announce'' special CPU features (floating point, etc).  So the trap
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instruction still switches to interrupt mode, but the CC register is not
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nearly as useful for telling the supervisor mode processor what trap is being
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executed.
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221
Modern timesharing systems also depend upon a {\bf Timer} interrupt
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to handle task swapping.  For the Zip CPU, this interrupt is handled
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external to the CPU as part of the CPU System, found in {\tt zipsystem.v}.
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The timer module itself is found in {\tt ziptimer.v}.
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226
\item {\bf Pipeline Stalls:} My original plan was to not support pipeline
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        stalls at all, but rather to require the compiler to properly schedule
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        all instructions so that stalls would never be necessary.  After trying
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        to build such an architecture, I gave up, having learned some things:
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        For example, in  order to facilitate interrupt handling and debug
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        stepping, the CPU needs to know what instructions have finished, and
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        which have not.  In other words, it needs to know where it can restart
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        the pipeline from.  Once restarted, it must act as though it had
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        never stopped.  This killed my idea of delayed branching, since what
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        would be the appropriate program counter to restart at?  The one the
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        CPU was going to branch to, or the ones in the delay slots?  This
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        also makes the idea of compressed instruction codes difficult, since,
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        again, where do you restart on interrupt?
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241
        So I switched to a model of discrete execution: Once an instruction
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        enters into either the ALU or memory unit, the instruction is
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        guaranteed to complete.  If the logic recognizes a branch or a
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        condition that would render the instruction entering into this stage
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        possibly inappropriate (i.e. a conditional branch preceding a store
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        instruction for example), then the pipeline stalls for one cycle
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        until the conditional branch completes.  Then, if it generates a new
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        PC address, the stages preceding are all wiped clean.
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250
        The discrete execution model allows such things as sleeping: if the
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        CPU is put to ``sleep,'' the ALU and memory stages stall and back up
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        everything before them.  Likewise, anything that has entered the ALU
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        or memory stage when the CPU is placed to sleep continues to completion.
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        To handle this logic, each pipeline stage has three control signals:
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        a valid signal, a stall signal, and a clock enable signal.  In
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        general, a stage stalls if it's contents are valid and the next step
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        is stalled.  This allows the pipeline to fill any time a later stage
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        stalls.
259
 
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        This approach is also different from other pipeline approaches.  Instead
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        of keeping the entire pipeline filled, each stage is treated
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        independently.  Therefore, individual stages may move forward as long
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        as the subsequent stage is available, regardless of whether the stage
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        behind it is filled.
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\item {\bf Verilog Modules:} When examining how other processors worked
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        here on open cores, many of them had one separate module per pipeline
268
        stage.  While this appeared to me to be a fascinating and commendable
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        idea, my own implementation didn't work out quite so nicely.
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271
        As an example, the decode module produces a {\em lot} of
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        control wires and registers.  Creating a module out of this, with
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        only the simplest of logic within it, seemed to be more a lesson
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        in passing wires around, rather than encapsulating logic.
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276
        Another example was the register writeback section.  I would love
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        this section to be a module in its own right, and many have made them
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        such.  However, other modules depend upon writeback results other
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        than just what's placed in the register (i.e., the control wires).
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        For these reasons, I didn't manage to fit this section into it's
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        own module.
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283
        The result is that the majority of the CPU code can be found in
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        the {\tt zipcpu.v} file.
285
\end{itemize}
286
 
287
With that introduction out of the way, let's move on to the instruction
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set.
289
 
290
\chapter{CPU Architecture}\label{chap:arch}
291
 
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The Zip CPU supports a set of two operand instructions, where the second operand
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(always a register) is the result.  The only exception is the store instruction,
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where the first operand (always a register) is the source of the data to be
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stored.
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\section{Simplified Bus}
298
The bus architecture of the Zip CPU is that of a simplified WISHBONE bus.
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It has been simplified in this fashion: all operations are 32--bit operations.
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The bus is neither little endian nor bit endian.  For this reason, all words
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are 32--bits.  All instructions are also 32--bits wide.  Everything has been
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built around the 32--bit word.
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\section{Register Set}
305
The Zip CPU supports two sets of sixteen 32-bit registers, a supervisor
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and a user set as shown in Fig.~\ref{fig:regset}.
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\begin{figure}\begin{center}
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\includegraphics[width=3.5in]{../gfx/regset.eps}
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\caption{Zip CPU Register File}\label{fig:regset}
310
\end{center}\end{figure}
311
The supervisor set is used in interrupt mode when interrupts are disabled,
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whereas the user set is used otherwise.  Of this register set, the Program
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Counter (PC) is register 15, whereas the status register (SR) or condition
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code register
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(CC) is register 14.  By convention, the stack pointer will be register 13 and
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noted as (SP)--although there is nothing special about this register other
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than this convention.
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The CPU can access both register sets via move instructions from the
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supervisor state, whereas the user state can only access the user registers.
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321
The status register is special, and bears further mention.  The lower
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10 bits of the status register form a set of CPU state and condition codes.
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Writes to other bits of this register are preserved.
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Of the condition codes, the bottom four bits are the current flags:
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                Zero (Z),
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                Carry (C),
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                Negative (N),
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                and Overflow (V).
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331
The next bit is a clock enable (0 to enable) or sleep bit (1 to put
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        the CPU to sleep).  Setting this bit will cause the CPU to
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        wait for an interrupt (if interrupts are enabled), or to
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        completely halt (if interrupts are disabled).
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The sixth bit is a global interrupt enable bit (GIE).  When this
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        sixth bit is a `1' interrupts will be enabled, else disabled.  When
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        interrupts are disabled, the CPU will be in supervisor mode, otherwise
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        it is in user mode.  Thus, to execute a context switch, one only
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        need enable or disable interrupts.  (When an interrupt line goes
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        high, interrupts will automatically be disabled, as the CPU goes
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        and deals with its context switch.)  Special logic has been added to
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        keep the user mode from setting the sleep register and clearing the
344
        GIE register at the same time, with clearing the GIE register taking
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        precedence.
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The seventh bit is a step bit.  This bit can be
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        set from supervisor mode only.  After setting this bit, should
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        the supervisor mode process switch to user mode, it would then
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        accomplish one instruction in user mode before returning to supervisor
351
        mode.  Then, upon return to supervisor mode, this bit will
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        be automatically cleared.  This bit has no effect on the CPU while in
353
        supervisor mode.
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355
        This functionality was added to enable a userspace debugger
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        functionality on a user process, working through supervisor mode
357
        of course.
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The eighth bit is a break enable bit.  This controls whether a break
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instruction in user mode will halt the processor for an external debugger
362
(break enabled), or whether the break instruction will simply send send the
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CPU into interrupt mode.  Encountering a break in supervisor mode will
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halt the CPU independent of the break enable bit.  This bit can only be set
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within supervisor mode.
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% Should break enable be a supervisor mode bit, while the break enable bit
368
% in user mode is a break has taken place bit?
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%
370
 
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This functionality was added to enable an external debugger to
372
        set and manage breakpoints.
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374
The ninth bit is reserved for a floating point enable bit.  When set, the
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arithmetic for the next instruction will be sent to a floating point unit.
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Such a unit may later be added as an extension to the Zip CPU.  If the
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CPU does not support floating point instructions, this bit will never be set.
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The instruction set could also be simply extended to allow other data types
379
in this fashion, such as two by 16--bit vector operations or four by 8--bit
380
vector operations.
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382
The tenth bit is a trap bit.  It is set whenever the user requests a soft
383
interrupt, and cleared on any return to userspace command.  This allows the
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supervisor, in supervisor mode, to determine whether it got to supervisor
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mode from a trap or from an external interrupt or both.
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These status register bits are summarized in Tbl.~\ref{tbl:ccbits}.
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\begin{table}
389
\begin{center}
390
\begin{tabular}{l|l}
391
Bit & Meaning \\\hline
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9 & Soft trap, set on a trap from user mode, cleared when returning to user mode\\\hline
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8 & (Reserved for) Floating point enable \\\hline
394
7 & Halt on break, to support an external debugger \\\hline
395
6 & Step, single step the CPU in user mode\\\hline
396
5 & GIE, or Global Interrupt Enable \\\hline
397
4 & Sleep \\\hline
398
3 & V, or overflow bit.\\\hline
399
2 & N, or negative bit.\\\hline
400
1 & C, or carry bit.\\\hline
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402
\end{tabular}
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\caption{Condition Code / Status Register Bits}\label{tbl:ccbits}
404
\end{center}\end{table}
405
 
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\section{Conditional Instructions}
407
Most, although not quite all, instructions are conditionally executed.  From
408
the four condition code flags, eight conditions are defined.  These are shown
409
in Tbl.~\ref{tbl:conditions}.
410
\begin{table}
411
\begin{center}
412
\begin{tabular}{l|l|l}
413
Code & Mneumonic & Condition \\\hline
414
3'h0 & None & Always execute the instruction \\
415
3'h1 & {\tt .Z} & Only execute when 'Z' is set \\
416
3'h2 & {\tt .NE} & Only execute when 'Z' is not set \\
417
3'h3 & {\tt .GE} & Greater than or equal ('N' not set, 'Z' irrelevant) \\
418
3'h4 & {\tt .GT} & Greater than ('N' not set, 'Z' not set) \\
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3'h5 & {\tt .LT} & Less than ('N' set) \\
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3'h6 & {\tt .C} & Carry set\\
421
3'h7 & {\tt .V} & Overflow set\\
422
\end{tabular}
423
\caption{Conditions for conditional operand execution}\label{tbl:conditions}
424
\end{center}
425
\end{table}
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There is no condition code for less than or equal, not C or not V.  Sorry,
427
I ran out of space in 3--bits.  Using these conditions will take an extra
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instruction and a pipeline stall.  (Ex: \hbox{\em (Stall)}; \hbox{\tt TST \$4,CC;} \hbox{\tt STO.NZ R0,(R1)})
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430
\section{Operand B}
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Many instruction forms have a 21-bit source ``Operand B'' associated with them.
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This Operand B is either equal to a register plus a signed immediate offset,
433
or an immediate offset by itself.  This value is encoded as shown in
434
Tbl.~\ref{tbl:opb}.
435
\begin{table}\begin{center}
436
\begin{tabular}{|l|l|l|}\hline
437
Bit 20 & 19 \ldots 16 & 15 \ldots 0 \\\hline
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1'b0 & \multicolumn{2}{l|}{20--bit Signed Immediate value} \\\hline
439
1'b1 & 4-bit Register & 16--bit Signed immediate offset \\\hline
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\end{tabular}
441
\caption{Bit allocation for Operand B}\label{tbl:opb}
442
\end{center}\end{table}
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Sixteen and twenty bit immediate values don't make sense for all instructions.
445
For example, what is the point of a 20--bit immediate when executing a 16--bit
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multiply?  Likewise, why have a 16--bit immediate when adding to a logical
447
or arithmetic shift?  In these cases, the extra bits are reserved for future
448
instruction possibilities.
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\section{Address Modes}
451
The ZIP CPU supports two addressing modes: register plus immediate, and
452
immediate address.  Addresses are therefore encoded in the same fashion as
453
Operand B's, shown above.
454
 
