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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%
%% Filename: 	spec.tex
%%
%% Project:	Zip CPU -- a small, lightweight, RISC CPU soft core
%%
%% Purpose:	This LaTeX file contains all of the documentation/description
%%		currently provided with this Zip CPU soft core.  It supersedes
%%		any information about the instruction set or CPUs found
%%		elsewhere.  It's not nearly as interesting, though, as the PDF
%%		file it creates, so I'd recommend reading that before diving
%%		into this file.  You should be able to find the PDF file in
%%		the SVN distribution together with this PDF file and a copy of
%%		just type 'make' in the doc directory and it (should) build
%%		without a problem.
%%
%%
%% Creator:	Dan Gisselquist
%%		Gisselquist Technology, LLC
%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%
%% Copyright (C) 2015, Gisselquist Technology, LLC
%%
%% This program is free software (firmware): you can redistribute it and/or
%% modify it under the terms of  the GNU General Public License as published
%% by the Free Software Foundation, either version 3 of the License, or (at
%% your option) any later version.
%%
%% This program is distributed in the hope that it will be useful, but WITHOUT
%% ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
%% FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
%% for more details.
%%
%% You should have received a copy of the GNU General Public License along
%% with this program.  (It's in the $(ROOT)/doc directory, run make with no %% target there if the PDF file isn't present.) If not, see %% <http://www.gnu.org/licenses/> for a copy. %% %% License: GPL, v3, as defined and found on www.gnu.org, %% http://www.gnu.org/licenses/gpl.html %% %% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \documentclass{gqtekspec} \project{Zip CPU} \title{Specification} \author{Dan Gisselquist, Ph.D.} \email{dgisselq (at) opencores.org} \revision{Rev.~0.3} \begin{document} \pagestyle{gqtekspecplain} \titlepage \begin{license} Copyright (C) \theyear\today, Gisselquist Technology, LLC This project is free software (firmware): you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see \hbox{<http://www.gnu.org/licenses/>} for a copy. \end{license} \begin{revisionhistory} 0.3 & 8/22/2015 & Gisselquist & First completed draft\\\hline 0.2 & 8/19/2015 & Gisselquist & Still Draft, more complete \\\hline 0.1 & 8/17/2015 & Gisselquist & Incomplete First Draft \\\hline \end{revisionhistory} % Revision History % Table of Contents, named Contents \tableofcontents \listoffigures \listoftables \begin{preface} Many people have asked me why I am building the Zip CPU. ARM processors are good and effective. Xilinx makes and markets Microblaze, Altera Nios, and both have better toolsets than the Zip CPU will ever have. OpenRISC is also available, RISC--V may be replacing it. Why build a new processor? The easiest, most obvious answer is the simple one: Because I can. There's more to it, though. There's a lot that I would like to do with a processor, and I want to be able to do it in a vendor independent fashion. I would like to be able to generate Verilog code that can run equivalently on both Xilinx and Altera chips, and that can be easily ported from one manufacturer's chipsets to another. Even more, before purchasing a chip or a board, I would like to know that my soft core works. I would like to build a test bench to test components with, and Verilator is my chosen test bench. This forces me to use all Verilog, and it prevents me from using any proprietary cores. For this reason, Microblaze and Nios are out of the question. Why not OpenRISC? That's a hard question. The OpenRISC team has done some wonderful work on an amazing processor, and I'll have to admit that I am envious of what they've accomplished. I would like to port binutils to the Zip CPU, as I would like to port GCC and GDB. They are way ahead of me. The OpenRISC processor, however, is complex and hefty at about 4,500 LUTs. It has a lot of features of modern CPUs within it that ... well, let's just say it's not the little guy on the block. The Zip CPU is lighter weight, costing only about 2,300 LUTs with no peripherals, and 3,200 LUTs with some very basic peripherals. My final reason is that I'm building the Zip CPU as a learning experience. The Zip CPU has allowed me to learn a lot about how CPUs work on a very micro level. For the first time, I am beginning to understand many of the Computer Architecture lessons from years ago. To summarize: Because I can, because it is open source, because it is light weight, and as an exercise in learning. \end{preface} \chapter{Introduction} \pagenumbering{arabic} \setcounter{page}{1} The original goal of the ZIP CPU was to be a very simple CPU. You might think of it as a poor man's alternative to the OpenRISC architecture. For this reason, all instructions have been designed to be as simple as possible, and are all designed to be executed in one instruction cycle per instruction, barring pipeline stalls. Indeed, even the bus has been simplified to a constant 32-bit width, with no option for more or less. This has resulted in the choice to drop push and pop instructions, pre-increment and post-decrement addressing modes, and more. For those who like buzz words, the Zip CPU is: \begin{itemize} \item A 32-bit CPU: All registers are 32-bits, addresses are 32-bits, instructions are 32-bits wide, etc. \item A RISC CPU. There is no microcode for executing instructions. All instructions are designed to be completed in one clock cycle. \item A Load/Store architecture. (Only load and store instructions can access memory.) \item Wishbone compliant. All peripherals are accessed just like memory across this bus. \item A Von-Neumann architecture. (The instructions and data share a common bus.) \item A pipelined architecture, having stages for {\bf Prefetch}, {\bf Decode}, {\bf Read-Operand}, the {\bf ALU/Memory} unit, and {\bf Write-back}. See Fig.~\ref{fig:cpu} \begin{figure}\begin{center} \includegraphics[width=3.5in]{../gfx/cpu.eps} \caption{Zip CPU internal pipeline architecture}\label{fig:cpu} \end{center}\end{figure} for a diagram of this structure. \item Completely open source, licensed under the GPL.\footnote{Should you need a copy of the Zip CPU licensed under other terms, please contact me.} \end{itemize} Now, however, that I've worked on the Zip CPU for a while, it is not nearly as simple as I originally hoped. Worse, I've had to adjust to create capabilities that I was never expecting to need. These include: \begin{itemize} \item {\bf External Debug:} Once placed upon an FPGA, some external means is still necessary to debug this CPU. That means that there needs to be an external register that can control the CPU: reset it, halt it, step it, and tell whether it is running or not. My chosen interface includes a second register similar to this control register. This second register allows the external controller or debugger to examine registers internal to the CPU. \item {\bf Internal Debug:} Being able to run a debugger from within a user process requires an ability to step a user process from within a debugger. It also requires a break instruction that can be substituted for any other instruction, and substituted back. The break is actually difficult: the break instruction cannot be allowed to execute. That way, upon a break, the debugger should be able to jump back into the user process to step the instruction that would've been at the break point initially, and then to replace the break after passing it. Incidentally, this break messes with the prefetch cache and the pipeline: if you change an instruction partially through the pipeline, the whole pipeline needs to be cleansed. Likewise if you change an instruction in memory, you need to make sure the cache is reloaded with the new instruction. \item {\bf Prefetch Cache:} My original implementation had a very simple prefetch stage. Any time the PC changed the prefetch would go and fetch the new instruction. While this was perhaps this simplest approach, it cost roughly five clocks for every instruction. This was deemed unacceptable, as I wanted a CPU that could execute instructions in one cycle. I therefore have a prefetch cache that issues pipelined wishbone accesses to memory and then pushes instructions at the CPU. Sadly, this accounts for about 20\% of the logic in the entire CPU, or 15\% of the logic in the entire system. \item {\bf Operating System:} In order to support an operating system, interrupts and so forth, the CPU needs to support supervisor and user modes, as well as a means of switching between them. For example, the user needs a means of executing a system call. This is the purpose of the {\bf trap'} instruction. This instruction needs to place the CPU into supervisor mode (here equivalent to disabling interrupts), as well as handing it a parameter such as identifying which O/S function was called. My initial approach to building a trap instruction was to create an external peripheral which, when written to, would generate an interrupt and could return the last value written to it. In practice, this approach didn't work at all: the CPU executed two instructions while waiting for the trap interrupt to take place. Since then, I've decided to keep the rest of the CC register for that purpose so that a write to the CC register, with the GIE bit cleared, could be used to execute a trap. This has other problems, though, primarily in the limitation of the uses of the CC register. In particular, the CC register is the best place to put CPU state information and to announce'' special CPU features (floating point, etc). So the trap instruction still switches to interrupt mode, but the CC register is not nearly as useful for telling the supervisor mode processor what trap is being executed. Modern timesharing systems also depend upon a {\bf Timer} interrupt to handle task swapping. For the Zip CPU, this interrupt is handled external to the CPU as part of the CPU System, found in {\tt zipsystem.v}. The timer module itself is found in {\tt ziptimer.v}. \item {\bf Pipeline Stalls:} My original plan was to not support pipeline stalls at all, but rather to require the compiler to properly schedule all instructions so that stalls would never be necessary. After trying to build such an architecture, I gave up, having learned some things: For example, in order to facilitate interrupt handling and debug stepping, the CPU needs to know what instructions have finished, and which have not. In other words, it needs to know where it can restart the pipeline from. Once restarted, it must act as though it had never stopped. This killed my idea of delayed branching, since what would be the appropriate program counter to restart at? The one the CPU was going to branch to, or the ones in the delay slots? This also makes the idea of compressed instruction codes difficult, since, again, where do you restart on interrupt? So I switched to a model of discrete execution: Once an instruction enters into either the ALU or memory unit, the instruction is guaranteed to complete. If the logic recognizes a branch or a condition that would render the instruction entering into this stage possibly inappropriate (i.e. a conditional branch preceding a store instruction for example), then the pipeline stalls for one cycle until the conditional branch completes. Then, if it generates a new PC address, the stages preceding are all wiped clean. The discrete execution model allows such things as sleeping: if the CPU is put to sleep,'' the ALU and memory stages stall and back up everything before them. Likewise, anything that has entered the ALU or memory stage when the CPU is placed to sleep continues to completion. To handle this logic, each pipeline stage has three control signals: a valid signal, a stall signal, and a clock enable signal. In general, a stage stalls if it's contents are valid and the next step is stalled. This allows the pipeline to fill any time a later stage stalls. This approach is also different from other pipeline approaches. Instead of keeping the entire pipeline filled, each stage is treated independently. Therefore, individual stages may move forward as long as the subsequent stage is available, regardless of whether the stage behind it is filled. \item {\bf Verilog Modules:} When examining how other processors worked here on open cores, many of them had one separate module per pipeline stage. While this appeared to me to be a fascinating and commendable idea, my own implementation didn't work out quite so nicely. As an example, the decode module produces a {\em lot} of control wires and registers. Creating a module out of this, with only the simplest of logic within it, seemed to be more a lesson in passing wires around, rather than encapsulating logic. Another example was the register writeback section. I would love this section to be a module in its own right, and many have made them such. However, other modules depend upon writeback results other than just what's placed in the register (i.e., the control wires). For these reasons, I didn't manage to fit this section into it's own module. The result is that the majority of the CPU code can be found in the {\tt zipcpu.v} file. \end{itemize} With that introduction out of the way, let's move on to the instruction set. \chapter{CPU Architecture}\label{chap:arch} The Zip CPU supports a set of two operand instructions, where the second operand (always a register) is the result. The only exception is the store instruction, where the first operand (always a register) is the source of the data to be stored. \section{Simplified Bus} The bus architecture of the Zip CPU is that of a simplified WISHBONE bus. It has been simplified in this fashion: all operations are 32--bit operations. The bus is neither little endian nor bit endian. For this reason, all words are 32--bits. All instructions are also 32--bits wide. Everything has been built around the 32--bit word. \section{Register Set} The Zip CPU supports two sets of sixteen 32-bit registers, a supervisor and a user set as shown in Fig.~\ref{fig:regset}. \begin{figure}\begin{center} \includegraphics[width=3.5in]{../gfx/regset.eps} \caption{Zip CPU Register File}\label{fig:regset} \end{center}\end{figure} The supervisor set is used in interrupt mode when interrupts are disabled, whereas the user set is used otherwise. Of this register set, the Program Counter (PC) is register 15, whereas the status register (SR) or condition code register (CC) is register 14. By convention, the stack pointer will be register 13 and noted as (SP)--although there is nothing special about this register other than this convention. The CPU can access both register sets via move instructions from the supervisor state, whereas the user state can only access the user registers. The status register is special, and bears further mention. The lower 10 bits of the status register form a set of CPU state and condition codes. Writes to other bits of this register are preserved. Of the condition codes, the bottom four bits are the current flags: Zero (Z), Carry (C), Negative (N), and Overflow (V). The next bit is a clock enable (0 to enable) or sleep bit (1 to put the CPU to sleep). Setting this bit will cause the CPU to wait for an interrupt (if interrupts are enabled), or to completely halt (if interrupts are disabled). The sixth bit is a global interrupt enable bit (GIE). When this sixth bit is a 1' interrupts will be enabled, else disabled. When interrupts are disabled, the CPU will be in supervisor mode, otherwise it is in user mode. Thus, to execute a context switch, one only need enable or disable interrupts. (When an interrupt line goes high, interrupts will automatically be disabled, as the CPU goes and deals with its context switch.) Special logic has been added to keep the user mode from setting the sleep register and clearing the GIE register at the same time, with clearing the GIE register taking precedence. The seventh bit is a step bit. This bit can be set from supervisor mode only. After setting this bit, should the supervisor mode process switch to user mode, it would then accomplish one instruction in user mode before returning to supervisor mode. Then, upon return to supervisor mode, this bit will be automatically cleared. This bit has no effect on the CPU while in supervisor mode. This functionality was added to enable a userspace debugger functionality on a user process, working through supervisor mode of course. The eighth bit is a break enable bit. This controls whether a break instruction in user mode will halt the processor for an external debugger (break enabled), or whether the break instruction will simply send send the CPU into interrupt mode. Encountering a break in supervisor mode will halt the CPU independent of the break enable bit. This bit can only be set within supervisor mode. % Should break enable be a supervisor mode bit, while the break enable bit % in user mode is a break has taken place bit? % This functionality was added to enable an external debugger to set and manage breakpoints. The ninth bit is reserved for a floating point enable bit. When set, the arithmetic for the next instruction will be sent to a floating point unit. Such a unit may later be added as an extension to the Zip CPU. If the CPU does not support floating point instructions, this bit will never be set. The instruction set could also be simply extended to allow other data types in this fashion, such as two by 16--bit vector operations or four by 8--bit vector operations. The tenth bit is a trap bit. It is set whenever the user requests a soft interrupt, and cleared on any return to userspace command. This allows the supervisor, in supervisor mode, to determine whether it got to supervisor mode from a trap or from an external interrupt or both. These status register bits are summarized in Tbl.~\ref{tbl:ccbits}. \begin{table} \begin{center} \begin{tabular}{l|l} Bit & Meaning \\\hline 9 & Soft trap, set on a trap from user mode, cleared when returning to user mode\\\hline 8 & (Reserved for) Floating point enable \\\hline 7 & Halt on break, to support an external debugger \\\hline 6 & Step, single step the CPU in user mode\\\hline 5 & GIE, or Global Interrupt Enable \\\hline 4 & Sleep \\\hline 3 & V, or overflow bit.\\\hline 2 & N, or negative bit.\\\hline 1 & C, or carry bit.\\\hline 0 & Z, or zero bit. \\\hline \end{tabular} \caption{Condition Code / Status Register Bits}\label{tbl:ccbits} \end{center}\end{table} \section{Conditional Instructions} Most, although not quite all, instructions are conditionally executed. From the four condition code flags, eight conditions are defined. These are shown in Tbl.~\ref{tbl:conditions}. \begin{table} \begin{center} \begin{tabular}{l|l|l} Code & Mneumonic & Condition \\\hline 3'h0 & None & Always execute the instruction \\ 3'h1 & {\tt .Z} & Only execute when 'Z' is set \\ 3'h2 & {\tt .NE} & Only execute when 'Z' is not set \\ 3'h3 & {\tt .GE} & Greater than or equal ('N' not set, 'Z' irrelevant) \\ 3'h4 & {\tt .GT} & Greater than ('N' not set, 'Z' not set) \\ 3'h5 & {\tt .LT} & Less than ('N' set) \\ 3'h6 & {\tt .C} & Carry set\\ 3'h7 & {\tt .V} & Overflow set\\ \end{tabular} \caption{Conditions for conditional operand execution}\label{tbl:conditions} \end{center} \end{table} There is no condition code for less than or equal, not C or not V. Sorry, I ran out of space in 3--bits. Using these conditions will take an extra instruction and a pipeline stall. (Ex: \hbox{\em (Stall)}; \hbox{\tt TST \$4,CC;} \hbox{\tt STO.NZ R0,(R1)})

