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/spec.tex
45,18 → 45,24
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\documentclass{gqtekspec}
\usepackage{import}
\usepackage{bytefield}
% \graphicspath{{../gfx}}
\project{Zip CPU}
\title{Specification}
\author{Dan Gisselquist, Ph.D.}
\email{dgisselq (at) opencores.org}
\revision{Rev.~0.6}
\definecolor{webred}{rgb}{0.2,0,0}
\definecolor{webgreen}{rgb}{0,0.2,0}
\revision{Rev.~0.7}
\definecolor{webred}{rgb}{0.5,0,0}
\definecolor{webgreen}{rgb}{0,0.4,0}
\usepackage[dvips,ps2pdf,colorlinks=true,
anchorcolor=black,pagecolor=webgreen,pdfpagelabels,hypertexnames,
anchorcolor=black,pdfpagelabels,hypertexnames,
pdfauthor={Dan Gisselquist},
pdfsubject={Zip CPU}]{hyperref}
\hypersetup{
colorlinks = true,
linkcolor = webred,
citecolor = webgreen
}
\begin{document}
\pagestyle{gqtekspecplain}
\titlepage
78,6 → 84,7
copy.
\end{license}
\begin{revisionhistory}
0.7 & 12/22/2015 & Gisselquist & New Instruction Set Architecture \\\hline
0.6 & 11/17/2015 & Gisselquist & Added graphics to illustrate pipeline discussion.\\\hline
0.5 & 9/29/2015 & Gisselquist & Added pipelined memory access discussion.\\\hline
0.4 & 9/19/2015 & Gisselquist & Added DMA controller, improved stall information, and self--assessment info.\\\hline
139,11 → 146,11
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.
possible, and the base instructions 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}
158,8 → 165,11
\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}
{\bf Decode}, {\bf Read-Operand}, a
combined stage containing the {\bf ALU},
{\bf Memory}, {\bf Divide}, and {\bf Floating Point}
units, and then the final {\bf Write-back} stage.
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}
183,7 → 193,7
 
Most other approaches to soft core CPU's employ a Harvard architecture.
This allows these other CPU's to have two separate bus structures: one for the
program fetch, and the other for thememory. The Zip CPU is fairly unique in
program fetch, and the other for the memory. The Zip CPU is fairly unique in
its approach because it uses a von Neumann architecture. This was done for
simplicity. By using a von Neumann architecture, only one bus needs to be
implemented within any FPGA. This helps to minimize real-estate, while
191,12 → 201,13
degrade the overall instructions per clock count.
 
Soft core's within an FPGA have an additional characteristic regarding
memory access: it is slow. Memory on chip may be accessed at a single
cycle per access, but small FPGA's have a limited amount of memory on chip.
Going off chip, however, is expensive. Two examples will prove this point. On
memory access: it is slow. While memory on chip may be accessed at a single
cycle per access, small FPGA's often have only a limited amount of memory on
chip. Going off chip, however, is expensive. Two examples will prove this
point. On
the XuLA2 board, Flash can be accessed at 128~cycles per 32--bit word,
or 64~cycles per subsequent word in a pipelined architecture. Likewise, the
SDRAM chip on the XuLA2 board allows 6~cycle access for a write, 10~cycles
SDRAM chip on the XuLA2 board allows a 6~cycle access for a write, 10~cycles
per read, and 2~cycles for any subsequent pipelined access read or write.
Either way you look at it, this memory access will be slow and this doesn't
account for any logic delays should the bus implementation logic get
222,11 → 233,13
be created on chip to support the SwiC if necessary. As an example, a simple
30-bit peripheral could easily support reversing 30-bit numbers: a read from
the peripheral returns it's bit--reversed address. This is cheap within an
FPGA, but expensive in instructions.
FPGA, but expensive in instructions. Reading from another 16--bit peripheral
might calculate a sine function, where the 16--bit address internal to the
peripheral was the angle of the sine wave.
 