455
A lot of long hard thought was put into whether to allow pre/post increment
456
and decrement addressing modes.  Finding no way to use these operators without
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taking two or more clocks per instruction,\footnote{The two clocks figure
458
comes from the design of the register set, allowing only one write per clock.
459
That write is either from the memory unit or the ALU, but never both.} these
460
addressing modes have been
461 21 dgisselq
removed from the realm of possibilities.  This means that the Zip CPU has no
462
native way of executing push, pop, return, or jump to subroutine operations.
463 24 dgisselq
Each of these instructions can be emulated with a set of instructions from the
464
existing set.
465 21 dgisselq
 
466
\section{Move Operands}
467
The previous set of operands would be perfect and complete, save only that
468 24 dgisselq
the CPU needs access to non--supervisory registers while in supervisory mode.
469
Therefore, the MOV instruction is special and offers access to these registers
470
\ldots when in supervisory mode.  To keep the compiler simple, the extra bits
471
are ignored in non-supervisory mode (as though they didn't exist), rather than
472
being mapped to new instructions or additional capabilities.  The bits
473
indicating which register set each register lies within are the A-Usr and
474
B-Usr bits.  When set to a one, these refer to a user mode register.  When set
475
to a zero, these refer to a register in the current mode, whether user or
476
supervisor.  Further, because a load immediate instruction exists, there is no
477
move capability between an immediate and a register: all moves come from either
478
a register or a register plus an offset.
479 21 dgisselq
 
480 24 dgisselq
This actually leads to a bit of a problem: since the MOV instruction encodes
481
which register set each register is coming from or moving to, how shall a
482
compiler or assembler know how to compile a MOV instruction without knowing
483
the mode of the CPU at the time?  For this reason, the compiler will assume
484
all MOV registers are supervisor registers, and display them as normal.
485
Anything with the user bit set will be treated as a user register.  The CPU
486
will quietly ignore the supervisor bits while in user mode, and anything
487
marked as a user register will always be valid.  (Did I just say that in the
488
last paragraph?)
489 21 dgisselq
 
490
\section{Multiply Operations}
491 24 dgisselq
The Zip CPU supports two Multiply operations, a
492 21 dgisselq
16x16 bit signed multiply (MPYS) and the same but unsigned (MPYU).  In both
493
cases, the operand is a register plus a 16-bit immediate, subject to the
494
rule that the register cannot be the PC or CC registers.  The PC register
495
field has been stolen to create a multiply by immediate instruction.  The
496
CC register field is reserved.
497
 
498
\section{Floating Point}
499 32 dgisselq
The ZIP CPU does not support floating point operations.  However, the
500
instruction set reserves two possibilities for future floating point
501
operations.
502 21 dgisselq
 
503 32 dgisselq
The first floating point operation hole in the instruction set involves
504
setting the floating point bit in the CC register.  The next instruction
505
will simply interpret its operands as floating point instructions.
506
Not all instructions, however, have floating point equivalents.  Further, the
507
immediate fields do not apply in floating point mode, and must be set to
508
zero.  Not all instructions make sense as floating point operations.
509
Therefore, only the CMP, SUB, ADD, and MPY instructions may be issued as
510
floating point instructions.  Other instructions allow the examining of the
511
floating point bit in the CC register.  In all cases, the floating point bit
512
is cleared one instruction after it is set.
513 21 dgisselq
 
514 32 dgisselq
The other possibility for floating point operations involves exploiting the
515
hole in the instruction set that the NOOP and BREAK instructions reside within.
516
These two instructions use 24--bits of address space.  A simple adjustment
517
to this space could create instructions with 4--bit register addresses for
518
each register, a 3--bit field for conditional execution, and a 2--bit field
519
for which operation.  In this fashion, such a floating point capability would
520
only fill 13--bits of the 24--bit field, still leaving lots of room for
521
expansion.
522
 
523
In both cases, the Zip CPU would support 32--bit single precision floats
524
only.
525
 
526
The current architecture does not support a floating point not-implemented
527
interrupt.  Any soft floating point emulation must be done deliberately.
528
 
529 21 dgisselq
\section{Native Instructions}
530
The instruction set for the Zip CPU is summarized in
531
Tbl.~\ref{tbl:zip-instructions}.
532
\begin{table}\begin{center}
533
\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
534
Op Code & \multicolumn{8}{c|}{31\ldots24} & \multicolumn{8}{c|}{23\ldots 16}
535
        & \multicolumn{8}{c|}{15\ldots 8} & \multicolumn{8}{c|}{7\ldots 0}
536
        & Sets CC? \\\hline
537
CMP(Sub) & \multicolumn{4}{l|}{4'h0}
538
                & \multicolumn{4}{l|}{D. Reg}
539
                & \multicolumn{3}{l|}{Cond.}
540
                & \multicolumn{21}{l|}{Operand B}
541
                & Yes \\\hline
542 24 dgisselq
TST(And) & \multicolumn{4}{l|}{4'h1}
543 21 dgisselq
                & \multicolumn{4}{l|}{D. Reg}
544
                & \multicolumn{3}{l|}{Cond.}
545
                & \multicolumn{21}{l|}{Operand B}
546
        & Yes \\\hline
547
MOV & \multicolumn{4}{l|}{4'h2}
548
                & \multicolumn{4}{l|}{D. Reg}
549
                & \multicolumn{3}{l|}{Cond.}
550
                & A-Usr
551
                & \multicolumn{4}{l|}{B-Reg}
552
                & B-Usr
553
                & \multicolumn{15}{l|}{15'bit signed offset}
554
                & \\\hline
555
LODI & \multicolumn{4}{l|}{4'h3}
556
                & \multicolumn{4}{l|}{R. Reg}
557
                & \multicolumn{24}{l|}{24'bit Signed Immediate}
558
                & \\\hline
559
NOOP & \multicolumn{4}{l|}{4'h4}
560
                & \multicolumn{4}{l|}{4'he}
561
                & \multicolumn{24}{l|}{24'h00}
562
                & \\\hline
563
BREAK & \multicolumn{4}{l|}{4'h4}
564
                & \multicolumn{4}{l|}{4'he}
565
                & \multicolumn{24}{l|}{24'h01}
566
                & \\\hline
567
{\em Rsrd} & \multicolumn{4}{l|}{4'h4}
568
                & \multicolumn{4}{l|}{4'he}
569
                & \multicolumn{24}{l|}{24'bits, but not 0 or 1.}
570
                & \\\hline
571
LODIHI & \multicolumn{4}{l|}{4'h4}
572
                & \multicolumn{4}{l|}{4'hf}
573
                & \multicolumn{3}{l|}{Cond.}
574
                & 1'b1
575
                & \multicolumn{4}{l|}{R. Reg}
576
                & \multicolumn{16}{l|}{16-bit Immediate}
577
                & \\\hline
578
LODILO & \multicolumn{4}{l|}{4'h4}
579
                & \multicolumn{4}{l|}{4'hf}
580
                & \multicolumn{3}{l|}{Cond.}
581
                & 1'b0
582
                & \multicolumn{4}{l|}{R. Reg}
583
                & \multicolumn{16}{l|}{16-bit Immediate}
584
                & \\\hline
585
16-b MPYU & \multicolumn{4}{l|}{4'h4}
586
                & \multicolumn{4}{l|}{R. Reg}
587
                & \multicolumn{3}{l|}{Cond.}
588
                & 1'b0 & \multicolumn{4}{l|}{Reg}
589
                & \multicolumn{16}{l|}{16-bit Offset}
590
                & Yes \\\hline
591
16-b MPYU(I) & \multicolumn{4}{l|}{4'h4}
592
                & \multicolumn{4}{l|}{R. Reg}
593
                & \multicolumn{3}{l|}{Cond.}
594
                & 1'b0 & \multicolumn{4}{l|}{4'hf}
595
                & \multicolumn{16}{l|}{16-bit Offset}
596
                & Yes \\\hline
597
16-b MPYS & \multicolumn{4}{l|}{4'h4}
598
                & \multicolumn{4}{l|}{R. Reg}
599
                & \multicolumn{3}{l|}{Cond.}
600
                & 1'b1 & \multicolumn{4}{l|}{Reg}
601
                & \multicolumn{16}{l|}{16-bit Offset}
602
                & Yes \\\hline
603
16-b MPYS(I) & \multicolumn{4}{l|}{4'h4}
604
                & \multicolumn{4}{l|}{R. Reg}
605
                & \multicolumn{3}{l|}{Cond.}
606
                & 1'b1 & \multicolumn{4}{l|}{4'hf}
607
                & \multicolumn{16}{l|}{16-bit Offset}
608
                & Yes \\\hline
609
ROL & \multicolumn{4}{l|}{4'h5}
610
                & \multicolumn{4}{l|}{R. Reg}
611
                & \multicolumn{3}{l|}{Cond.}
612
                & \multicolumn{21}{l|}{Operand B, truncated to low order 5 bits}
613
                & \\\hline
614
LOD & \multicolumn{4}{l|}{4'h6}
615
                & \multicolumn{4}{l|}{R. Reg}
616
                & \multicolumn{3}{l|}{Cond.}
617
                & \multicolumn{21}{l|}{Operand B address}
618
                & \\\hline
619
STO & \multicolumn{4}{l|}{4'h7}
620
                & \multicolumn{4}{l|}{D. Reg}
621
                & \multicolumn{3}{l|}{Cond.}
622
                & \multicolumn{21}{l|}{Operand B address}
623
                & \\\hline
624
SUB & \multicolumn{4}{l|}{4'h8}
625
        &       \multicolumn{4}{l|}{R. Reg}
626
        &       \multicolumn{3}{l|}{Cond.}
627 32 dgisselq
        &       \multicolumn{21}{l|}{Operand B}
628 21 dgisselq
        & Yes \\\hline
629
AND & \multicolumn{4}{l|}{4'h9}
630
        &       \multicolumn{4}{l|}{R. Reg}
631
        &       \multicolumn{3}{l|}{Cond.}
632
        &       \multicolumn{21}{l|}{Operand B}
633
        & Yes \\\hline
634
ADD & \multicolumn{4}{l|}{4'ha}
635
        &       \multicolumn{4}{l|}{R. Reg}
636
        &       \multicolumn{3}{l|}{Cond.}
637
        &       \multicolumn{21}{l|}{Operand B}
638
        & Yes \\\hline
639
OR & \multicolumn{4}{l|}{4'hb}
640
        &       \multicolumn{4}{l|}{R. Reg}
641
        &       \multicolumn{3}{l|}{Cond.}
642
        &       \multicolumn{21}{l|}{Operand B}
643
        & Yes \\\hline
644
XOR & \multicolumn{4}{l|}{4'hc}
645
        &       \multicolumn{4}{l|}{R. Reg}
646
        &       \multicolumn{3}{l|}{Cond.}
647
        &       \multicolumn{21}{l|}{Operand B}
648
        & Yes \\\hline
649
LSL/ASL & \multicolumn{4}{l|}{4'hd}
650
        &       \multicolumn{4}{l|}{R. Reg}
651
        &       \multicolumn{3}{l|}{Cond.}
652 33 dgisselq
        &       \multicolumn{21}{l|}{Operand B, imm. truncated to 6 bits}
653 21 dgisselq
        & Yes \\\hline
654
ASR & \multicolumn{4}{l|}{4'he}
655
        &       \multicolumn{4}{l|}{R. Reg}
656
        &       \multicolumn{3}{l|}{Cond.}
657 33 dgisselq
        &       \multicolumn{21}{l|}{Operand B, imm. truncated to 6 bits}
658 21 dgisselq
        & Yes \\\hline
659
LSR & \multicolumn{4}{l|}{4'hf}
660
        &       \multicolumn{4}{l|}{R. Reg}
661
        &       \multicolumn{3}{l|}{Cond.}
662 33 dgisselq
        &       \multicolumn{21}{l|}{Operand B, imm. truncated to 6 bits}
663 21 dgisselq
        & Yes \\\hline
664
\end{tabular}
665
\caption{Zip CPU Instruction Set}\label{tbl:zip-instructions}
666
\end{center}\end{table}
667
 