\section{Operand B}
Many instruction forms have a 21-bit source Operand B'' associated with them.
This Operand B is either equal to a register plus a signed immediate offset,
or an immediate offset by itself.  This value is encoded as shown in
Tbl.~\ref{tbl:opb}.
\begin{table}\begin{center}
\begin{tabular}{|l|l|l|}\hline
Bit 20 & 19 \ldots 16 & 15 \ldots 0 \\\hline
1'b0 & \multicolumn{2}{l|}{20--bit Signed Immediate value} \\\hline
1'b1 & 4-bit Register & 16--bit Signed immediate offset \\\hline
\end{tabular}
\caption{Bit allocation for Operand B}\label{tbl:opb}
\end{center}\end{table}

Sixteen and twenty bit immediate values don't make sense for all instructions.
For example, what is the point of a 20--bit immediate when executing a 16--bit
multiply?  Likewise, why have a 16--bit immediate when adding to a logical
or arithmetic shift?  In these cases, the extra bits are reserved for future
instruction possibilities.

The ZIP CPU supports two addressing modes: register plus immediate, and
Operand B's, shown above.

A lot of long hard thought was put into whether to allow pre/post increment
and decrement addressing modes.  Finding no way to use these operators without
taking two or more clocks per instruction,\footnote{The two clocks figure
comes from the design of the register set, allowing only one write per clock.
That write is either from the memory unit or the ALU, but never both.} these
removed from the realm of possibilities.  This means that the Zip CPU has no
native way of executing push, pop, return, or jump to subroutine operations.
Each of these instructions can be emulated with a set of instructions from the
existing set.

\section{Move Operands}
The previous set of operands would be perfect and complete, save only that
Therefore, the MOV instruction is special and offers access to these registers
\ldots when in supervisory mode.  To keep the compiler simple, the extra bits
are ignored in non-supervisory mode (as though they didn't exist), rather than
being mapped to new instructions or additional capabilities.  The bits
indicating which register set each register lies within are the A-Usr and
B-Usr bits.  When set to a one, these refer to a user mode register.  When set
to a zero, these refer to a register in the current mode, whether user or
supervisor.  Further, because a load immediate instruction exists, there is no
move capability between an immediate and a register: all moves come from either
a register or a register plus an offset.