Indeed, anything that must be done fast within an FPGA is likely to already
be done--elsewhere in the fabric. This leaves the CPU with the role of handling
sequential tasks that need a lot of state.
be done--elsewhere in the fabric. This leaves the CPU with the simple role
of solely handling sequential tasks that need a lot of state.
 
This means that the SwiC needs to live within a very unique environment,
separate and different from the traditional SoC. That isn't to say that a
263,17 → 276,30
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 Prefetch Cache:} My original implementation, {\tt prefetch}, 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.
 
My second implementation, {\tt pipefetch}, attempted to make the most
use of pipelined memory. When a new CPU address was issued, it would
start reading
memory in a pipelined fashion, and issuing instructions as soon as they
were ready. This cache was a sliding window in memory. This suffered
from some difficult performance problems, though. If the CPU was
alternating between two diverse sections of code, both could never be
in the cache at the same time--causing lots of cache misses. Further,
the extra logic to implement this window cost an extra clock cycle
in the cache implementation, slowing down branches.
 
The Zip CPU now has a third cache implementation, {\tt pfcache}. This
new implementation takes only a single cycle per access, but costs a
full cache line miss on any miss. While configurable, a full cache
line miss might mean that the CPU needs to read 256~instructions from
memory before it can execute the first one of them.
 
\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,
302,6 → 328,19
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 Bus Errors:} My original implementation had no logic to handle
what would happen if the CPU attempted to read or write a non-existent
memory address. This changed after I needed to troubleshoot a failure
caused by a subroutine return to a non-existent address.
 
My next problem bus problem was caused by a misbehaving peripheral.
Whenever the CPU attempted to read from or write to this peripheral,
the peripheral would take control of the wishbone bus and not return
it. For example, it might never return an {\tt ACK} to signal
the end of the bus transaction. This led to the implementation of
a wishbone bus watchdog that would create a bus error if any particular
bus action didn't complete in a timely fashion.
 
\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
394,6 → 433,8
\includegraphics[width=4in]{../gfx/stacking.eps}
\caption{Instructions can stack up behind a stalled instruction}\label{fig:stacking}
\end{center}\end{figure}
However, if a pipeline hazard is detected, a stage can stall in order
to prevent the previous from moving forward.
 
This approach is also different from other pipeline approaches.
Instead of keeping the entire pipeline filled, each stage is treated
400,26 → 441,6
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
452,7 → 473,8
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.
than this convention. Also by convention register~12 will point to a global
offset table, and may be abbreviated as the (GBL) register.
The CPU can access both register sets via move instructions from the
supervisor state, whereas the user state can only access the user registers.
 
460,8 → 482,10
Fig.~\ref{tbl:cc-register},
\begin{table}\begin{center}
\begin{bitlist}
31\ldots 11 & R/W & Reserved for future uses\\\hline
10 & R & (Reserved for) Bus-Error Flag\\\hline
31\ldots 13 & R/W & Reserved for future uses\\\hline
12 & R & (Reserved for) Floating Point Exception\\\hline
11 & R & Division by Zero Exception\\\hline
10 & R & Bus-Error Flag\\\hline
9 & R & Trap, or user interrupt, Flag. Cleared on return to userspace.\\\hline
8 & R & Illegal Instruction Flag\\\hline
7 & R/W & Break--Enable\\\hline
484,11 → 508,21
Carry (C),
Negative (N),
and Overflow (V).
On those instructions that set the flags, these flags will be set based upon
the output of the instruction. If the result is zero, the Z flag will be set.
If the high order bit is set, the N flag will be set. If the instruction
caused a bit to fall off the end, the carry bit will be set. Finally, if
the instruction causes a signed integer overflow, the V flag will be set
afterwards.
 