668
As you can see, there's lots of room for instruction set expansion.  The
669 24 dgisselq
NOOP and BREAK instructions are the only instructions within one particular
670 32 dgisselq
24--bit hole.  This spaces are reserved for future enhancements.  For example,
671
floating point operations, consisting of a 3-bit floating point operation,
672
two 4-bit registers, no immediate offset, and a 3-bit condition would fit
673
nicely into 14--bits of this address space--making it so that the floating
674
point bit in the CC register need not be used.
675 21 dgisselq
 
676
\section{Derived Instructions}
677
The ZIP CPU supports many other common instructions, but not all of them
678 24 dgisselq
are single cycle instructions.  The derived instruction tables,
679 21 dgisselq
Tbls.~\ref{tbl:derived-1}, \ref{tbl:derived-2}, and~\ref{tbl:derived-3},
680
help to capture some of how these other instructions may be implemented on
681
the ZIP CPU.  Many of these instructions will have assembly equivalents,
682
such as the branch instructions, to facilitate working with the CPU.
683
\begin{table}\begin{center}
684
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
685
Mapped & Actual  & Notes \\\hline
686
\parbox[t]{1.4in}{ADD Ra,Rx\\ADDC Rb,Ry}
687
        & \parbox[t]{1.5in}{Add Ra,Rx\\ADD.C \$1,Ry\\Add Rb,Ry}
688
        & Add with carry \\\hline
689
BRA.Cond +/-\$Addr
690 33 dgisselq
        & \hbox{MOV.cond \$Addr+PC,PC}
691 24 dgisselq
        & Branch or jump on condition.  Works for 15--bit
692
                signed address offsets.\\\hline
693 21 dgisselq
BRA.Cond +/-\$Addr
694
        & \parbox[t]{1.5in}{LDI \$Addr,Rx \\ ADD.cond Rx,PC}
695
        & Branch/jump on condition.  Works for
696
        23 bit address offsets, but costs a register, an extra instruction,
697 33 dgisselq
        and sets the flags. \\\hline
698 21 dgisselq
BNC PC+\$Addr
699
        & \parbox[t]{1.5in}{Test \$Carry,CC \\ MOV.Z PC+\$Addr,PC}
700
        & Example of a branch on an unsupported
701
                condition, in this case a branch on not carry \\\hline
702
BUSY & MOV \$-1(PC),PC & Execute an infinite loop \\\hline
703
CLRF.NZ Rx
704
        & XOR.NZ Rx,Rx
705
        & Clear Rx, and flags, if the Z-bit is not set \\\hline
706
CLR Rx
707
        & LDI \$0,Rx
708
        & Clears Rx, leaves flags untouched.  This instruction cannot be
709
                conditional. \\\hline
710
EXCH.W Rx
711
        & ROL \$16,Rx
712
        & Exchanges the top and bottom 16'bit words of Rx \\\hline
713
HALT
714
        & Or \$SLEEP,CC
715
        & Executed while in interrupt mode.  In user mode this is simply a
716 33 dgisselq
        wait until interrupt instruction. \\\hline
717 21 dgisselq
INT & LDI \$0,CC
718
        & Since we're using the CC register as a trap vector as well, this
719
        executes TRAP \#0. \\\hline
720
IRET
721
        & OR \$GIE,CC
722
        & Also an RTU instruction (Return to Userspace) \\\hline
723
JMP R6+\$Addr
724
        & MOV \$Addr(R6),PC
725
        & \\\hline
726
JSR PC+\$Addr
727
        & \parbox[t]{1.5in}{SUB \$1,SP \\\
728
        MOV \$3+PC,R0 \\
729
        STO R0,1(SP) \\
730
        MOV \$Addr+PC,PC \\
731
        ADD \$1,SP}
732 24 dgisselq
        & Jump to Subroutine. Note the required cleanup instruction after
733
        returning. \\\hline
734 21 dgisselq
JSR PC+\$Addr
735
        & \parbox[t]{1.5in}{MOV \$3+PC,R12 \\ MOV \$addr+PC,PC}
736
        &This is the high speed
737
        version of a subroutine call, necessitating a register to hold the
738
        last PC address.  In its favor, this method doesn't suffer the
739
        mandatory memory access of the other approach. \\\hline
740
LDI.l \$val,Rx
741
        & \parbox[t]{1.5in}{LDIHI (\$val$>>$16)\&0x0ffff, Rx \\
742
                        LDILO (\$val \& 0x0ffff)}
743
        & Sadly, there's not enough instruction
744
                space to load a complete immediate value into any register.
745
                Therefore, fully loading any register takes two cycles.
746
                The LDIHI (load immediate high) and LDILO (load immediate low)
747
                instructions have been created to facilitate this. \\\hline
748
\end{tabular}
749
\caption{Derived Instructions}\label{tbl:derived-1}
750
\end{center}\end{table}
751
\begin{table}\begin{center}
752
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
753
Mapped & Actual  & Notes \\\hline
754
LOD.b \$addr,Rx
755
        & \parbox[t]{1.5in}{%
756
        LDI     \$addr,Ra \\
757
        LDI     \$addr,Rb \\
758
        LSR     \$2,Ra \\
759
        AND     \$3,Rb \\
760
        LOD     (Ra),Rx \\
761
        LSL     \$3,Rb \\
762
        SUB     \$32,Rb \\
763
        ROL     Rb,Rx \\
764
        AND \$0ffh,Rx}
765
        & \parbox[t]{3in}{This CPU is designed for 32'bit word
766
        length instructions.  Byte addressing is not supported by the CPU or
767
        the bus, so it therefore takes more work to do.
768
 
769
        Note also that in this example, \$Addr is a byte-wise address, where
770 24 dgisselq
        all other addresses in this document are 32-bit wordlength addresses.
771
        For this reason,
772 21 dgisselq
        we needed to drop the bottom two bits.  This also limits the address
773
        space of character accesses using this method from 16 MB down to 4MB.}
774
                \\\hline
775
\parbox[t]{1.5in}{LSL \$1,Rx\\ LSLC \$1,Ry}
776
        & \parbox[t]{1.5in}{LSL \$1,Ry \\
777
        LSL \$1,Rx \\
778
        OR.C \$1,Ry}
779
        & Logical shift left with carry.  Note that the
780
        instruction order is now backwards, to keep the conditions valid.
781 33 dgisselq
        That is, LSL sets the carry flag, so if we did this the other way
782 21 dgisselq
        with Rx before Ry, then the condition flag wouldn't have been right
783
        for an OR correction at the end. \\\hline
784
\parbox[t]{1.5in}{LSR \$1,Rx \\ LSRC \$1,Ry}
785
        & \parbox[t]{1.5in}{CLR Rz \\
786
        LSR \$1,Ry \\
787
        LDIHI.C \$8000h,Rz \\
788
        LSR \$1,Rx \\
789
        OR Rz,Rx}
790
        & Logical shift right with carry \\\hline
791
NEG Rx & \parbox[t]{1.5in}{XOR \$-1,Rx \\ ADD \$1,Rx} & \\\hline
792
NOOP & NOOP & While there are many
793
        operations that do nothing, such as MOV Rx,Rx, or OR \$0,Rx, these
794
        operations have consequences in that they might stall the bus if
795
        Rx isn't ready yet.  For this reason, we have a dedicated NOOP
796
        instruction. \\\hline
797
NOT Rx & XOR \$-1,Rx & \\\hline
798
POP Rx
799
        & \parbox[t]{1.5in}{LOD \$-1(SP),Rx \\ ADD \$1,SP}
800
        & Note
801
        that for interrupt purposes, one can never depend upon the value at
802
        (SP).  Hence you read from it, then increment it, lest having
803 33 dgisselq
        incremented it first something then comes along and writes to that
804 21 dgisselq
        value before you can read the result. \\\hline
805
PUSH Rx
806 33 dgisselq
        & \parbox[t]{1.5in}{SUB \$1,SP \\
807 21 dgisselq
        STO Rx,\$1(SP)}
808
        & \\\hline
809
RESET
810
        & \parbox[t]{1in}{STO \$1,\$watchdog(R12)\\NOOP\\NOOP}
811
        & \parbox[t]{3in}{This depends upon the peripheral base address being
812
        in R12.
813
 