This actually leads to a bit of a problem: since the MOV instruction encodes
which register set each register is coming from or moving to, how shall a
compiler or assembler know how to compile a MOV instruction without knowing
the mode of the CPU at the time?  For this reason, the compiler will assume
all MOV registers are supervisor registers, and display them as normal.
Anything with the user bit set will be treated as a user register.  The CPU
will quietly ignore the supervisor bits while in user mode, and anything
marked as a user register will always be valid.  (Did I just say that in the
last paragraph?)

\section{Multiply Operations}
The Zip CPU supports two Multiply operations, a
16x16 bit signed multiply (MPYS) and the same but unsigned (MPYU).  In both
cases, the operand is a register plus a 16-bit immediate, subject to the
rule that the register cannot be the PC or CC registers.  The PC register
field has been stolen to create a multiply by immediate instruction.  The
CC register field is reserved.

\section{Floating Point}
The ZIP CPU does not support floating point operations.  However, the
instruction set reserves two possibilities for future floating point
operations.

The first floating point operation hole in the instruction set involves
setting the floating point bit in the CC register.  The next instruction
will simply interpret its operands as floating point instructions.
Not all instructions, however, have floating point equivalents.  Further, the
immediate fields do not apply in floating point mode, and must be set to
zero.  Not all instructions make sense as floating point operations.
Therefore, only the CMP, SUB, ADD, and MPY instructions may be issued as
floating point instructions.  Other instructions allow the examining of the
floating point bit in the CC register.  In all cases, the floating point bit
is cleared one instruction after it is set.

The other possibility for floating point operations involves exploiting the
hole in the instruction set that the NOOP and BREAK instructions reside within.
to this space could create instructions with 4--bit register addresses for
each register, a 3--bit field for conditional execution, and a 2--bit field
for which operation.  In this fashion, such a floating point capability would
only fill 13--bits of the 24--bit field, still leaving lots of room for
expansion.

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

The current architecture does not support a floating point not-implemented
interrupt.  Any soft floating point emulation must be done deliberately.

\section{Native Instructions}
The instruction set for the Zip CPU is summarized in
Tbl.~\ref{tbl:zip-instructions}.
\begin{table}\begin{center}
\begin{tabular}{|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|l|c|}\hline
Op Code & \multicolumn{8}{c|}{31\ldots24} & \multicolumn{8}{c|}{23\ldots 16}
& \multicolumn{8}{c|}{15\ldots 8} & \multicolumn{8}{c|}{7\ldots 0}
& Sets CC? \\\hline
CMP(Sub) & \multicolumn{4}{l|}{4'h0}
& \multicolumn{4}{l|}{D. Reg}
& \multicolumn{3}{l|}{Cond.}
& \multicolumn{21}{l|}{Operand B}
& Yes \\\hline
TST(And) & \multicolumn{4}{l|}{4'h1}
& \multicolumn{4}{l|}{D. Reg}
& \multicolumn{3}{l|}{Cond.}
& \multicolumn{21}{l|}{Operand B}
& Yes \\\hline
MOV & \multicolumn{4}{l|}{4'h2}
& \multicolumn{4}{l|}{D. Reg}
& \multicolumn{3}{l|}{Cond.}
& A-Usr
& \multicolumn{4}{l|}{B-Reg}
& B-Usr
& \multicolumn{15}{l|}{15'bit signed offset}
& \\\hline
LODI & \multicolumn{4}{l|}{4'h3}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{24}{l|}{24'bit Signed Immediate}
& \\\hline
NOOP & \multicolumn{4}{l|}{4'h4}
& \multicolumn{4}{l|}{4'he}
& \multicolumn{24}{l|}{24'h00}
& \\\hline
BREAK & \multicolumn{4}{l|}{4'h4}
& \multicolumn{4}{l|}{4'he}
& \multicolumn{24}{l|}{24'h01}
& \\\hline
{\em Rsrd} & \multicolumn{4}{l|}{4'h4}
& \multicolumn{4}{l|}{4'he}
& \multicolumn{24}{l|}{24'bits, but not 0 or 1.}
& \\\hline
LODIHI & \multicolumn{4}{l|}{4'h4}
& \multicolumn{4}{l|}{4'hf}
& \multicolumn{3}{l|}{Cond.}
& 1'b1
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{16}{l|}{16-bit Immediate}
& \\\hline
LODILO & \multicolumn{4}{l|}{4'h4}
& \multicolumn{4}{l|}{4'hf}
& \multicolumn{3}{l|}{Cond.}
& 1'b0
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{16}{l|}{16-bit Immediate}
& \\\hline
16-b MPYU & \multicolumn{4}{l|}{4'h4}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{3}{l|}{Cond.}
& 1'b0 & \multicolumn{4}{l|}{Reg}
& \multicolumn{16}{l|}{16-bit Offset}
& Yes \\\hline
16-b MPYU(I) & \multicolumn{4}{l|}{4'h4}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{3}{l|}{Cond.}
& 1'b0 & \multicolumn{4}{l|}{4'hf}
& \multicolumn{16}{l|}{16-bit Offset}
& Yes \\\hline
16-b MPYS & \multicolumn{4}{l|}{4'h4}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{3}{l|}{Cond.}
& 1'b1 & \multicolumn{4}{l|}{Reg}
& \multicolumn{16}{l|}{16-bit Offset}
& Yes \\\hline
16-b MPYS(I) & \multicolumn{4}{l|}{4'h4}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{3}{l|}{Cond.}
& 1'b1 & \multicolumn{4}{l|}{4'hf}
& \multicolumn{16}{l|}{16-bit Offset}
& Yes \\\hline
ROL & \multicolumn{4}{l|}{4'h5}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{3}{l|}{Cond.}
& \multicolumn{21}{l|}{Operand B, truncated to low order 5 bits}
& \\\hline
LOD & \multicolumn{4}{l|}{4'h6}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{3}{l|}{Cond.}
& \\\hline
STO & \multicolumn{4}{l|}{4'h7}
& \multicolumn{4}{l|}{D. Reg}
& \multicolumn{3}{l|}{Cond.}
& \\\hline
SUB & \multicolumn{4}{l|}{4'h8}
&	\multicolumn{4}{l|}{R. Reg}
&	\multicolumn{3}{l|}{Cond.}
&	\multicolumn{21}{l|}{Operand B}
& Yes \\\hline
AND & \multicolumn{4}{l|}{4'h9}
&	\multicolumn{4}{l|}{R. Reg}
&	\multicolumn{3}{l|}{Cond.}
&	\multicolumn{21}{l|}{Operand B}
& Yes \\\hline
&	\multicolumn{4}{l|}{R. Reg}
&	\multicolumn{3}{l|}{Cond.}
&	\multicolumn{21}{l|}{Operand B}
& Yes \\\hline
OR & \multicolumn{4}{l|}{4'hb}
&	\multicolumn{4}{l|}{R. Reg}
&	\multicolumn{3}{l|}{Cond.}
&	\multicolumn{21}{l|}{Operand B}
& Yes \\\hline
XOR & \multicolumn{4}{l|}{4'hc}
&	\multicolumn{4}{l|}{R. Reg}
&	\multicolumn{3}{l|}{Cond.}
&	\multicolumn{21}{l|}{Operand B}
& Yes \\\hline
LSL/ASL & \multicolumn{4}{l|}{4'hd}
&	\multicolumn{4}{l|}{R. Reg}
&	\multicolumn{3}{l|}{Cond.}
&	\multicolumn{21}{l|}{Operand B, imm. truncated to 6 bits}
& Yes \\\hline
ASR & \multicolumn{4}{l|}{4'he}
&	\multicolumn{4}{l|}{R. Reg}
&	\multicolumn{3}{l|}{Cond.}
&	\multicolumn{21}{l|}{Operand B, imm. truncated to 6 bits}
& Yes \\\hline
LSR & \multicolumn{4}{l|}{4'hf}
&	\multicolumn{4}{l|}{R. Reg}
&	\multicolumn{3}{l|}{Cond.}
&	\multicolumn{21}{l|}{Operand B, imm. truncated to 6 bits}
& Yes \\\hline
\end{tabular}
\caption{Zip CPU Instruction Set}\label{tbl:zip-instructions}
\end{center}\end{table}

As you can see, there's lots of room for instruction set expansion.  The
NOOP and BREAK instructions are the only instructions within one particular
24--bit hole.  This spaces are reserved for future enhancements.  For example,
floating point operations, consisting of a 3-bit floating point operation,
two 4-bit registers, no immediate offset, and a 3-bit condition would fit
nicely into 14--bits of this address space--making it so that the floating
point bit in the CC register need not be used.