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 next bit is a sleep bit. Set this bit to one to disable instruction
execution and place the CPU to sleep, or to zero to keep the pipeline
running. Setting this bit will cause the CPU to wait for an interrupt
(if interrupts are enabled), or to completely halt (if interrupts are
disabled). In order to prevent users from halting the CPU, only the
supervisor is allowed to both put the CPU to sleep and disable
interrupts. Any user attempt to do so will simply result in a switch
to supervisor mode.
 
The sixth bit is a global interrupt enable bit (GIE). When this
sixth bit is a `1' interrupts will be enabled, else disabled. When
501,13 → 535,12
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.
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
539,56 → 572,204
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 & Illegal instruction error flag \\\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
\section{Instruction Format}
All Zip CPU instructions fit in one of the formats shown in
Fig.~\ref{fig:iset-format}.
\begin{figure}\begin{center}
\begin{bytefield}[endianness=big]{32}
\bitheader{0-31}\\
\begin{leftwordgroup}{Standard}\bitbox{1}{0}\bitbox{4}{DR}
\bitbox[lrt]{5}{OpCode}
\bitbox[lrt]{3}{Cnd}
\bitbox{1}{0}
\bitbox{18}{18-bit Signed Immediate} \\
\bitbox{1}{0}\bitbox{4}{DR}
\bitbox[lrb]{5}{}
\bitbox[lrb]{3}{}
\bitbox{1}{1}
\bitbox{4}{BR}
\bitbox{14}{14-bit Signed Immediate}\end{leftwordgroup} \\
\begin{leftwordgroup}{MOV}\bitbox{1}{0}\bitbox{4}{DR}
\bitbox[lrt]{5}{5'hf}
\bitbox[lrt]{3}{Cnd}
\bitbox{1}{A}
\bitbox{4}{BR}
\bitbox{1}{B}
\bitbox{13}{13-bit Signed Immediate}\end{leftwordgroup} \\
\begin{leftwordgroup}{LDI}\bitbox{1}{0}\bitbox{4}{DR}
\bitbox{4}{4'hb}
\bitbox{23}{23-bit Signed Immediate}\end{leftwordgroup} \\
\begin{leftwordgroup}{NOOP}\bitbox{1}{0}\bitbox{3}{3'h7}
\bitbox{1}{}
\bitbox{2}{11}
\bitbox{3}{xxx}
\bitbox{22}{Ignored}
\end{leftwordgroup} \\
\begin{leftwordgroup}{VLIW}\bitbox{1}{1}\bitbox[lrt]{4}{DR}
\bitbox[lrt]{5}{OpCode}
\bitbox[lrt]{3}{Cnd}
\bitbox{1}{0}
\bitbox{4}{Imm.}
\bitbox{14}{---} \\
\bitbox{1}{1}\bitbox[lr]{4}{}
\bitbox[lrb]{5}{}
\bitbox[lr]{3}{}
\bitbox{1}{1}
\bitbox{4}{BR}
\bitbox{14}{---} \\
\bitbox{1}{1}\bitbox[lrb]{4}{}
\bitbox{4}{4'hb}
\bitbox{1}{}
\bitbox[lrb]{3}{}
\bitbox{5}{5'b Imm}
\bitbox{14}{---} \\
\bitbox{1}{1}\bitbox{9}{---}
\bitbox[lrt]{3}{Cnd}
\bitbox{5}{---}
\bitbox[lrt]{4}{DR}
\bitbox[lrt]{5}{OpCode}
\bitbox{1}{0}
\bitbox{4}{Imm}
\\
\bitbox{1}{1}\bitbox{9}{---}
\bitbox[lr]{3}{}
\bitbox{5}{---}
\bitbox[lr]{4}{}
\bitbox[lrb]{5}{}
\bitbox{1}{1}
\bitbox{4}{Reg} \\
\bitbox{1}{1}\bitbox{9}{---}
\bitbox[lrb]{3}{}
\bitbox{5}{---}
\bitbox[lrb]{4}{}
\bitbox{4}{4'hb}
\bitbox{1}{}
\bitbox{5}{5'b Imm}
\end{leftwordgroup} \\
\end{bytefield}
\caption{Zip Instruction Set Format}\label{fig:iset-format}
\end{center}\end{figure}
The basic format is that some operation, defined by the OpCode, is applied
if a condition, Cnd, is true in order to produce a result which is placed in
the destination register, or DR. The Load 23--bit signed immediate instruction
is different in that it requires no conditions, and uses only a 4-bit opcode.
 