814
        Another opportunity might be to jump to the reset address from within
815
        supervisor mode.}\\\hline
816 24 dgisselq
RET & \parbox[t]{1.5in}{LOD \$-1(SP),PC}
817
        & Note that this depends upon the calling context to clean up the
818
        stack, as outlined for the JSR instruction.  \\\hline
819 21 dgisselq
\end{tabular}
820
\caption{Derived Instructions, continued}\label{tbl:derived-2}
821
\end{center}\end{table}
822
\begin{table}\begin{center}
823
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
824
RET & MOV R12,PC
825
        & This is the high(er) speed version, that doesn't touch the stack.
826
        As such, it doesn't suffer a stall on memory read/write to the stack.
827
        \\\hline
828
STEP Rr,Rt
829
        & \parbox[t]{1.5in}{LSR \$1,Rr \\ XOR.C Rt,Rr}
830
        & Step a Galois implementation of a Linear Feedback Shift Register, Rr,
831
                using taps Rt \\\hline
832
STO.b Rx,\$addr
833
        & \parbox[t]{1.5in}{%
834
        LDI \$addr,Ra \\
835
        LDI \$addr,Rb \\
836
        LSR \$2,Ra \\
837
        AND \$3,Rb \\
838
        SUB \$32,Rb \\
839
        LOD (Ra),Ry \\
840
        AND \$0ffh,Rx \\
841
        AND \$-0ffh,Ry \\
842
        ROL Rb,Rx \\
843
        OR Rx,Ry \\
844
        STO Ry,(Ra) }
845
        & \parbox[t]{3in}{This CPU and it's bus are {\em not} optimized
846
        for byte-wise operations.
847
 
848
        Note that in this example, \$addr is a
849
        byte-wise address, whereas in all of our other examples it is a
850
        32-bit word address. This also limits the address space
851
        of character accesses from 16 MB down to 4MB.F
852
        Further, this instruction implies a byte ordering,
853
        such as big or little endian.} \\\hline
854
SWAP Rx,Ry
855
        & \parbox[t]{1.5in}{
856
        XOR Ry,Rx \\
857
        XOR Rx,Ry \\
858
        XOR Ry,Rx}
859
        & While no extra registers are needed, this example
860
        does take 3-clocks. \\\hline
861
TRAP \#X
862
        & LDILO \$x,CC
863
        & This approach uses the unused bits of the CC register as a TRAP
864 24 dgisselq
        address.  The user will need to make certain
865 21 dgisselq
        that the SLEEP and GIE bits are not set in \$x.  LDI would also work,
866
        however using LDILO permits the use of conditional traps.  (i.e.,
867
        trap if the zero flag is set.)  Should you wish to trap off of a
868
        register value, you could equivalently load \$x into the register and
869
        then MOV it into the CC register. \\\hline
870
TST Rx
871
        & TST \$-1,Rx
872
        & Set the condition codes based upon Rx.  Could also do a CMP \$0,Rx,
873
        ADD \$0,Rx, SUB \$0,Rx, etc, AND \$-1,Rx, etc.  The TST and CMP
874
        approaches won't stall future pipeline stages looking for the value
875
        of Rx. \\\hline
876
WAIT
877
        & Or \$SLEEP,CC
878
        & Wait 'til interrupt.  In an interrupts disabled context, this
879
        becomes a HALT instruction.
880
\end{tabular}
881
\caption{Derived Instructions, continued}\label{tbl:derived-3}
882
\end{center}\end{table}
883
\iffalse
884
\fi
885
\section{Pipeline Stages}
886 32 dgisselq
As mentioned in the introduction, and highlighted in Fig.~\ref{fig:cpu},
887
the Zip CPU supports a five stage pipeline.
888 21 dgisselq
\begin{enumerate}
889
\item {\bf Prefetch}: Read instruction from memory (cache if possible).  This
890
        stage is actually pipelined itself, and so it will stall if the PC
891
        ever changes.  Stalls are also created here if the instruction isn't
892
        in the prefetch cache.
893
\item {\bf Decode}: Decode instruction into op code, register(s) to read, and
894 32 dgisselq
        immediate offset.  This stage also determines whether the flags will
895
        be set or whether the result will be written back.
896 21 dgisselq
\item {\bf Read Operands}: Read registers and apply any immediate values to
897 24 dgisselq
        them.  There is no means of detecting or flagging arithmetic overflow
898
        or carry when adding the immediate to the operand.  This stage will
899
        stall if any source operand is pending.
900 21 dgisselq
\item Split into two tracks: An {\bf ALU} which will accomplish a simple
901
        instruction, and the {\bf MemOps} stage which accomplishes memory
902
        read/write.
903
        \begin{itemize}
904
        \item Loads stall instructions that access the register until it is
905
                written to the register set.
906
        \item Condition codes are available upon completion
907
        \item Issuing an instruction to the memory while the memory is busy will
908 32 dgisselq
                stall the entire pipeline.  If the bus deadlocks, only a reset
909
                will release the CPU.  (Watchdog timer, anyone?)
910 24 dgisselq
        \item The Zip CPU currently has no means of reading and acting on any
911
        error conditions on the bus.
912 21 dgisselq
        \end{itemize}
913 32 dgisselq
\item {\bf Write-Back}: Conditionally write back the result to the register
914
        set, applying the condition.  This routine is bi-re-entrant: either the
915 21 dgisselq
        memory or the simple instruction may request a register write.
916
\end{enumerate}
917
 
918 24 dgisselq
The Zip CPU does not support out of order execution.  Therefore, if the memory
919
unit stalls, every other instruction stalls.  Memory stores, however, can take
920 32 dgisselq
place concurrently with ALU operations, although memory reads cannot.
921 24 dgisselq
 
922 33 dgisselq
\iffalse
923
 
924 21 dgisselq
\section{Pipeline Logic}
925
How the CPU handles some instruction combinations can be telling when
926
determining what happens in the pipeline.  The following lists some examples:
927
\begin{itemize}
928
\item {\bf Delayed Branching}
929
 
930 33 dgisselq
        I had originally hoped to implement delayed branching.   My goal
931
        was that the compiler would handle any pipeline stall conditions so
932
        that the pipeline logic could be simpler within the CPU.  I ran into
933
        two problems with this.
934 21 dgisselq
 
935 33 dgisselq
        The first problem has to deal with debug mode.  When the debugger
936
        single steps an instruction, that instruction goes to completion.
937
        This means that if the instruction moves a value to the PC register,
938
        the PC register would now contain that value, indicating that the
939
        next instruction would be on the other side of the branch.  There's
940
        just no easy way around this: the entire CPU state must be captured
941
        by the registers, to include the program counter.  What value should
942
        the program counter be equal to?  The branch?  Fine.  The address
943
        you are branching to?  Fine.  The address of the delay slot?  Problem.
944 21 dgisselq
 
945 33 dgisselq
        The second problem with delayed branching is the idea of suspending
946
        processing for an interrupt.  Which address should the CPU return
947
        to upon completing the interrupt processing?  The branch?  Good.  The
948
        address after the branch?  Also good.  The address of the delay slot?
949
        Not so good.
950
 
951
        If you then add into this mess the idea that, if the CPU is running
952
        from a really slow memory such as the flash, the delay slot may never
953
        be filled before the branch is determined, then this makes even less
954
        sense.
955
 
956
        For all of these reasons, this CPU does not support delayed branching.
957
 
958 21 dgisselq
\item {\bf Register Result:} {\tt MOV R0,R1; MOV R1,R2 }
959
 
960 33 dgisselq
        What value does R2 get, the value of R1 before the first move or the
961
        value of R0?  The Zip CPU has been optimized so that neither of these
962
        instructions require a pipeline stall--unless an immediate were to
963
        be added to R1 in the second instruction.
964 21 dgisselq
 
965
        The ZIP CPU architecture requires that R2 must equal R0 at the end of
966 32 dgisselq
        this operation.  Even better, such combinations do not (normally)
967
        stall the pipeline.
968 21 dgisselq
 
969 33 dgisselq
\item {\bf Condition Codes Result:} {\tt CMP R0,R1;} {\tt MOV.EQ \$x,PC}
970 21 dgisselq
 
971
        At issue is the same item as above, save that the CMP instruction
972 33 dgisselq
        updates the flags that the MOV instruction depends upon.
973 21 dgisselq
 
974
        The Zip CPU architecture requires that condition codes must be updated
975
        and available immediately for the next instruction without stalling the
976
        pipeline.
977
 
978
\item {\bf Condition Codes Register Result:} {\tt CMP R0,R1; MOV CC,R2}
979
 
980
        At issue is the
981
        fact that the logic supporting the CC register is more complicated than
982
        the logic supporting any other register.
983
 
984 32 dgisselq
        The ZIP CPU will stall for a cycle cycle on this instruction.
985 33 dgisselq
\item {\bf Condition Codes Register Operand:} {\tt MOV R0,R1; MOV CC,R2}
986 21 dgisselq
 
987 33 dgisselq
        Unlike the previous case, this move prior to reading the {\tt CC}
988
        register does not impact the {\tt CC} register.  Therefore, this
989
        does not stall the bus, whereas the previous one would.
990 21 dgisselq
\end{itemize}
991
 
992
As I've studied  this, I find several approaches to handling pipeline
993
        issues.  These approaches (and their consequences) are listed below.
994
 
995
\begin{itemize}
996 33 dgisselq
\item {\bf All issued instructions complete, stages stall individually}
997 21 dgisselq
 
998
        What about a slow pre-fetch?
999
 
1000
        Nominally, this works well: any issued instruction
1001
        just runs to completion.  If there are four issued instructions in the
1002
        pipeline, with the writeback instruction being a write-to-PC
1003
        instruction, the other three instructions naturally finish.
1004
 
1005
        This approach fails when reading instructions from the flash,
1006
        since such reads require N clocks to clocks to complete.  Thus
1007
        there may be only one instruction in the pipeline if reading from flash,
1008
        or a full pipeline if reading from cache.  Each of these approaches
1009
        would produce a different response.
1010
 
1011 33 dgisselq
        For this reason, the Zip CPU works off of a different basis: All
1012
        instructions that enter either the ALU or the memory unit will
1013
        complete.  Stages still stall individually.
1014
 
1015 21 dgisselq
\item {\bf Issued instructions may be canceled}
1016
 
1017 33 dgisselq
        The problem here is that
1018
        memory operations cannot be canceled: even reads may have side effects
1019 21 dgisselq
        on peripherals that cannot be canceled later.  Further, in the case of
1020
        an interrupt, it's difficult to know what to cancel.  What happens in
1021
        a \hbox{\tt MOV.C \$x,PC} followed by a \hbox{\tt MOV \$y,PC}
1022 33 dgisselq
        instruction?  Which get canceled?
1023 21 dgisselq
 