\section{Derived Instructions}
The ZIP CPU supports many other common instructions, but not all of them
are single cycle instructions.  The derived instruction tables,
Tbls.~\ref{tbl:derived-1}, \ref{tbl:derived-2}, and~\ref{tbl:derived-3},
help to capture some of how these other instructions may be implemented on
the ZIP CPU.  Many of these instructions will have assembly equivalents,
such as the branch instructions, to facilitate working with the CPU.
\begin{table}\begin{center}
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
Mapped & Actual  & Notes \\\hline
& \parbox[t]{1.5in}{Add Ra,Rx\\ADD.C \$1,Ry\\Add Rb,Ry} & Add with carry \\\hline BRA.Cond +/-\$Addr
& \hbox{MOV.cond \$Addr+PC,PC} & Branch or jump on condition. Works for 15--bit signed address offsets.\\\hline BRA.Cond +/-\$Addr
& \parbox[t]{1.5in}{LDI \$Addr,Rx \\ ADD.cond Rx,PC} & Branch/jump on condition. Works for 23 bit address offsets, but costs a register, an extra instruction, and sets the flags. \\\hline BNC PC+\$Addr
& \parbox[t]{1.5in}{Test \$Carry,CC \\ MOV.Z PC+\$Addr,PC}
& Example of a branch on an unsupported
condition, in this case a branch on not carry \\\hline
BUSY & MOV \$-1(PC),PC & Execute an infinite loop \\\hline CLRF.NZ Rx & XOR.NZ Rx,Rx & Clear Rx, and flags, if the Z-bit is not set \\\hline CLR Rx & LDI \$0,Rx
& Clears Rx, leaves flags untouched.  This instruction cannot be
conditional. \\\hline
EXCH.W Rx
& ROL \$16,Rx & Exchanges the top and bottom 16'bit words of Rx \\\hline HALT & Or \$SLEEP,CC
& Executed while in interrupt mode.  In user mode this is simply a
wait until interrupt instruction. \\\hline
INT & LDI \$0,CC & Since we're using the CC register as a trap vector as well, this executes TRAP \#0. \\\hline IRET & OR \$GIE,CC
JMP R6+\$Addr & MOV \$Addr(R6),PC
& \\\hline
JSR PC+\$Addr & \parbox[t]{1.5in}{SUB \$1,SP \\\
MOV \$3+PC,R0 \\ STO R0,1(SP) \\ MOV \$Addr+PC,PC \\
ADD \$1,SP} & Jump to Subroutine. Note the required cleanup instruction after returning. \\\hline JSR PC+\$Addr
& \parbox[t]{1.5in}{MOV \$3+PC,R12 \\ MOV \$addr+PC,PC}
&This is the high speed
version of a subroutine call, necessitating a register to hold the
last PC address.  In its favor, this method doesn't suffer the
mandatory memory access of the other approach. \\\hline
LDI.l \$val,Rx & \parbox[t]{1.5in}{LDIHI (\$val$>>$16)\&0x0ffff, Rx \\
LDILO (\$val \& 0x0ffff)} & Sadly, there's not enough instruction space to load a complete immediate value into any register. Therefore, fully loading any register takes two cycles. The LDIHI (load immediate high) and LDILO (load immediate low) instructions have been created to facilitate this. \\\hline \end{tabular} \caption{Derived Instructions}\label{tbl:derived-1} \end{center}\end{table} \begin{table}\begin{center} \begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline Mapped & Actual & Notes \\\hline LOD.b \$addr,Rx
& \parbox[t]{1.5in}{%
LDI	\$addr,Ra \\ LDI \$addr,Rb \\
LSR	\$2,Ra \\ AND \$3,Rb \\
LOD	(Ra),Rx \\
LSL	\$3,Rb \\ SUB \$32,Rb \\
ROL	Rb,Rx \\
AND \$0ffh,Rx} & \parbox[t]{3in}{This CPU is designed for 32'bit word length instructions. Byte addressing is not supported by the CPU or the bus, so it therefore takes more work to do. Note also that in this example, \$Addr is a byte-wise address, where
For this reason,
we needed to drop the bottom two bits.  This also limits the address
space of character accesses using this method from 16 MB down to 4MB.}
\\\hline
\parbox[t]{1.5in}{LSL \$1,Rx\\ LSLC \$1,Ry}
& \parbox[t]{1.5in}{LSL \$1,Ry \\ LSL \$1,Rx \\
OR.C \$1,Ry} & Logical shift left with carry. Note that the instruction order is now backwards, to keep the conditions valid. That is, LSL sets the carry flag, so if we did this the other way with Rx before Ry, then the condition flag wouldn't have been right for an OR correction at the end. \\\hline \parbox[t]{1.5in}{LSR \$1,Rx \\ LSRC \$1,Ry} & \parbox[t]{1.5in}{CLR Rz \\ LSR \$1,Ry \\
LDIHI.C \$8000h,Rz \\ LSR \$1,Rx \\
OR Rz,Rx}
& Logical shift right with carry \\\hline
NEG Rx & \parbox[t]{1.5in}{XOR \$-1,Rx \\ ADD \$1,Rx} & \\\hline
NOOP & NOOP & While there are many
operations that do nothing, such as MOV Rx,Rx, or OR \$0,Rx, these operations have consequences in that they might stall the bus if Rx isn't ready yet. For this reason, we have a dedicated NOOP instruction. \\\hline NOT Rx & XOR \$-1,Rx & \\\hline
POP Rx
& \parbox[t]{1.5in}{LOD \$-1(SP),Rx \\ ADD \$1,SP}
& Note
that for interrupt purposes, one can never depend upon the value at
(SP).  Hence you read from it, then increment it, lest having
incremented it first something then comes along and writes to that
value before you can read the result. \\\hline
PUSH Rx
& \parbox[t]{1.5in}{SUB \$1,SP \\ STO Rx,\$1(SP)}
& \\\hline
RESET
& \parbox[t]{1in}{STO \$1,\$watchdog(R12)\\NOOP\\NOOP}
& \parbox[t]{3in}{This depends upon the peripheral base address being
in R12.

supervisor mode.}\\\hline
RET & \parbox[t]{1.5in}{LOD \$-1(SP),PC} & Note that this depends upon the calling context to clean up the stack, as outlined for the JSR instruction. \\\hline \end{tabular} \caption{Derived Instructions, continued}\label{tbl:derived-2} \end{center}\end{table} \begin{table}\begin{center} \begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline RET & MOV R12,PC & This is the high(er) speed version, that doesn't touch the stack. As such, it doesn't suffer a stall on memory read/write to the stack. \\\hline STEP Rr,Rt & \parbox[t]{1.5in}{LSR \$1,Rr \\ XOR.C Rt,Rr}
& Step a Galois implementation of a Linear Feedback Shift Register, Rr,
using taps Rt \\\hline
STO.b Rx,\$addr & \parbox[t]{1.5in}{% LDI \$addr,Ra \\
LDI \$addr,Rb \\ LSR \$2,Ra \\
AND \$3,Rb \\ SUB \$32,Rb \\
LOD (Ra),Ry \\
AND \$0ffh,Rx \\ AND \$-0ffh,Ry \\
ROL Rb,Rx \\
OR Rx,Ry \\
STO Ry,(Ra) }
& \parbox[t]{3in}{This CPU and it's bus are {\em not} optimized
for byte-wise operations.

Note that in this example, \$addr is a byte-wise address, whereas in all of our other examples it is a 32-bit word address. This also limits the address space of character accesses from 16 MB down to 4MB.F Further, this instruction implies a byte ordering, such as big or little endian.} \\\hline SWAP Rx,Ry & \parbox[t]{1.5in}{ XOR Ry,Rx \\ XOR Rx,Ry \\ XOR Ry,Rx} & While no extra registers are needed, this example does take 3-clocks. \\\hline TRAP \#X & LDILO \$x,CC
& This approach uses the unused bits of the CC register as a TRAP
address.  The user will need to make certain
that the SLEEP and GIE bits are not set in \$x. LDI would also work, however using LDILO permits the use of conditional traps. (i.e., trap if the zero flag is set.) Should you wish to trap off of a register value, you could equivalently load \$x into the register and
then MOV it into the CC register. \\\hline
TST Rx
& TST \$-1,Rx & Set the condition codes based upon Rx. Could also do a CMP \$0,Rx,
ADD \$0,Rx, SUB \$0,Rx, etc, AND \$-1,Rx, etc. The TST and CMP approaches won't stall future pipeline stages looking for the value of Rx. \\\hline WAIT & Or \$SLEEP,CC
& Wait 'til interrupt.  In an interrupts disabled context, this
becomes a HALT instruction.
\end{tabular}
\caption{Derived Instructions, continued}\label{tbl:derived-3}
\end{center}\end{table}
\iffalse
\fi
\section{Pipeline Stages}
As mentioned in the introduction, and highlighted in Fig.~\ref{fig:cpu},
the Zip CPU supports a five stage pipeline.
\begin{enumerate}
\item {\bf Prefetch}: Read instruction from memory (cache if possible).  This
stage is actually pipelined itself, and so it will stall if the PC
ever changes.  Stalls are also created here if the instruction isn't
in the prefetch cache.
\item {\bf Decode}: Decode instruction into op code, register(s) to read, and
immediate offset.  This stage also determines whether the flags will
be set or whether the result will be written back.
\item {\bf Read Operands}: Read registers and apply any immediate values to
them.  There is no means of detecting or flagging arithmetic overflow
or carry when adding the immediate to the operand.  This stage will
stall if any source operand is pending.
\item Split into two tracks: An {\bf ALU} which will accomplish a simple
instruction, and the {\bf MemOps} stage which accomplishes memory
\begin{itemize}
\item Loads stall instructions that access the register until it is
written to the register set.
\item Condition codes are available upon completion
\item Issuing an instruction to the memory while the memory is busy will
stall the entire pipeline.  If the bus deadlocks, only a reset
will release the CPU.  (Watchdog timer, anyone?)
\item The Zip CPU currently has no means of reading and acting on any
error conditions on the bus.
\end{itemize}
\item {\bf Write-Back}: Conditionally write back the result to the register
set, applying the condition.  This routine is bi-re-entrant: either the
memory or the simple instruction may request a register write.
\end{enumerate}

The Zip CPU does not support out of order execution.  Therefore, if the memory
unit stalls, every other instruction stalls.  Memory stores, however, can take
place concurrently with ALU operations, although memory reads cannot.