This is actually a second version of instruction set definition, given certain
lessons learned. For example, the original instruction set had the following
problems:
\begin{enumerate}
\item No opcodes were available for divide or floating point extensions to be
made available. Although there was space in the instruction set to
add these types of instructions, this instruction space was going to
require extra logic to use.
\item The carveouts for instructions such as NOOP and LDIHI/LDILO required
extra logic to process.
\item The instruction set wasn't very compact. One bus operation was required
for every instruction.
\end{enumerate}
This second version was designed with two criteria. The first was that the
new instruction set needed to be compatible, at the assembly language level,
with the previous instruction set. Thus, it must be able to support all of
the previous menumonics and more. This was achieved with the sole exception
that instruction immediates are generally two bits shorter than before.
(One bit was lost to the VLIW bit in front, another from changing from 4--bit
to 5--bit opcodes.) Second, the new instruction set needed to be a drop--in
replacement for the decoder, modifying nothing else. This was almost achieved,
save for two issues: the ALU unit needed to be replaced since the OpCodes
were reordered, and some condition code logic needed to be adjusted since the
condition codes were renumbered as well. In the end, maximum reuse of the
existing RTL (Verilog) code was achieved in this upgrade.
 
As of this second version of the Zip CPU instruction set, the Zip CPU also
supports a very long instruction word (VLIW) set of instructions. These
instruction formats pack two instructions into a single instuction word,
trading immediate instruction space to do this, but in just about all other
respects these are identical to two standard instructions. Other than
instruction format, the only basic difference is that the CPU will not switch
to interrupt mode in between the two instructions. Likewise a new job given
to the assembler is that of automatically packing as many instructions as
possible into the VLIW format. Where necessary to place both VLIW instructions
on the same line, they will be separated by a vertical bar.
 
\section{Instruction OpCodes}
With a 5--bit opcode field, there are 32--possible instructions as shown in
Tbl.~\ref{tbl:iset-opcodes}.
\begin{table}\begin{center}
\begin{tabular}{|l|l|l|c|} \hline \rowcolor[gray]{0.85}
OpCode & & Instruction &Sets CC \\\hline\hline
5'h00 & SUB & Subtract & \\\cline{1-3}
5'h01 & AND & Bitwise And & \\\cline{1-3}
5'h02 & ADD & Add two numbers & \\\cline{1-3}
5'h03 & OR & Bitwise Or & Y \\\cline{1-3}
5'h04 & XOR & Bitwise Exclusive Or & \\\cline{1-3}
5'h05 & LSR & Logical Shift Right & \\\cline{1-3}
5'h06 & LSL & Logical Shift Left & \\\cline{1-3}
5'h07 & ASR & Arithmetic Shift Right & \\\hline
5'h08 & LDIHI & Load Immediate High & N \\\cline{1-3}
5'h09 & LDILO & Load Immediate Low & \\\hline
5'h0a & MPYU & Unsigned 16--bit Multiply & \\\cline{1-3}
5'h0b & MPYS & Signed 16--bit Multiply & Y \\\cline{1-3}
5'h0c & BREV & Bit Reverse & \\\cline{1-3}
5'h0d & POPC& Population Count & \\\cline{1-3}
5'h0e & ROL & Rotate left & \\\hline
5'h0f & MOV & Move register & N \\\hline
5'h10 & CMP & Compare & Y \\\cline{1-3}
5'h11 & TST & Test (AND w/o setting result) & \\\hline
5'h12 & LOD & Load from memory & N \\\cline{1-3}
5'h13 & STO & Store a register into memory & \\\hline\hline
5'h14 & DIVU & Divide, unsigned & Y \\\cline{1-3}
5'h15 & DIVS & Divide, signed & \\\hline\hline
5'h16/7 & LDI & Load 23--bit signed immediate & N \\\hline\hline
5'h18 & FPADD & Floating point add & \\\cline{1-3}
5'h19 & FPSUB & Floating point subtract & \\\cline{1-3}
5'h1a & FPMPY & Floating point multiply & Y \\\cline{1-3}
5'h1b & FPDIV & Floating point divide & \\\cline{1-3}
5'h1c & FPCVT & Convert integer to floating point & \\\cline{1-3}
5'h1d & FPINT & Convert to integer & \\\hline
5'h1e & & {\em Reserved for future use} &\\\hline
5'h1f & & {\em Reserved for future use} &\\\hline
\end{tabular}
\caption{Condition Code / Status Register Bits}\label{tbl:ccbits}
\caption{Zip CPU OpCodes}\label{tbl:iset-opcodes}
\end{center}\end{table}
%
Of these opcodes, the {\tt BREV} and {\tt POPC} are experimental, and may be
replaced later, and two floating point instruction opcodes are reserved for
future use.
 