1024 33 dgisselq
        Because it isn't clear what would need to be canceled, the Zip CPU
1025
        will not permit this combination.  A MOV to the PC register will be
1026
        followed by a stall, and possibly many stalls, so that the second
1027
        move to PC will never be executed.
1028 21 dgisselq
 
1029
\item {\bf All issued instructions complete.}
1030
 
1031 33 dgisselq
        In this example, we try all issued instructions complete, but the
1032
        entire pipeline stalls if one stage is not filled.  In this approach,
1033
        though, we again struggle with the problems associated with
1034
        delayed branching.  Upon attempting to restart the processor, where
1035
        do you restart it from?
1036 21 dgisselq
 
1037
\item {\bf Memory instructions must complete}
1038
 
1039 32 dgisselq
        All instructions that enter into the memory module {\em must}
1040 21 dgisselq
        complete.  Issued instructions from the prefetch, decode, or operand
1041
        read stages may or may not complete.  Jumps into code must be valid,
1042
        so that interrupt returns may be valid.  All instructions entering the
1043
        ALU complete.
1044
 
1045
        This looks to be the simplest approach.
1046
        While the logic may be difficult, this appears to be the only
1047
        re-entrant approach.
1048
 
1049
        A {\tt new\_pc} flag will be high anytime the PC changes in an
1050
        unpredictable way (i.e., it doesn't increment).  This includes jumps
1051
        as well as interrupts and interrupt returns.  Whenever this flag may
1052
        go high, memory operations and ALU operations will stall until the
1053
        result is known.  When the flag does go high, anything in the prefetch,
1054
        decode, and read-op stage will be invalidated.
1055
 
1056
\end{itemize}
1057 33 dgisselq
\fi
1058 21 dgisselq
 
1059 32 dgisselq
\section{Pipeline Stalls}
1060
The processing pipeline can and will stall for a variety of reasons.  Some of
1061
these are obvious, some less so.  These reasons are listed below:
1062
\begin{itemize}
1063
\item When the prefetch cache is exhausted
1064 21 dgisselq
 
1065 32 dgisselq
This should be obvious.  If the prefetch cache doesn't have the instruction
1066
in memory, the entire pipeline must stall until enough of the prefetch cache
1067
is loaded to support the next instruction.
1068 21 dgisselq
 
1069 32 dgisselq
\item While waiting for the pipeline to load following any taken branch, jump,
1070
        return from interrupt or switch to interrupt context (6 clocks)
1071
 
1072
If the PC suddenly changes, the pipeline is subsequently cleared and needs to
1073
be reloaded.  Given that there are five stages to the pipeline, that accounts
1074
for five of the six delay clocks.  The last clock is lost in the prefetch
1075
stage which needs at least one clock with a valid PC before it can produce
1076
a new output.  Hence, six clocks will always be lost anytime the pipeline needs
1077
to be cleared.
1078
 
1079
\item When reading from a prior register while also adding an immediate offset
1080
\begin{enumerate}
1081
\item\ {\tt OPCODE ?,RA}
1082
\item\ {\em (stall)}
1083
\item\ {\tt OPCODE I+RA,RB}
1084
\end{enumerate}
1085
 
1086
Since the addition of the immediate register within OpB decoding gets applied
1087
during the read operand stage so that it can be nicely settled before the ALU,
1088
any instruction that will write back an operand must be separated from the
1089
opcode that will read and apply an immediate offset by one instruction.  The
1090
good news is that this stall can easily be mitigated by proper scheduling.
1091
 
1092
\item When writing to the CC or PC Register
1093
\begin{enumerate}
1094
\item\ {\tt OPCODE RA,PC} {\em Ex: a branch opcode}
1095
\item\ {\em (stall, even if jump not taken)}
1096
\item\ {\tt OPCODE RA,RB}
1097
\end{enumerate}
1098
Since branches take place in the writeback stage, the Zip CPU will stall the
1099
pipeline for one clock anytime there may be a possible jump.  This prevents
1100
an instruction from executing a memory access after the jump but before the
1101
jump is recognized.
1102
 
1103 33 dgisselq
This stall cannot be mitigated through scheduling.
1104
 
1105 32 dgisselq
\item When reading from the CC register after setting the flags
1106
\begin{enumerate}
1107
\item\ {\tt ALUOP RA,RB}
1108
\item\ {\em (stall}
1109
\item\ {\tt TST sys.ccv,CC}
1110
\item\ {\tt BZ somewhere}
1111
\end{enumerate}
1112
 
1113
The reason for this stall is simply performance.  Many of the flags are
1114
determined via combinatorial logic after the writeback instruction is
1115
determined.  Trying to then place these into the input for one of the operands
1116
created a time delay loop that would no longer execute in a single 100~MHz
1117
clock cycle.  (The time delay of the multiply within the ALU wasn't helping
1118
either \ldots).
1119
 
1120 33 dgisselq
This stall may be eliminated via proper scheduling, by placing an instruction
1121
that does not set flags in between the ALU operation and the instruction
1122
that references the CC register.  For example, {\tt MOV \$addr+PC,uPC}
1123
followed by an {\tt RTU} ({\tt OR \$GIE,CC}) instruction will not incur
1124
this stall, whereas an {\tt OR \$BREAKEN,CC} followed by an {\tt OR \$STEP,CC}
1125
will incur the stall.
1126
 
1127 32 dgisselq
\item When waiting for a memory read operation to complete
1128
\begin{enumerate}
1129
\item\ {\tt LOD address,RA}
1130
\item\ {\em (multiple stalls, bus dependent, 7 clocks best)}
1131
\item\ {\tt OPCODE I+RA,RB}
1132
\end{enumerate}
1133
 
1134
Remember, the ZIP CPU does not support out of order execution.  Therefore,
1135
anytime the memory unit becomes busy both the memory unit and the ALU must
1136
stall until the memory unit is cleared.  This is especially true of a load
1137 33 dgisselq
instruction, which must still write its operand back to the register file.
1138
Store instructions are different, since they can be busy with no impact on
1139
later ALU write back operations.  Hence, only loads stall the pipeline.
1140 32 dgisselq
 
1141
This also assumes that the memory being accessed is a single cycle memory.
1142
Slower memories, such as the Quad SPI flash, will take longer--perhaps even
1143 33 dgisselq
as long as forty clocks.   During this time the CPU and the external bus
1144 32 dgisselq
will be busy, and unable to do anything else.
1145
 
1146
\item Memory operation followed by a memory operation
1147
\begin{enumerate}
1148
\item\ {\tt STO address,RA}
1149
\item\ {\em (multiple stalls, bus dependent, 7 clocks best)}
1150
\item\ {\tt LOD address,RB}
1151
\item\ {\em (multiple stalls, bus dependent, 7 clocks best)}
1152
\end{enumerate}
1153
 
1154
In this case, the LOD instruction cannot start until the STALL is finished.
1155
With proper scheduling, it is possible to do something in the ALU while the
1156
STO is busy, but otherwise this pipeline will stall waiting for it to complete.
1157
 
1158
Note that even though the Wishbone bus can support pipelined accesses at
1159
one access per clock, only the prefetch stage can take advantage of this.
1160
Load and Store instructions are stuck at one wishbone cycle per instruction.
1161
\end{itemize}
1162
 
1163
 
1164 21 dgisselq
\chapter{Peripherals}\label{chap:periph}
1165 24 dgisselq
 
1166
While the previous chapter describes a CPU in isolation, the Zip System
1167
includes a minimum set of peripherals as well.  These peripherals are shown
1168
in Fig.~\ref{fig:zipsystem}
1169
\begin{figure}\begin{center}
1170
\includegraphics[width=3.5in]{../gfx/system.eps}
1171
\caption{Zip System Peripherals}\label{fig:zipsystem}
1172
\end{center}\end{figure}
1173
and described here.  They are designed to make
1174
the Zip CPU more useful in an Embedded Operating System environment.
1175
 
1176 21 dgisselq
\section{Interrupt Controller}
1177 24 dgisselq
 
1178
Perhaps the most important peripheral within the Zip System is the interrupt
1179
controller.  While the Zip CPU itself can only handle one interrupt, and has
1180
only the one interrupt state: disabled or enabled, the interrupt controller
1181
can make things more interesting.
1182
 
1183
The Zip System interrupt controller module supports up to 15 interrupts, all
1184
controlled from one register.  Bit~31 of the interrupt controller controls
1185
overall whether interrupts are enabled (1'b1) or disabled (1'b0).  Bits~16--30
1186
control whether individual interrupts are enabled (1'b0) or disabled (1'b0).
1187
Bit~15 is an indicator showing whether or not any interrupt is active, and
1188
bits~0--15 indicate whether or not an individual interrupt is active.
1189
 
1190
The interrupt controller has been designed so that bits can be controlled
1191
individually without having any knowledge of the rest of the controller
1192
setting.  To enable an interrupt, write to the register with the high order
1193
global enable bit set and the respective interrupt enable bit set.  No other
1194
bits will be affected.  To disable an interrupt, write to the register with
1195
the high order global enable bit cleared and the respective interrupt enable
1196
bit set.  To clear an interrupt, write a `1' to that interrupts status pin.
1197
Zero's written to the register have no affect, save that a zero written to the
1198
master enable will disable all interrupts.
1199
 
1200
As an example, suppose you wished to enable interrupt \#4.  You would then
1201
write to the register a {\tt 0x80100010} to enable interrupt \#4 and to clear
1202
any past active state.  When you later wish to disable this interrupt, you would
1203
write a {\tt 0x00100010} to the register.  As before, this both disables the
1204
interrupt and clears the active indicator.  This also has the side effect of
1205
disabling all interrupts, so a second write of {\tt 0x80000000} may be necessary
1206
to re-enable any other interrupts.
1207
 
1208
The Zip System currently hosts two interrupt controllers, a primary and a
1209
secondary.  The primary interrupt controller has one interrupt line which may
1210
come from an external interrupt controller, and one interrupt line from the
1211
secondary controller.  Other primary interrupts include the system timers,
1212
the jiffies interrupt, and the manual cache interrupt.  The secondary interrupt
1213
controller maintains an interrupt state for all of the processor accounting
1214
counters.
1215
 