\iffalse

\section{Pipeline Logic}
How the CPU handles some instruction combinations can be telling when
determining what happens in the pipeline.  The following lists some examples:
\begin{itemize}
\item {\bf Delayed Branching}

I had originally hoped to implement delayed branching.   My goal
was that the compiler would handle any pipeline stall conditions so
that the pipeline logic could be simpler within the CPU.  I ran into
two problems with this.

The first problem has to deal with debug mode.  When the debugger
single steps an instruction, that instruction goes to completion.
This means that if the instruction moves a value to the PC register,
the PC register would now contain that value, indicating that the
next instruction would be on the other side of the branch.  There's
just no easy way around this: the entire CPU state must be captured
by the registers, to include the program counter.  What value should
the program counter be equal to?  The branch?  Fine.  The address
you are branching to?  Fine.  The address of the delay slot?  Problem.

The second problem with delayed branching is the idea of suspending
processing for an interrupt.  Which address should the CPU return
to upon completing the interrupt processing?  The branch?  Good.  The
address after the branch?  Also good.  The address of the delay slot?
Not so good.

If you then add into this mess the idea that, if the CPU is running
from a really slow memory such as the flash, the delay slot may never
be filled before the branch is determined, then this makes even less
sense.

For all of these reasons, this CPU does not support delayed branching.

\item {\bf Register Result:} {\tt MOV R0,R1; MOV R1,R2 }

What value does R2 get, the value of R1 before the first move or the
value of R0?  The Zip CPU has been optimized so that neither of these
instructions require a pipeline stall--unless an immediate were to
be added to R1 in the second instruction.

The ZIP CPU architecture requires that R2 must equal R0 at the end of
this operation.  Even better, such combinations do not (normally)
stall the pipeline.

\item {\bf Condition Codes Result:} {\tt CMP R0,R1;} {\tt MOV.EQ \$x,PC} At issue is the same item as above, save that the CMP instruction updates the flags that the MOV instruction depends upon. The Zip CPU architecture requires that condition codes must be updated and available immediately for the next instruction without stalling the pipeline. \item {\bf Condition Codes Register Result:} {\tt CMP R0,R1; MOV CC,R2} At issue is the fact that the logic supporting the CC register is more complicated than the logic supporting any other register. The ZIP CPU will stall for a cycle cycle on this instruction. \item {\bf Condition Codes Register Operand:} {\tt MOV R0,R1; MOV CC,R2} Unlike the previous case, this move prior to reading the {\tt CC} register does not impact the {\tt CC} register. Therefore, this does not stall the bus, whereas the previous one would. \end{itemize} As I've studied this, I find several approaches to handling pipeline issues. These approaches (and their consequences) are listed below. \begin{itemize} \item {\bf All issued instructions complete, stages stall individually} What about a slow pre-fetch? Nominally, this works well: any issued instruction just runs to completion. If there are four issued instructions in the pipeline, with the writeback instruction being a write-to-PC instruction, the other three instructions naturally finish. This approach fails when reading instructions from the flash, since such reads require N clocks to clocks to complete. Thus there may be only one instruction in the pipeline if reading from flash, or a full pipeline if reading from cache. Each of these approaches would produce a different response. For this reason, the Zip CPU works off of a different basis: All instructions that enter either the ALU or the memory unit will complete. Stages still stall individually. \item {\bf Issued instructions may be canceled} The problem here is that memory operations cannot be canceled: even reads may have side effects on peripherals that cannot be canceled later. Further, in the case of an interrupt, it's difficult to know what to cancel. What happens in a \hbox{\tt MOV.C \$x,PC} followed by a \hbox{\tt MOV \$y,PC} instruction? Which get canceled? Because it isn't clear what would need to be canceled, the Zip CPU will not permit this combination. A MOV to the PC register will be followed by a stall, and possibly many stalls, so that the second move to PC will never be executed. \item {\bf All issued instructions complete.} In this example, we try all issued instructions complete, but the entire pipeline stalls if one stage is not filled. In this approach, though, we again struggle with the problems associated with delayed branching. Upon attempting to restart the processor, where do you restart it from? \item {\bf Memory instructions must complete} All instructions that enter into the memory module {\em must} complete. Issued instructions from the prefetch, decode, or operand read stages may or may not complete. Jumps into code must be valid, so that interrupt returns may be valid. All instructions entering the ALU complete. This looks to be the simplest approach. While the logic may be difficult, this appears to be the only re-entrant approach. A {\tt new\_pc} flag will be high anytime the PC changes in an unpredictable way (i.e., it doesn't increment). This includes jumps as well as interrupts and interrupt returns. Whenever this flag may go high, memory operations and ALU operations will stall until the result is known. When the flag does go high, anything in the prefetch, decode, and read-op stage will be invalidated. \end{itemize} \fi \section{Pipeline Stalls} The processing pipeline can and will stall for a variety of reasons. Some of these are obvious, some less so. These reasons are listed below: \begin{itemize} \item When the prefetch cache is exhausted This should be obvious. If the prefetch cache doesn't have the instruction in memory, the entire pipeline must stall until enough of the prefetch cache is loaded to support the next instruction. \item While waiting for the pipeline to load following any taken branch, jump, return from interrupt or switch to interrupt context (6 clocks) If the PC suddenly changes, the pipeline is subsequently cleared and needs to be reloaded. Given that there are five stages to the pipeline, that accounts for five of the six delay clocks. The last clock is lost in the prefetch stage which needs at least one clock with a valid PC before it can produce a new output. Hence, six clocks will always be lost anytime the pipeline needs to be cleared. \item When reading from a prior register while also adding an immediate offset \begin{enumerate} \item\ {\tt OPCODE ?,RA} \item\ {\em (stall)} \item\ {\tt OPCODE I+RA,RB} \end{enumerate} Since the addition of the immediate register within OpB decoding gets applied during the read operand stage so that it can be nicely settled before the ALU, any instruction that will write back an operand must be separated from the opcode that will read and apply an immediate offset by one instruction. The good news is that this stall can easily be mitigated by proper scheduling. \item When writing to the CC or PC Register \begin{enumerate} \item\ {\tt OPCODE RA,PC} {\em Ex: a branch opcode} \item\ {\em (stall, even if jump not taken)} \item\ {\tt OPCODE RA,RB} \end{enumerate} Since branches take place in the writeback stage, the Zip CPU will stall the pipeline for one clock anytime there may be a possible jump. This prevents an instruction from executing a memory access after the jump but before the jump is recognized. This stall cannot be mitigated through scheduling. \item When reading from the CC register after setting the flags \begin{enumerate} \item\ {\tt ALUOP RA,RB} \item\ {\em (stall} \item\ {\tt TST sys.ccv,CC} \item\ {\tt BZ somewhere} \end{enumerate} The reason for this stall is simply performance. Many of the flags are determined via combinatorial logic after the writeback instruction is determined. Trying to then place these into the input for one of the operands created a time delay loop that would no longer execute in a single 100~MHz clock cycle. (The time delay of the multiply within the ALU wasn't helping either \ldots). This stall may be eliminated via proper scheduling, by placing an instruction that does not set flags in between the ALU operation and the instruction that references the CC register. For example, {\tt MOV \$addr+PC,uPC}
followed by an {\tt RTU} ({\tt OR \$GIE,CC}) instruction will not incur this stall, whereas an {\tt OR \$BREAKEN,CC} followed by an {\tt OR \$STEP,CC} will incur the stall. \item When waiting for a memory read operation to complete \begin{enumerate} \item\ {\tt LOD address,RA} \item\ {\em (multiple stalls, bus dependent, 7 clocks best)} \item\ {\tt OPCODE I+RA,RB} \end{enumerate} Remember, the ZIP CPU does not support out of order execution. Therefore, anytime the memory unit becomes busy both the memory unit and the ALU must stall until the memory unit is cleared. This is especially true of a load instruction, which must still write its operand back to the register file. Store instructions are different, since they can be busy with no impact on later ALU write back operations. Hence, only loads stall the pipeline. This also assumes that the memory being accessed is a single cycle memory. Slower memories, such as the Quad SPI flash, will take longer--perhaps even as long as forty clocks. During this time the CPU and the external bus will be busy, and unable to do anything else. \item Memory operation followed by a memory operation \begin{enumerate} \item\ {\tt STO address,RA} \item\ {\em (multiple stalls, bus dependent, 7 clocks best)} \item\ {\tt LOD address,RB} \item\ {\em (multiple stalls, bus dependent, 7 clocks best)} \end{enumerate} In this case, the LOD instruction cannot start until the STALL is finished. With proper scheduling, it is possible to do something in the ALU while the STO is busy, but otherwise this pipeline will stall waiting for it to complete. Note that even though the Wishbone bus can support pipelined accesses at one access per clock, only the prefetch stage can take advantage of this. Load and Store instructions are stuck at one wishbone cycle per instruction. \end{itemize} \chapter{Peripherals}\label{chap:periph} While the previous chapter describes a CPU in isolation, the Zip System includes a minimum set of peripherals as well. These peripherals are shown in Fig.~\ref{fig:zipsystem} \begin{figure}\begin{center} \includegraphics[width=3.5in]{../gfx/system.eps} \caption{Zip System Peripherals}\label{fig:zipsystem} \end{center}\end{figure} and described here. They are designed to make the Zip CPU more useful in an Embedded Operating System environment. \section{Interrupt Controller} Perhaps the most important peripheral within the Zip System is the interrupt controller. While the Zip CPU itself can only handle one interrupt, and has only the one interrupt state: disabled or enabled, the interrupt controller can make things more interesting. The Zip System interrupt controller module supports up to 15 interrupts, all controlled from one register. Bit~31 of the interrupt controller controls overall whether interrupts are enabled (1'b1) or disabled (1'b0). Bits~16--30 control whether individual interrupts are enabled (1'b0) or disabled (1'b0). Bit~15 is an indicator showing whether or not any interrupt is active, and bits~0--15 indicate whether or not an individual interrupt is active. The interrupt controller has been designed so that bits can be controlled individually without having any knowledge of the rest of the controller setting. To enable an interrupt, write to the register with the high order global enable bit set and the respective interrupt enable bit set. No other bits will be affected. To disable an interrupt, write to the register with the high order global enable bit cleared and the respective interrupt enable bit set. To clear an interrupt, write a 1' to that interrupts status pin. Zero's written to the register have no affect, save that a zero written to the master enable will disable all interrupts. As an example, suppose you wished to enable interrupt \#4. You would then write to the register a {\tt 0x80100010} to enable interrupt \#4 and to clear any past active state. When you later wish to disable this interrupt, you would write a {\tt 0x00100010} to the register. As before, this both disables the interrupt and clears the active indicator. This also has the side effect of disabling all interrupts, so a second write of {\tt 0x80000000} may be necessary to re-enable any other interrupts. The Zip System currently hosts two interrupt controllers, a primary and a secondary. The primary interrupt controller has one interrupt line which may come from an external interrupt controller, and one interrupt line from the secondary controller. Other primary interrupts include the system timers, the jiffies interrupt, and the manual cache interrupt. The secondary interrupt controller maintains an interrupt state for all of the processor accounting counters. \section{Counter} The Zip Counter is a very simple counter: it just counts. It cannot be halted. When it rolls over, it issues an interrupt. Writing a value to the counter just sets the current value, and it starts counting again from that value. Eight counters are implemented in the Zip System for process accounting. This may change in the future, as nothing as yet uses these counters. \section{Timer} The Zip Timer is also very simple: it simply counts down to zero. When it transitions from a one to a zero it creates an interrupt. Writing any non-zero value to the timer starts the timer. If the high order bit is set when writing to the timer, the timer becomes an interval timer and reloads its last start time on any interrupt. Hence, to mark seconds, one might set the timer to 100~million (the number of clocks per second), and set the high bit. Ever after, the timer will interrupt the CPU once per second (assuming a 100~MHz clock). This reload capability also limits the maximum timer value to$2^{31}-1$, rather than$2^{32}-1$. \section{Watchdog Timer} The watchdog timer is no different from any of the other timers, save for one critical difference: the interrupt line from the watchdog timer is tied to the reset line of the CPU. Hence writing a 1' to the watchdog timer will always reset the CPU. To stop the Watchdog timer, write a 0' to it. To start it, write any other number to it---as with the other timers. While the watchdog timer supports interval mode, it doesn't make as much sense as it did with the other timers. \section{Jiffies} This peripheral is motivated by the Linux use of jiffies' whereby a process can request to be put to sleep until a certain number of jiffies' have elapsed. Using this interface, the CPU can read the number of jiffies' from the peripheral (it only has the one location in address space), add the sleep length to it, and write the result back to the peripheral. The zipjiffies peripheral will record the value written to it only if it is nearer the current counter value than the last current waiting interrupt time. If no other interrupts are waiting, and this time is in the future, it will be enabled. (There is currently no way to disable a jiffie interrupt once set, other than to disable the interrupt line in the interrupt controller.) The processor may then place this sleep request into a list among other sleep requests. Once the timer expires, it would write the next Jiffy request to the peripheral and wake up the process whose timer had expired. Indeed, the Jiffies register is nothing more than a glorified counter with an interrupt. Unlike the other counters, the Jiffies register cannot be set. Writes to the jiffies register create an interrupt time. When the Jiffies register later equals the value written to it, an interrupt will be asserted and the register then continues counting as though no interrupt had taken place. The purpose of this register is to support alarm times within a CPU. To set an alarm for a particular process$N$clocks in advance, read the current Jiffies value, and$N\$, and write it back to the Jiffies register.  The
O/S must also keep track of values written to the Jiffies register.  Thus,
when an alarm' trips, it should be removed from the list of alarms, the list
should be sorted, and the next alarm in terms of Jiffies should be written
to the register.