\section{Conditional Instructions}
Most, although not quite all, instructions may be conditionally executed. From
the four condition code flags, eight conditions are defined. These are shown
in Tbl.~\ref{tbl:conditions}.
\begin{table}
\begin{center}
Most, although not quite all, instructions may be conditionally executed.
The 23--bit load immediate instruction, together with the {\tt NOOP},
{\tt BREAK}, and {\tt LOCK} instructions are the only exception to this rule.
 
From the four condition code flags, eight conditions are defined for standard
instructions. 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'h1 & {\tt .LT} & Less than ('N' set) \\
3'h2 & {\tt .Z} & Only execute when 'Z' is set \\
3'h3 & {\tt .NZ} & Only execute when 'Z' is not set \\
3'h4 & {\tt .GT} & Greater than ('N' not set, 'Z' not set) \\
3'h5 & {\tt .LT} & Less than ('N' set) \\
3'h5 & {\tt .GE} & Greater than or equal ('N' not set, 'Z' irrelevant) \\
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. Conditioning on a non--supported condition
is still possible, but it will take an extra instruction and a pipeline stall. (Ex: \hbox{\em (Stall)}; \hbox{\tt TST \$4,CC;} \hbox{\tt STO.NZ R0,(R1)})
As an alternative, the condition may often be reversed, recovering those
extra two clocks. Thus instead of \hbox{\tt CMP Rx,Ry;}
\hbox{\tt BNV label} you can issue a \hbox{\tt CMP Ry,Rx;} \hbox{\tt BV label}.
\end{center}\end{table}
There is no condition code for less than or equal, not C or not V---there
just wasn't enough space in 3--bits. Conditioning on a non--supported
condition is still possible, but it will take an extra instruction and a
pipeline stall. (Ex: \hbox{\em (Stall)}; \hbox{\tt TST \$4,CC;} \hbox{\tt
STO.NZ R0,(R1)}) As an alternative, it is often possible to reverse the
condition, and thus recovering those extra two clocks. Thus instead of
\hbox{\tt CMP Rx,Ry;} \hbox{\tt BNV label} you can issue a
\hbox{\tt CMP Ry,Rx;} \hbox{\tt BV label}.
 