1216 21 dgisselq
\section{Counter}
1217
 
1218
The Zip Counter is a very simple counter: it just counts.  It cannot be
1219
halted.  When it rolls over, it issues an interrupt.  Writing a value to the
1220
counter just sets the current value, and it starts counting again from that
1221
value.
1222
 
1223
Eight counters are implemented in the Zip System for process accounting.
1224
This may change in the future, as nothing as yet uses these counters.
1225
 
1226
\section{Timer}
1227
 
1228
The Zip Timer is also very simple: it simply counts down to zero.  When it
1229
transitions from a one to a zero it creates an interrupt.
1230
 
1231
Writing any non-zero value to the timer starts the timer.  If the high order
1232
bit is set when writing to the timer, the timer becomes an interval timer and
1233
reloads its last start time on any interrupt.  Hence, to mark seconds, one
1234
might set the timer to 100~million (the number of clocks per second), and
1235
set the high bit.  Ever after, the timer will interrupt the CPU once per
1236 24 dgisselq
second (assuming a 100~MHz clock).  This reload capability also limits the
1237
maximum timer value to $2^{31}-1$, rather than $2^{32}-1$.
1238 21 dgisselq
 
1239
\section{Watchdog Timer}
1240
 
1241
The watchdog timer is no different from any of the other timers, save for one
1242
critical difference: the interrupt line from the watchdog
1243
timer is tied to the reset line of the CPU.  Hence writing a `1' to the
1244
watchdog timer will always reset the CPU.
1245 32 dgisselq
To stop the Watchdog timer, write a `0' to it.  To start it,
1246 21 dgisselq
write any other number to it---as with the other timers.
1247
 
1248
While the watchdog timer supports interval mode, it doesn't make as much sense
1249
as it did with the other timers.
1250
 
1251
\section{Jiffies}
1252
 
1253
This peripheral is motivated by the Linux use of `jiffies' whereby a process
1254
can request to be put to sleep until a certain number of `jiffies' have
1255
elapsed.  Using this interface, the CPU can read the number of `jiffies'
1256
from the peripheral (it only has the one location in address space), add the
1257 24 dgisselq
sleep length to it, and write the result back to the peripheral.  The zipjiffies
1258 21 dgisselq
peripheral will record the value written to it only if it is nearer the current
1259
counter value than the last current waiting interrupt time.  If no other
1260
interrupts are waiting, and this time is in the future, it will be enabled.
1261
(There is currently no way to disable a jiffie interrupt once set, other
1262 24 dgisselq
than to disable the interrupt line in the interrupt controller.)  The processor
1263 21 dgisselq
may then place this sleep request into a list among other sleep requests.
1264
Once the timer expires, it would write the next Jiffy request to the peripheral
1265
and wake up the process whose timer had expired.
1266
 
1267
Indeed, the Jiffies register is nothing more than a glorified counter with
1268
an interrupt.  Unlike the other counters, the Jiffies register cannot be set.
1269
Writes to the jiffies register create an interrupt time.  When the Jiffies
1270
register later equals the value written to it, an interrupt will be asserted
1271
and the register then continues counting as though no interrupt had taken
1272
place.
1273
 
1274
The purpose of this register is to support alarm times within a CPU.  To
1275
set an alarm for a particular process $N$ clocks in advance, read the current
1276
Jiffies value, and $N$, and write it back to the Jiffies register.  The
1277
O/S must also keep track of values written to the Jiffies register.  Thus,
1278 32 dgisselq
when an `alarm' trips, it should be removed from the list of alarms, the list
1279 21 dgisselq
should be sorted, and the next alarm in terms of Jiffies should be written
1280
to the register.
1281
 
1282 24 dgisselq
\section{Manual Cache}
1283
 
1284
The manual cache is an experimental setting that may not remain with the Zip
1285
CPU for very long.  It is designed to facilitate running from FLASH or ROM
1286 32 dgisselq
memory, although the pipeline prefetch cache really makes this need obsolete.
1287
The manual
1288 24 dgisselq
cache works by copying data from a wishbone address (range) into the cache
1289
register, and then by making that memory available as memory to the Zip System.
1290
It is a {\em manual cache} because the processor must first specify what
1291
memory to copy, and then once copied the processor can only access the cache
1292
memory by the cache memory location.  There is no transparency.  It is perhaps
1293
best described as a combination DMA controller and local memory.
1294
 
1295
Worse, this cache is likely going to be removed from the ZipSystem.  Having used
1296
the ZipSystem now for some time, I have yet to find a need or use for the manual
1297
cache.  I will likely replace this peripheral with a proper DMA controller.
1298
 
1299 21 dgisselq
\chapter{Operation}\label{chap:ops}
1300
 
1301 33 dgisselq
The Zip CPU, and even the Zip System, is not a System on a Chip (SoC).  It
1302
needs to be connected to its operational environment in order to be used.
1303
Specifically, some per system adjustments need to be made:
1304
\begin{enumerate}
1305
\item The Zip System depends upon an external 32-bit Wishbone bus.  This
1306
        must exist, and must be connected to the Zip CPU for it to work.
1307
\item The Zip System needs to be told of its {\tt RESET\_ADDRESS}.  This is
1308
        the program counter of the first instruction following a reset.
1309
\item If you want the Zip System to start up on its own, you will need to
1310
        set the {\tt START\_HALTED} parameter to zero.  Otherwise, if you
1311
        wish to manually start the CPU, that is if upon reset you want the
1312
        CPU start start in its halted, reset state, then set this parameter to
1313
        one.
1314
\item The third parameter to set is the number of interrupts you will be
1315
        providing from external to the CPU.  This can be anything from one
1316
        to nine, but it cannot be zero.  (Wire this line to a 1'b0 if you
1317
        do not wish to support any external interrupts.)
1318
\item Finally, you need to place into some wishbone accessible address, whether
1319
        RAM or (more likely) ROM, the initial instructions for the CPU.
1320
\end{enumerate}
1321
If you have enabled your CPU to start automatically, then upon power up the
1322
CPU will immediately start executing your instructions.
1323
 
1324
This is, however, not how I have used the Zip CPU.  I have instead used the
1325
ZIP CPU in a more controlled environment.  For me, the CPU starts in a
1326
halted state, and waits to be told to start.  Further, the RESET address is a
1327
location in RAM.  After bringing up the board I am using, and further the
1328
bus that is on it, the RAM memory is then loaded externally with the program
1329
I wish the Zip System to run.  Once the RAM is loaded, I release the CPU.
1330
The CPU then runs until its halt condition, at which point its task is
1331
complete.
1332
 
1333
Eventually, I intend to place an operating system onto the ZipSystem, I'm
1334
just not there yet.
1335
 
1336
 
1337 21 dgisselq
\chapter{Registers}\label{chap:regs}
1338
 
1339 24 dgisselq
The ZipSystem registers fall into two categories, ZipSystem internal registers
1340
accessed via the ZipCPU shown in Tbl.~\ref{tbl:zpregs},
1341
\begin{table}[htbp]
1342
\begin{center}\begin{reglist}
1343 32 dgisselq
PIC   & \scalebox{0.8}{\tt 0xc0000000} & 32 & R/W & Primary Interrupt Controller \\\hline
1344
WDT   & \scalebox{0.8}{\tt 0xc0000001} & 32 & R/W & Watchdog Timer \\\hline
1345
CCHE  & \scalebox{0.8}{\tt 0xc0000002} & 32 & R/W & Manual Cache Controller \\\hline
1346
CTRIC & \scalebox{0.8}{\tt 0xc0000003} & 32 & R/W & Secondary Interrupt Controller \\\hline
1347
TMRA  & \scalebox{0.8}{\tt 0xc0000004} & 32 & R/W & Timer A\\\hline
1348
TMRB  & \scalebox{0.8}{\tt 0xc0000005} & 32 & R/W & Timer B\\\hline
1349
TMRC  & \scalebox{0.8}{\tt 0xc0000006} & 32 & R/W & Timer C\\\hline
1350
JIFF  & \scalebox{0.8}{\tt 0xc0000007} & 32 & R/W & Jiffies \\\hline
1351
MTASK  & \scalebox{0.8}{\tt 0xc0000008} & 32 & R/W & Master Task Clock Counter \\\hline
1352
MMSTL  & \scalebox{0.8}{\tt 0xc0000009} & 32 & R/W & Master Stall Counter \\\hline
1353
MPSTL  & \scalebox{0.8}{\tt 0xc000000a} & 32 & R/W & Master Pre--Fetch Stall Counter \\\hline
1354
MICNT  & \scalebox{0.8}{\tt 0xc000000b} & 32 & R/W & Master Instruction Counter\\\hline
1355
UTASK  & \scalebox{0.8}{\tt 0xc000000c} & 32 & R/W & User Task Clock Counter \\\hline
1356
UMSTL  & \scalebox{0.8}{\tt 0xc000000d} & 32 & R/W & User Stall Counter \\\hline
1357
UPSTL  & \scalebox{0.8}{\tt 0xc000000e} & 32 & R/W & User Pre--Fetch Stall Counter \\\hline
1358
UICNT  & \scalebox{0.8}{\tt 0xc000000f} & 32 & R/W & User Instruction Counter\\\hline
1359
% Cache  & \scalebox{0.8}{\tt 0xc0100000} & & & Base address of the Cache memory\\\hline
1360 24 dgisselq
\end{reglist}
1361
\caption{Zip System Internal/Peripheral Registers}\label{tbl:zpregs}
1362
\end{center}\end{table}
1363 33 dgisselq
and the two debug registers shown in Tbl.~\ref{tbl:dbgregs}.
1364 24 dgisselq
\begin{table}[htbp]
1365
\begin{center}\begin{reglist}
1366
ZIPCTRL & 0 & 32 & R/W & Debug Control Register \\\hline
1367
ZIPDATA & 1 & 32 & R/W & Debug Data Register \\\hline
1368
\end{reglist}
1369
\caption{Zip System Debug Registers}\label{tbl:dbgregs}
1370
\end{center}\end{table}
1371
 
1372 33 dgisselq
\section{Peripheral Registers}
1373
The peripheral registers, listed in Tbl.~\ref{tbl:zpregs}, are shown in the
1374
CPU's address space.  These may be accessed by the CPU at these addresses,
1375
and when so accessed will respond as described in Chapt.~\ref{chap:periph}.
1376
These registers will be discussed briefly again here.
1377 24 dgisselq
 