\section{Manual Cache}

The manual cache is an experimental setting that may not remain with the Zip
CPU for very long.  It is designed to facilitate running from FLASH or ROM
memory, although the pipeline prefetch cache really makes this need obsolete.
The manual
cache works by copying data from a wishbone address (range) into the cache
register, and then by making that memory available as memory to the Zip System.
It is a {\em manual cache} because the processor must first specify what
memory to copy, and then once copied the processor can only access the cache
memory by the cache memory location.  There is no transparency.  It is perhaps
best described as a combination DMA controller and local memory.

Worse, this cache is likely going to be removed from the ZipSystem.  Having used
the ZipSystem now for some time, I have yet to find a need or use for the manual
cache.  I will likely replace this peripheral with a proper DMA controller.

\chapter{Operation}\label{chap:ops}

The Zip CPU, and even the Zip System, is not a System on a Chip (SoC).  It
needs to be connected to its operational environment in order to be used.
\begin{enumerate}
\item The Zip System depends upon an external 32-bit Wishbone bus.  This
must exist, and must be connected to the Zip CPU for it to work.
\item The Zip System needs to be told of its {\tt RESET\_ADDRESS}.  This is
the program counter of the first instruction following a reset.
\item If you want the Zip System to start up on its own, you will need to
set the {\tt START\_HALTED} parameter to zero.  Otherwise, if you
wish to manually start the CPU, that is if upon reset you want the
CPU start start in its halted, reset state, then set this parameter to
one.
\item The third parameter to set is the number of interrupts you will be
providing from external to the CPU.  This can be anything from one
to nine, but it cannot be zero.  (Wire this line to a 1'b0 if you
do not wish to support any external interrupts.)
\item Finally, you need to place into some wishbone accessible address, whether
RAM or (more likely) ROM, the initial instructions for the CPU.
\end{enumerate}
If you have enabled your CPU to start automatically, then upon power up the
CPU will immediately start executing your instructions.

This is, however, not how I have used the Zip CPU.  I have instead used the
ZIP CPU in a more controlled environment.  For me, the CPU starts in a
halted state, and waits to be told to start.  Further, the RESET address is a
location in RAM.  After bringing up the board I am using, and further the
bus that is on it, the RAM memory is then loaded externally with the program
I wish the Zip System to run.  Once the RAM is loaded, I release the CPU.
The CPU then runs until its halt condition, at which point its task is
complete.

Eventually, I intend to place an operating system onto the ZipSystem, I'm
just not there yet.

\chapter{Registers}\label{chap:regs}

The ZipSystem registers fall into two categories, ZipSystem internal registers
accessed via the ZipCPU shown in Tbl.~\ref{tbl:zpregs},
\begin{table}[htbp]
\begin{center}\begin{reglist}
PIC   & \scalebox{0.8}{\tt 0xc0000000} & 32 & R/W & Primary Interrupt Controller \\\hline
WDT   & \scalebox{0.8}{\tt 0xc0000001} & 32 & R/W & Watchdog Timer \\\hline
CCHE  & \scalebox{0.8}{\tt 0xc0000002} & 32 & R/W & Manual Cache Controller \\\hline
CTRIC & \scalebox{0.8}{\tt 0xc0000003} & 32 & R/W & Secondary Interrupt Controller \\\hline
TMRA  & \scalebox{0.8}{\tt 0xc0000004} & 32 & R/W & Timer A\\\hline
TMRB  & \scalebox{0.8}{\tt 0xc0000005} & 32 & R/W & Timer B\\\hline
TMRC  & \scalebox{0.8}{\tt 0xc0000006} & 32 & R/W & Timer C\\\hline
JIFF  & \scalebox{0.8}{\tt 0xc0000007} & 32 & R/W & Jiffies \\\hline
MTASK  & \scalebox{0.8}{\tt 0xc0000008} & 32 & R/W & Master Task Clock Counter \\\hline
MMSTL  & \scalebox{0.8}{\tt 0xc0000009} & 32 & R/W & Master Stall Counter \\\hline
MPSTL  & \scalebox{0.8}{\tt 0xc000000a} & 32 & R/W & Master Pre--Fetch Stall Counter \\\hline
MICNT  & \scalebox{0.8}{\tt 0xc000000b} & 32 & R/W & Master Instruction Counter\\\hline
UTASK  & \scalebox{0.8}{\tt 0xc000000c} & 32 & R/W & User Task Clock Counter \\\hline
UMSTL  & \scalebox{0.8}{\tt 0xc000000d} & 32 & R/W & User Stall Counter \\\hline
UPSTL  & \scalebox{0.8}{\tt 0xc000000e} & 32 & R/W & User Pre--Fetch Stall Counter \\\hline
UICNT  & \scalebox{0.8}{\tt 0xc000000f} & 32 & R/W & User Instruction Counter\\\hline
% Cache  & \scalebox{0.8}{\tt 0xc0100000} & & & Base address of the Cache memory\\\hline
\end{reglist}
\caption{Zip System Internal/Peripheral Registers}\label{tbl:zpregs}
\end{center}\end{table}
and the two debug registers shown in Tbl.~\ref{tbl:dbgregs}.
\begin{table}[htbp]
\begin{center}\begin{reglist}
ZIPCTRL & 0 & 32 & R/W & Debug Control Register \\\hline
ZIPDATA & 1 & 32 & R/W & Debug Data Register \\\hline
\end{reglist}
\caption{Zip System Debug Registers}\label{tbl:dbgregs}
\end{center}\end{table}

\section{Peripheral Registers}
The peripheral registers, listed in Tbl.~\ref{tbl:zpregs}, are shown in the
CPU's address space.  These may be accessed by the CPU at these addresses,
and when so accessed will respond as described in Chapt.~\ref{chap:periph}.
These registers will be discussed briefly again here.