1378 33 dgisselq
The Zip CPU Interrupt controller has four different types of bits, as shown in
1379
Tbl.~\ref{tbl:picbits}.
1380
\begin{table}\begin{center}
1381
\begin{bitlist}
1382
31 & R/W & Master Interrupt Enable\\\hline
1383
30\ldots 16 & R/W & Interrupt Enables, write '1' to change\\\hline
1384
15 & R & Current Master Interrupt State\\\hline
1385
15\ldots 0 & R/W & Input Interrupt states, write '1' to clear\\\hline
1386
\end{bitlist}
1387
\caption{Interrupt Controller Register Bits}\label{tbl:picbits}
1388
\end{center}\end{table}
1389
The high order bit, or bit--31, is the master interrupt enable bit.  When this
1390
bit is set, then any time an interrupt occurs the CPU will be interrupted and
1391
will switch to supervisor mode, etc.
1392
 
1393
Bits 30~\ldots 16 are interrupt enable bits.  Should the interrupt line go
1394
ghile while enabled, an interrupt will be generated.  To set an interrupt enable
1395
bit, one needs to write the master interrupt enable while writing a `1' to this
1396
the bit.  To clear, one need only write a `0' to the master interrupt enable,
1397
while leaving this line high.
1398
 
1399
Bits 15\ldots 0 are the current state of the interrupt vector.  Interrupt lines
1400
trip when they go high, and remain tripped until they are acknowledged.  If
1401
the interrupt goes high for longer than one pulse, it may be high when a clear
1402
is requested.  If so, the interrupt will not clear.  The line must go low
1403
again before the status bit can be cleared.
1404
 
1405
As an example, consider the following scenario where the Zip CPU supports four
1406
interrupts, 3\ldots0.
1407
\begin{enumerate}
1408
\item The Supervisor will first, while in the interrupts disabled mode,
1409
        write a {\tt 32'h800f000f} to the controller.  The supervisor may then
1410
        switch to the user state with interrupts enabled.
1411
\item When an interrupt occurs, the supervisor will switch to the interrupt
1412
        state.  It will then cycle through the interrupt bits to learn which
1413
        interrupt handler to call.
1414
\item If the interrupt handler expects more interrupts, it will clear its
1415
        current interrupt when it is done handling the interrupt in question.
1416
        To do this, it will write a '1' to the low order interrupt mask,
1417
        such as writing a {\tt 32'h80000001}.
1418
\item If the interrupt handler does not expect any more interrupts, it will
1419
        instead clear the interrupt from the controller by writing a
1420
        {\tt 32'h00010001} to the controller.
1421
\item Once all interrupts have been handled, the supervisor will write a
1422
        {\tt 32'h80000000} to the interrupt register to re-enable interrupt
1423
        generation.
1424
\item The supervisor should also check the user trap bit, and possible soft
1425
        interrupt bits here, but this action has nothing to do with the
1426
        interrupt control register.
1427
\item The supervisor will then leave interrupt mode, possibly adjusting
1428
        whichever task is running, by executing a return from interrupt
1429
        command.
1430
\end{enumerate}
1431
 
1432
Leaving the interrupt controller, we show the timer registers bit definitions
1433
in Tbl.~\ref{tbl:tmrbits}.
1434
\begin{table}\begin{center}
1435
\begin{bitlist}
1436
31 & R/W & Auto-Reload\\\hline
1437
30\ldots 0 & R/W & Current timer value\\\hline
1438
\end{bitlist}
1439
\caption{Timer Register Bits}\label{tbl:tmrbits}
1440
\end{center}\end{table}
1441
As you may recall, the timer just counts down to zero and then trips an
1442
interrupt.  Writing to the current timer value sets that value, and reading
1443
from it returns that value.  Writing to the current timer value while also
1444
setting the auto--reload bit will send the timer into an auto--reload mode.
1445
In this mode, upon setting its interrupt bit for one cycle, the timer will
1446
also reset itself back to the value of the timer that was written to it when
1447
the auto--reload option was written to it.  To clear and stop the timer,
1448
just simply write a `32'h00' to this register.
1449
 
1450
The Jiffies register is somewhat similar in that the register always changes.
1451
In this case, the register counts up, whereas the timer always counted down.
1452
Reads from this register, as shown in Tbl.~\ref{tbl:jiffybits},
1453
\begin{table}\begin{center}
1454
\begin{bitlist}
1455
31\ldots 0 & R & Current jiffy value\\\hline
1456
31\ldots 0 & W & Value/time of next interrupt\\\hline
1457
\end{bitlist}
1458
\caption{Jiffies Register Bits}\label{tbl:jiffybits}
1459
\end{center}\end{table}
1460
always return the time value contained in the register.  Writes greater than
1461
the current Jiffy value, that is where the new value minus the old value is
1462
greater than zero while ignoring truncation, will set a new Jiffy interrupt
1463
time.  At that time, the Jiffy vector will clear, and another interrupt time
1464
may either be written to it, or it will just continue counting without
1465
activating any more interrupts.
1466
 
1467
The Zip CPU also supports several counter peripherals, mostly in the way of
1468
process accounting.  This peripherals have a single register associated with
1469
them, shown in Tbl.~\ref{tbl:ctrbits}.
1470
\begin{table}\begin{center}
1471
\begin{bitlist}
1472
31\ldots 0 & R/W & Current counter value\\\hline
1473
\end{bitlist}
1474
\caption{Counter Register Bits}\label{tbl:ctrbits}
1475
\end{center}\end{table}
1476
Writes to this register set the new counter value.  Reads read the current
1477
counter value.
1478
 
1479
The current design operation of these counters is that of performance counting.
1480
Two sets of four registers are available for keeping track of performance.
1481
The first is a task counter.  This just counts clock ticks.  The second
1482
counter is a prefetch stall counter, then an master stall counter.  These
1483
allow the CPU to be evaluated as to how efficient it is.  The fourth and
1484
final counter is an instruction counter, which counts how many instructions the
1485
CPU has issued.
1486
 
1487
It is envisioned that these counters will be used as follows: First, every time
1488
a master counter rolls over, the supervisor (Operating System) will record
1489
the fact.  Second, whenever activating a user task, the Operating System will
1490
set the four user counters to zero.  When the user task has completed, the
1491
Operating System will read the timers back off, to determine how much of the
1492
CPU the process had consumed.
1493
 
1494
\section{Debug Port Registers}
1495
Accessing the Zip System via the debug port isn't as straight forward as
1496
accessing the system via the wishbone bus.  The debug port itself has been
1497
reduced to two addresses, as outlined earlier in Tbl.~\ref{tbl:dbgregs}.
1498
Access to the Zip System begins with the Debug Control register, shown in
1499
Tbl.~\ref{tbl:dbgctrl}.
1500
\begin{table}\begin{center}
1501
\begin{bitlist}
1502
31\ldots 14 & R & Reserved\\\hline
1503
13 & R & CPU GIE setting\\\hline
1504
12 & R & CPU is sleeping\\\hline
1505
11 & W & Command clear PF cache\\\hline
1506
10 & R/W & Command HALT, Set to '1' to halt the CPU\\\hline
1507
9 & R & Stall Status, '1' if CPU is busy\\\hline
1508
8 & R/W & Step Command, set to '1' to step the CPU\\\hline
1509
7 & R & Interrupt Request \\\hline
1510
6 & R/W & Command RESET \\\hline
1511
5\ldots 0 & R/W & Debug Register Address \\\hline
1512
\end{bitlist}
1513
\caption{Debug Control Register Bits}\label{tbl:dbgctrl}
1514
\end{center}\end{table}
1515
 
1516
The first step in debugging access is to determine whether or not the CPU
1517
is halted, and to halt it if not.  To do this, first write a '1' to the
1518
Command HALT bit.  This will halt the CPU and place it into debug mode.
1519
Once the CPU is halted, the stall status bit will drop to zero.  Thus,
1520
if bit 10 is high and bit 9 low, the debug port is open to examine the
1521
internal state of the CPU.
1522
 
1523
At this point, the external debugger may examine internal state information
1524
from within the CPU.  To do this, first write again to the command register
1525
a value (with command halt still high) containing the address of an internal
1526
register of interest in the bottom 6~bits.  Internal registers that may be
1527
accessed this way are listed in Tbl.~\ref{tbl:dbgaddrs}.
1528
\begin{table}\begin{center}
1529
\begin{reglist}
1530
sR0 & 0 & 32 & R/W & Supervisor Register R0 \\\hline
1531
sR1 & 0 & 32 & R/W & Supervisor Register R1 \\\hline
1532
sSP & 13 & 32 & R/W & Supervisor Stack Pointer\\\hline
1533
sCC & 14 & 32 & R/W & Supervisor Condition Code Register \\\hline
1534
sPC & 15 & 32 & R/W & Supervisor Program Counter\\\hline
1535
uR0 & 16 & 32 & R/W & User Register R0 \\\hline
1536
uR1 & 17 & 32 & R/W & User Register R1 \\\hline
1537
uSP & 29 & 32 & R/W & User Stack Pointer\\\hline
1538
uCC & 30 & 32 & R/W & User Condition Code Register \\\hline
1539
uPC & 31 & 32 & R/W & User Program Counter\\\hline
1540
PIC & 32 & 32 & R/W & Primary Interrupt Controller \\\hline
1541
WDT & 33 & 32 & R/W & Watchdog Timer\\\hline
1542
CCHE & 34 & 32 & R/W & Manual Cache Controller\\\hline
1543
CTRIC & 35 & 32 & R/W & Secondary Interrupt Controller\\\hline
1544
TMRA & 36 & 32 & R/W & Timer A\\\hline
1545
TMRB & 37 & 32 & R/W & Timer B\\\hline
1546
TMRC & 38 & 32 & R/W & Timer C\\\hline
1547
JIFF & 39 & 32 & R/W & Jiffies peripheral\\\hline
1548
MTASK & 40 & 32 & R/W & Master task clock counter\\\hline
1549
MMSTL & 41 & 32 & R/W & Master memory stall counter\\\hline
1550
MPSTL & 42 & 32 & R/W & Master Pre-Fetch Stall counter\\\hline
1551
MICNT & 43 & 32 & R/W & Master instruction counter\\\hline
1552
UTASK & 44 & 32 & R/W & User task clock counter\\\hline
1553
UMSTL & 45 & 32 & R/W & User memory stall counter\\\hline
1554
UPSTL & 46 & 32 & R/W & User Pre-Fetch Stall counter\\\hline
1555
UICNT & 47 & 32 & R/W & User instruction counter\\\hline
1556
\end{reglist}
1557
\caption{Debug Register Addresses}\label{tbl:dbgaddrs}
1558
\end{center}\end{table}
1559
Primarily, these ``registers'' include access to the entire CPU register
1560
set, as well as the 16~internal peripherals.  To read one of these registers
1561
once the address is set, simply issue a read from the data port.  To write
1562
one of these registers or peripheral ports, simply write to the data port
1563
after setting the proper address.
1564
 
1565
In this manner, all of the CPU's internal state may be read and adjusted.
1566
 
1567
As an example of how to use this, consider what would happen in the case
1568
of an external break point.  If and when the CPU hits a break point that
1569
causes it to halt, the Command HALT bit will activate on its own, the CPU
1570
will then raise an external interrupt line and wait for a debugger to examine
1571
its state.  After examining the state, the debugger will need to remove
1572
the breakpoint by writing a different instruction into memory and by writing
1573
to the command register while holding the clear cache, command halt, and
1574
step CPU bits high, (32'hd00).  The debugger may then replace the breakpoint
1575
now that the CPU has gone beyond it, and clear the cache again (32'h500).
1576
 
1577
To leave this debug mode, simply write a `32'h0' value to the command register.
1578
 
1579
\chapter{Wishbone Datasheets}\label{chap:wishbone}
1580 32 dgisselq
The Zip System supports two wishbone ports, a slave debug port and a master
1581 21 dgisselq
port for the system itself.  These are shown in Tbl.~\ref{tbl:wishbone-slave}
1582
\begin{table}[htbp]
1583
\begin{center}
1584
\begin{wishboneds}
1585
Revision level of wishbone & WB B4 spec \\\hline
1586
Type of interface & Slave, Read/Write, single words only \\\hline
1587 24 dgisselq
Address Width & 1--bit \\\hline
1588 21 dgisselq
Port size & 32--bit \\\hline
1589
Port granularity & 32--bit \\\hline
1590
Maximum Operand Size & 32--bit \\\hline
1591
Data transfer ordering & (Irrelevant) \\\hline
1592
Clock constraints & Works at 100~MHz on a Basys--3 board\\\hline
1593
Signal Names & \begin{tabular}{ll}
1594
                Signal Name & Wishbone Equivalent \\\hline
1595
                {\tt i\_clk} & {\tt CLK\_I} \\
1596
                {\tt i\_dbg\_cyc} & {\tt CYC\_I} \\
1597
                {\tt i\_dbg\_stb} & {\tt STB\_I} \\
1598
                {\tt i\_dbg\_we} & {\tt WE\_I} \\
1599
                {\tt i\_dbg\_addr} & {\tt ADR\_I} \\
1600
                {\tt i\_dbg\_data} & {\tt DAT\_I} \\
1601
                {\tt o\_dbg\_ack} & {\tt ACK\_O} \\
1602
                {\tt o\_dbg\_stall} & {\tt STALL\_O} \\
1603
                {\tt o\_dbg\_data} & {\tt DAT\_O}
1604
                \end{tabular}\\\hline
1605
\end{wishboneds}
1606 22 dgisselq
\caption{Wishbone Datasheet for the Debug Interface}\label{tbl:wishbone-slave}
1607 21 dgisselq
\end{center}\end{table}
1608
and Tbl.~\ref{tbl:wishbone-master} respectively.
1609
\begin{table}[htbp]
1610
\begin{center}
1611
\begin{wishboneds}
1612
Revision level of wishbone & WB B4 spec \\\hline
1613 24 dgisselq
Type of interface & Master, Read/Write, single cycle or pipelined\\\hline
1614
Address Width & 32--bit bits \\\hline
1615 21 dgisselq
Port size & 32--bit \\\hline
1616
Port granularity & 32--bit \\\hline
1617
Maximum Operand Size & 32--bit \\\hline
1618
Data transfer ordering & (Irrelevant) \\\hline
1619
Clock constraints & Works at 100~MHz on a Basys--3 board\\\hline
1620
Signal Names & \begin{tabular}{ll}
1621
                Signal Name & Wishbone Equivalent \\\hline
1622
                {\tt i\_clk} & {\tt CLK\_O} \\
1623
                {\tt o\_wb\_cyc} & {\tt CYC\_O} \\
1624
                {\tt o\_wb\_stb} & {\tt STB\_O} \\
1625
                {\tt o\_wb\_we} & {\tt WE\_O} \\
1626
                {\tt o\_wb\_addr} & {\tt ADR\_O} \\
1627
                {\tt o\_wb\_data} & {\tt DAT\_O} \\
1628
                {\tt i\_wb\_ack} & {\tt ACK\_I} \\
1629
                {\tt i\_wb\_stall} & {\tt STALL\_I} \\
1630
                {\tt i\_wb\_data} & {\tt DAT\_I}
1631
                \end{tabular}\\\hline
1632
\end{wishboneds}
1633 22 dgisselq
\caption{Wishbone Datasheet for the CPU as Master}\label{tbl:wishbone-master}
1634 21 dgisselq
\end{center}\end{table}
1635
I do not recommend that you connect these together through the interconnect.
1636 24 dgisselq
Rather, the debug port of the CPU should be accessible regardless of the state
1637
of the master bus.
1638 21 dgisselq
 
1639 24 dgisselq
You may wish to notice that neither the {\tt ERR} nor the {\tt RETRY} wires
1640
have been implemented.  What this means is that the CPU is currently unable
1641
to detect a bus error condition, and so may stall indefinitely (hang) should
1642
it choose to access a value not on the bus, or a peripheral that is not
1643
yet properly configured.
1644 21 dgisselq
 
1645
\chapter{Clocks}\label{chap:clocks}
1646
 
1647 32 dgisselq
This core is based upon the Basys--3 development board sold by Digilent.
1648
The Basys--3 development board contains one external 100~MHz clock, which is
1649
sufficient to run the ZIP CPU core.
1650 21 dgisselq
\begin{table}[htbp]
1651
\begin{center}
1652
\begin{clocklist}
1653
i\_clk & External & 100~MHz & 100~MHz & System clock.\\\hline
1654
\end{clocklist}
1655
\caption{List of Clocks}\label{tbl:clocks}
1656
\end{center}\end{table}
1657
I hesitate to suggest that the core can run faster than 100~MHz, since I have
1658
had struggled with various timing violations to keep it at 100~MHz.  So, for
1659
now, I will only state that it can run at 100~MHz.
1660
 
1661
 
1662
\chapter{I/O Ports}\label{chap:ioports}
1663 33 dgisselq
The I/O ports to the Zip CPU may be grouped into three categories.  The first
1664
is that of the master wishbone used by the CPU, then the slave wishbone used
1665
to command the CPU via a debugger, and then the rest.  The first two of these
1666
were already discussed in the wishbone chapter.  They are listed here
1667
for completeness in Tbl.~\ref{tbl:iowb-master}
1668
\begin{table}
1669
\begin{center}\begin{portlist}
1670
{\tt o\_wb\_cyc}   &  1 & Output & Indicates an active Wishbone cycle\\\hline
1671
{\tt o\_wb\_stb}   &  1 & Output & WB Strobe signal\\\hline
1672
{\tt o\_wb\_we}    &  1 & Output & Write enable\\\hline
1673
{\tt o\_wb\_addr}  & 32 & Output & Bus address \\\hline
1674
{\tt o\_wb\_data}  & 32 & Output & Data on WB write\\\hline
1675
{\tt i\_wb\_ack}   &  1 & Input  & Slave has completed a R/W cycle\\\hline
1676
{\tt i\_wb\_stall} &  1 & Input  & WB bus slave not ready\\\hline
1677
{\tt i\_wb\_data}  & 32 & Input  & Incoming bus data\\\hline
1678
\end{portlist}\caption{CPU Master Wishbone I/O Ports}\label{tbl:iowb-master}\end{center}\end{table}
1679
and~\ref{tbl:iowb-slave} respectively.
1680
\begin{table}
1681
\begin{center}\begin{portlist}
1682
{\tt i\_wb\_cyc}   &  1 & Input & Indicates an active Wishbone cycle\\\hline
1683
{\tt i\_wb\_stb}   &  1 & Input & WB Strobe signal\\\hline
1684
{\tt i\_wb\_we}    &  1 & Input & Write enable\\\hline
1685
{\tt i\_wb\_addr}  &  1 & Input & Bus address, command or data port \\\hline
1686
{\tt i\_wb\_data}  & 32 & Input & Data on WB write\\\hline
1687
{\tt o\_wb\_ack}   &  1 & Output  & Slave has completed a R/W cycle\\\hline
1688
{\tt o\_wb\_stall} &  1 & Output  & WB bus slave not ready\\\hline
1689
{\tt o\_wb\_data}  & 32 & Output  & Incoming bus data\\\hline
1690
\end{portlist}\caption{CPU Debug Wishbone I/O Ports}\label{tbl:iowb-slave}\end{center}\end{table}
1691 21 dgisselq
 
1692 33 dgisselq
There are only four other lines to the CPU: the external clock, external
1693
reset, incoming external interrupt line(s), and the outgoing debug interrupt
1694
line.  These are shown in Tbl.~\ref{tbl:ioports}.
1695
\begin{table}
1696
\begin{center}\begin{portlist}
1697
{\tt i\_clk} & 1 & Input & The master CPU clock \\\hline
1698
{\tt i\_rst} & 1 & Input &  Active high reset line \\\hline
1699
{\tt i\_ext\_int} & 1\ldots 6 & Input &  Incoming external interrupts \\\hline
1700
{\tt o\_ext\_int} & 1 & Output & CPU Halted interrupt \\\hline
1701
\end{portlist}\caption{I/O Ports}\label{tbl:ioports}\end{center}\end{table}
1702
The clock line was discussed briefly in Chapt.~\ref{chap:clocks}.  We
1703
typically run it at 100~MHz.  The reset line is an active high reset.  When
1704
asserted, the CPU will start running again from its reset address in
1705
memory.  Further, depending upon how the CPU is configured and specifically on
1706
the {\tt START\_HALTED} parameter, it may or may not start running
1707
automatically.  The {\tt i\_ext\_int} line is for an external interrupt.  This
1708
line may be as wide as 6~external interrupts, depending upon the setting of
1709
the {\tt EXTERNAL\_INTERRUPTS} line.  As currently configured, the ZipSystem
1710
only supports one such interrupt line by default.  For us, this line is the
1711
output of another interrupt controller, but that's a board specific setup
1712
detail.  Finally, the Zip System produces one external interrupt whenever
1713
the CPU halts to wait for the debugger.
1714
 
1715 21 dgisselq
% Appendices
1716
% Index
1717
\end{document}
1718
 
1719
 

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