The Zip CPU Interrupt controller has four different types of bits, as shown in
Tbl.~\ref{tbl:picbits}.
\begin{table}\begin{center}
\begin{bitlist}
31 & R/W & Master Interrupt Enable\\\hline
30\ldots 16 & R/W & Interrupt Enables, write '1' to change\\\hline
15 & R & Current Master Interrupt State\\\hline
15\ldots 0 & R/W & Input Interrupt states, write '1' to clear\\\hline
\end{bitlist}
\caption{Interrupt Controller Register Bits}\label{tbl:picbits}
\end{center}\end{table}
The high order bit, or bit--31, is the master interrupt enable bit.  When this
bit is set, then any time an interrupt occurs the CPU will be interrupted and
will switch to supervisor mode, etc.

Bits 30~\ldots 16 are interrupt enable bits.  Should the interrupt line go
ghile while enabled, an interrupt will be generated.  To set an interrupt enable
bit, one needs to write the master interrupt enable while writing a 1' to this
the bit.  To clear, one need only write a 0' to the master interrupt enable,
while leaving this line high.

Bits 15\ldots 0 are the current state of the interrupt vector.  Interrupt lines
trip when they go high, and remain tripped until they are acknowledged.  If
the interrupt goes high for longer than one pulse, it may be high when a clear
is requested.  If so, the interrupt will not clear.  The line must go low
again before the status bit can be cleared.

As an example, consider the following scenario where the Zip CPU supports four
interrupts, 3\ldots0.
\begin{enumerate}
\item The Supervisor will first, while in the interrupts disabled mode,
write a {\tt 32'h800f000f} to the controller.  The supervisor may then
switch to the user state with interrupts enabled.
\item When an interrupt occurs, the supervisor will switch to the interrupt
state.  It will then cycle through the interrupt bits to learn which
interrupt handler to call.
\item If the interrupt handler expects more interrupts, it will clear its
current interrupt when it is done handling the interrupt in question.
To do this, it will write a '1' to the low order interrupt mask,
such as writing a {\tt 32'h80000001}.
\item If the interrupt handler does not expect any more interrupts, it will
instead clear the interrupt from the controller by writing a
{\tt 32'h00010001} to the controller.
\item Once all interrupts have been handled, the supervisor will write a
{\tt 32'h80000000} to the interrupt register to re-enable interrupt
generation.
\item The supervisor should also check the user trap bit, and possible soft
interrupt bits here, but this action has nothing to do with the
interrupt control register.
\item The supervisor will then leave interrupt mode, possibly adjusting
whichever task is running, by executing a return from interrupt
command.
\end{enumerate}

Leaving the interrupt controller, we show the timer registers bit definitions
in Tbl.~\ref{tbl:tmrbits}.
\begin{table}\begin{center}
\begin{bitlist}
30\ldots 0 & R/W & Current timer value\\\hline
\end{bitlist}
\caption{Timer Register Bits}\label{tbl:tmrbits}
\end{center}\end{table}
As you may recall, the timer just counts down to zero and then trips an
interrupt.  Writing to the current timer value sets that value, and reading
from it returns that value.  Writing to the current timer value while also
setting the auto--reload bit will send the timer into an auto--reload mode.
In this mode, upon setting its interrupt bit for one cycle, the timer will
also reset itself back to the value of the timer that was written to it when
the auto--reload option was written to it.  To clear and stop the timer,
just simply write a 32'h00' to this register.

The Jiffies register is somewhat similar in that the register always changes.
In this case, the register counts up, whereas the timer always counted down.
Reads from this register, as shown in Tbl.~\ref{tbl:jiffybits},
\begin{table}\begin{center}
\begin{bitlist}
31\ldots 0 & R & Current jiffy value\\\hline
31\ldots 0 & W & Value/time of next interrupt\\\hline
\end{bitlist}
\caption{Jiffies Register Bits}\label{tbl:jiffybits}
\end{center}\end{table}
always return the time value contained in the register.  Writes greater than
the current Jiffy value, that is where the new value minus the old value is
greater than zero while ignoring truncation, will set a new Jiffy interrupt
time.  At that time, the Jiffy vector will clear, and another interrupt time
may either be written to it, or it will just continue counting without
activating any more interrupts.

The Zip CPU also supports several counter peripherals, mostly in the way of
process accounting.  This peripherals have a single register associated with
them, shown in Tbl.~\ref{tbl:ctrbits}.
\begin{table}\begin{center}
\begin{bitlist}
31\ldots 0 & R/W & Current counter value\\\hline
\end{bitlist}
\caption{Counter Register Bits}\label{tbl:ctrbits}
\end{center}\end{table}
Writes to this register set the new counter value.  Reads read the current
counter value.

The current design operation of these counters is that of performance counting.
Two sets of four registers are available for keeping track of performance.
The first is a task counter.  This just counts clock ticks.  The second
counter is a prefetch stall counter, then an master stall counter.  These
allow the CPU to be evaluated as to how efficient it is.  The fourth and
final counter is an instruction counter, which counts how many instructions the
CPU has issued.

It is envisioned that these counters will be used as follows: First, every time
a master counter rolls over, the supervisor (Operating System) will record
the fact.  Second, whenever activating a user task, the Operating System will
set the four user counters to zero.  When the user task has completed, the
Operating System will read the timers back off, to determine how much of the

\section{Debug Port Registers}
Accessing the Zip System via the debug port isn't as straight forward as
accessing the system via the wishbone bus.  The debug port itself has been
reduced to two addresses, as outlined earlier in Tbl.~\ref{tbl:dbgregs}.
Access to the Zip System begins with the Debug Control register, shown in
Tbl.~\ref{tbl:dbgctrl}.
\begin{table}\begin{center}
\begin{bitlist}
31\ldots 14 & R & Reserved\\\hline
13 & R & CPU GIE setting\\\hline
12 & R & CPU is sleeping\\\hline
11 & W & Command clear PF cache\\\hline
10 & R/W & Command HALT, Set to '1' to halt the CPU\\\hline
9 & R & Stall Status, '1' if CPU is busy\\\hline
8 & R/W & Step Command, set to '1' to step the CPU\\\hline
7 & R & Interrupt Request \\\hline
6 & R/W & Command RESET \\\hline
5\ldots 0 & R/W & Debug Register Address \\\hline
\end{bitlist}
\caption{Debug Control Register Bits}\label{tbl:dbgctrl}
\end{center}\end{table}

The first step in debugging access is to determine whether or not the CPU
is halted, and to halt it if not.  To do this, first write a '1' to the
Command HALT bit.  This will halt the CPU and place it into debug mode.
Once the CPU is halted, the stall status bit will drop to zero.  Thus,
if bit 10 is high and bit 9 low, the debug port is open to examine the
internal state of the CPU.

At this point, the external debugger may examine internal state information
from within the CPU.  To do this, first write again to the command register
a value (with command halt still high) containing the address of an internal
register of interest in the bottom 6~bits.  Internal registers that may be
accessed this way are listed in Tbl.~\ref{tbl:dbgaddrs}.
\begin{table}\begin{center}
\begin{reglist}
sR0 & 0 & 32 & R/W & Supervisor Register R0 \\\hline
sR1 & 0 & 32 & R/W & Supervisor Register R1 \\\hline
sSP & 13 & 32 & R/W & Supervisor Stack Pointer\\\hline
sCC & 14 & 32 & R/W & Supervisor Condition Code Register \\\hline
sPC & 15 & 32 & R/W & Supervisor Program Counter\\\hline
uR0 & 16 & 32 & R/W & User Register R0 \\\hline
uR1 & 17 & 32 & R/W & User Register R1 \\\hline
uSP & 29 & 32 & R/W & User Stack Pointer\\\hline
uCC & 30 & 32 & R/W & User Condition Code Register \\\hline
uPC & 31 & 32 & R/W & User Program Counter\\\hline
PIC & 32 & 32 & R/W & Primary Interrupt Controller \\\hline
WDT & 33 & 32 & R/W & Watchdog Timer\\\hline
CCHE & 34 & 32 & R/W & Manual Cache Controller\\\hline
CTRIC & 35 & 32 & R/W & Secondary Interrupt Controller\\\hline
TMRA & 36 & 32 & R/W & Timer A\\\hline
TMRB & 37 & 32 & R/W & Timer B\\\hline
TMRC & 38 & 32 & R/W & Timer C\\\hline
JIFF & 39 & 32 & R/W & Jiffies peripheral\\\hline
MTASK & 40 & 32 & R/W & Master task clock counter\\\hline
MMSTL & 41 & 32 & R/W & Master memory stall counter\\\hline
MPSTL & 42 & 32 & R/W & Master Pre-Fetch Stall counter\\\hline
MICNT & 43 & 32 & R/W & Master instruction counter\\\hline
UTASK & 44 & 32 & R/W & User task clock counter\\\hline
UMSTL & 45 & 32 & R/W & User memory stall counter\\\hline
UPSTL & 46 & 32 & R/W & User Pre-Fetch Stall counter\\\hline
UICNT & 47 & 32 & R/W & User instruction counter\\\hline
\end{reglist}
\end{center}\end{table}
Primarily, these registers'' include access to the entire CPU register
set, as well as the 16~internal peripherals.  To read one of these registers
once the address is set, simply issue a read from the data port.  To write
one of these registers or peripheral ports, simply write to the data port

In this manner, all of the CPU's internal state may be read and adjusted.

As an example of how to use this, consider what would happen in the case
of an external break point.  If and when the CPU hits a break point that
causes it to halt, the Command HALT bit will activate on its own, the CPU
will then raise an external interrupt line and wait for a debugger to examine
its state.  After examining the state, the debugger will need to remove
the breakpoint by writing a different instruction into memory and by writing
to the command register while holding the clear cache, command halt, and
step CPU bits high, (32'hd00).  The debugger may then replace the breakpoint
now that the CPU has gone beyond it, and clear the cache again (32'h500).

To leave this debug mode, simply write a 32'h0' value to the command register.

\chapter{Wishbone Datasheets}\label{chap:wishbone}
The Zip System supports two wishbone ports, a slave debug port and a master
port for the system itself.  These are shown in Tbl.~\ref{tbl:wishbone-slave}
\begin{table}[htbp]
\begin{center}
\begin{wishboneds}
Revision level of wishbone & WB B4 spec \\\hline
Type of interface & Slave, Read/Write, single words only \\\hline
Port size & 32--bit \\\hline
Port granularity & 32--bit \\\hline
Maximum Operand Size & 32--bit \\\hline
Data transfer ordering & (Irrelevant) \\\hline
Clock constraints & Works at 100~MHz on a Basys--3 board\\\hline
Signal Names & \begin{tabular}{ll}
Signal Name & Wishbone Equivalent \\\hline
{\tt i\_clk} & {\tt CLK\_I} \\
{\tt i\_dbg\_cyc} & {\tt CYC\_I} \\
{\tt i\_dbg\_stb} & {\tt STB\_I} \\
{\tt i\_dbg\_we} & {\tt WE\_I} \\
{\tt i\_dbg\_data} & {\tt DAT\_I} \\
{\tt o\_dbg\_ack} & {\tt ACK\_O} \\
{\tt o\_dbg\_stall} & {\tt STALL\_O} \\
{\tt o\_dbg\_data} & {\tt DAT\_O}
\end{tabular}\\\hline
\end{wishboneds}
\caption{Wishbone Datasheet for the Debug Interface}\label{tbl:wishbone-slave}
\end{center}\end{table}
and Tbl.~\ref{tbl:wishbone-master} respectively.
\begin{table}[htbp]
\begin{center}
\begin{wishboneds}
Revision level of wishbone & WB B4 spec \\\hline
Type of interface & Master, Read/Write, single cycle or pipelined\\\hline
Address Width & 32--bit bits \\\hline
Port size & 32--bit \\\hline
Port granularity & 32--bit \\\hline
Maximum Operand Size & 32--bit \\\hline
Data transfer ordering & (Irrelevant) \\\hline
Clock constraints & Works at 100~MHz on a Basys--3 board\\\hline
Signal Names & \begin{tabular}{ll}
Signal Name & Wishbone Equivalent \\\hline
{\tt i\_clk} & {\tt CLK\_O} \\
{\tt o\_wb\_cyc} & {\tt CYC\_O} \\
{\tt o\_wb\_stb} & {\tt STB\_O} \\
{\tt o\_wb\_we} & {\tt WE\_O} \\
{\tt o\_wb\_data} & {\tt DAT\_O} \\
{\tt i\_wb\_ack} & {\tt ACK\_I} \\
{\tt i\_wb\_stall} & {\tt STALL\_I} \\
{\tt i\_wb\_data} & {\tt DAT\_I}
\end{tabular}\\\hline
\end{wishboneds}
\caption{Wishbone Datasheet for the CPU as Master}\label{tbl:wishbone-master}
\end{center}\end{table}
I do not recommend that you connect these together through the interconnect.
Rather, the debug port of the CPU should be accessible regardless of the state
of the master bus.

You may wish to notice that neither the {\tt ERR} nor the {\tt RETRY} wires
have been implemented.  What this means is that the CPU is currently unable
to detect a bus error condition, and so may stall indefinitely (hang) should
it choose to access a value not on the bus, or a peripheral that is not
yet properly configured.

\chapter{Clocks}\label{chap:clocks}

This core is based upon the Basys--3 development board sold by Digilent.
The Basys--3 development board contains one external 100~MHz clock, which is
sufficient to run the ZIP CPU core.
\begin{table}[htbp]
\begin{center}
\begin{clocklist}
i\_clk & External & 100~MHz & 100~MHz & System clock.\\\hline
\end{clocklist}
\caption{List of Clocks}\label{tbl:clocks}
\end{center}\end{table}
I hesitate to suggest that the core can run faster than 100~MHz, since I have
had struggled with various timing violations to keep it at 100~MHz.  So, for
now, I will only state that it can run at 100~MHz.

\chapter{I/O Ports}\label{chap:ioports}
The I/O ports to the Zip CPU may be grouped into three categories.  The first
is that of the master wishbone used by the CPU, then the slave wishbone used
to command the CPU via a debugger, and then the rest.  The first two of these
were already discussed in the wishbone chapter.  They are listed here
for completeness in Tbl.~\ref{tbl:iowb-master}
\begin{table}
\begin{center}\begin{portlist}
{\tt o\_wb\_cyc}   &  1 & Output & Indicates an active Wishbone cycle\\\hline
{\tt o\_wb\_stb}   &  1 & Output & WB Strobe signal\\\hline
{\tt o\_wb\_we}    &  1 & Output & Write enable\\\hline
{\tt o\_wb\_data}  & 32 & Output & Data on WB write\\\hline
{\tt i\_wb\_ack}   &  1 & Input  & Slave has completed a R/W cycle\\\hline
{\tt i\_wb\_stall} &  1 & Input  & WB bus slave not ready\\\hline
{\tt i\_wb\_data}  & 32 & Input  & Incoming bus data\\\hline
\end{portlist}\caption{CPU Master Wishbone I/O Ports}\label{tbl:iowb-master}\end{center}\end{table}
and~\ref{tbl:iowb-slave} respectively.
\begin{table}
\begin{center}\begin{portlist}
{\tt i\_wb\_cyc}   &  1 & Input & Indicates an active Wishbone cycle\\\hline
{\tt i\_wb\_stb}   &  1 & Input & WB Strobe signal\\\hline
{\tt i\_wb\_we}    &  1 & Input & Write enable\\\hline
{\tt i\_wb\_addr}  &  1 & Input & Bus address, command or data port \\\hline
{\tt i\_wb\_data}  & 32 & Input & Data on WB write\\\hline
{\tt o\_wb\_ack}   &  1 & Output  & Slave has completed a R/W cycle\\\hline
{\tt o\_wb\_stall} &  1 & Output  & WB bus slave not ready\\\hline
{\tt o\_wb\_data}  & 32 & Output  & Incoming bus data\\\hline
\end{portlist}\caption{CPU Debug Wishbone I/O Ports}\label{tbl:iowb-slave}\end{center}\end{table}

There are only four other lines to the CPU: the external clock, external
reset, incoming external interrupt line(s), and the outgoing debug interrupt
line.  These are shown in Tbl.~\ref{tbl:ioports}.
\begin{table}
\begin{center}\begin{portlist}
{\tt i\_clk} & 1 & Input & The master CPU clock \\\hline
{\tt i\_rst} & 1 & Input &  Active high reset line \\\hline
{\tt i\_ext\_int} & 1\ldots 6 & Input &  Incoming external interrupts \\\hline
{\tt o\_ext\_int} & 1 & Output & CPU Halted interrupt \\\hline
\end{portlist}\caption{I/O Ports}\label{tbl:ioports}\end{center}\end{table}
The clock line was discussed briefly in Chapt.~\ref{chap:clocks}.  We
typically run it at 100~MHz.  The reset line is an active high reset.  When
asserted, the CPU will start running again from its reset address in
memory.  Further, depending upon how the CPU is configured and specifically on
the {\tt START\_HALTED} parameter, it may or may not start running
automatically.  The {\tt i\_ext\_int} line is for an external interrupt.  This
line may be as wide as 6~external interrupts, depending upon the setting of
the {\tt EXTERNAL\_INTERRUPTS} line.  As currently configured, the ZipSystem
only supports one such interrupt line by default.  For us, this line is the
output of another interrupt controller, but that's a board specific setup
detail.  Finally, the Zip System produces one external interrupt whenever
the CPU halts to wait for the debugger.

% Appendices
% Index
\end{document}

`