Conditionally executed ALU instructions will not further adjust the
Conditionally executed instructions will not further adjust the
condition codes, with the exception of \hbox{\tt CMP} and \hbox{\tt TST}
instructions. Conditional \hbox{\tt CMP} or \hbox{\tt TST} instructions
will adjust conditions whenever their conditionals are true. In this way,
will adjust conditions whenever they are executed. In this way,
multiple conditions may be evaluated without branches. For example, to do
something if \hbox{\tt R0} is one and \hbox{\tt R1} is two, one might try
code such as Tbl.~\ref{tbl:dbl-condition}.
606,32 → 787,56
\caption{An example of a double conditional}\label{tbl:dbl-condition}
\end{center}\end{table}
 
\section{Traditional Interrupt Handling}
In the case of VLIW instructions, only four conditions are defined as shown
in Tbl.~\ref{tbl:vliw-conditions}.
\begin{table}\begin{center}
\begin{tabular}{l|l|l}
Code & Mneumonic & Condition \\\hline
2'h0 & None & Always execute the instruction \\
2'h1 & {\tt .LT} & Less than ('N' set) \\
2'h2 & {\tt .Z} & Only execute when 'Z' is set \\
2'h3 & {\tt .NZ} & Only execute when 'Z' is not set \\
\end{tabular}
\caption{VLIW Conditions}\label{tbl:vliw-conditions}
\end{center}\end{table}
Further, the first bit is given a special meaning. If the first bit is set,
the conditions apply to the second half of the instruction, otherwise the
conditions will only apply to the first half of a conditional instruction.
 
\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}.
Many instruction forms have a 19-bit source ``Operand B'' associated with them.
This ``Operand B'' is shown in Fig.~\ref{fig:iset-format} as part of the
standard instructions. This Operand B is either equal to a register plus a
14--bit signed immediate offset, or an 18--bit signed 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}
\begin{bytefield}[endianness=big]{19}
\bitheader{0-18} \\
\bitbox{1}{0}\bitbox{18}{18-bit Signed Immediate} \\
\bitbox{1}{1}\bitbox{4}{Reg}\bitbox{14}{14-bit Signed Immediate}
\end{bytefield}
\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.
Fourteen and eighteen bit immediate values don't make sense for all
instructions. For example, what is the point of an 18--bit immediate when
executing a 16--bit multiply? Or a 16--bit load--immediate? In these cases,
the extra bits are simply ignored.
 
VLIW instructions still use the same operand B, only there was no room for any
instruction plus immediate addressing. Therefore, VLIW instructions have either
a register or a 4--bit signed immediate as their operand B. The only exception
is the load immediate instruction, which permits a 5--bit signed operand
B.\footnote{Although the space exists to extend this VLIW load immediate
instruction to six bits, the 5--bit limit was chosen to simplify the
disassembler. This may change in the future.}
 
\section{Address Modes}
The Zip CPU supports two addressing modes: register plus immediate, and
immediate address. Addresses are therefore encoded in the same fashion as
Operand B's, shown above.
Operand B's, shown above. Practically, the VLIW instruction set only offers
register addressing, necessitating a non--VLIW instruction for most memory
operations.
 
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
651,206 → 856,105
\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.
indicating which register set each register lies within are the A-User, marked
`A' in Fig.~\ref{fig:iset-format}, and B-User bits, marked as `B'. 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
This actually leads to a bit of a problem: since the {\tt 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.
Anything with the user bit set will be treated as a user register and displayed
special. Since the CPU quietly ignores the supervisor bits while in user mode,
anything marked as a user register will always be specific.
 
\section{Multiply Operations}
The Zip CPU supports two Multiply operations, a 16x16 bit signed multiply
({\tt MPYS}) and a 16x16 bit unsigned multiply ({\tt 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.
({\tt MPYS}) and a 16x16 bit unsigned multiply ({\tt MPYU}). A 32--bit
multiply, should it be desired, needs to be created via software from this
16x16 bit multiply.
 
\section{Floating Point}
The Zip CPU does not (yet) support floating point operations. However, the
instruction set reserves two possibilities for future floating point
operations.
\section{Divide Unit}
The Zip CPU also has a divide unit which can be built alongside the ALU.
This divide unit provides the Zip CPU with its first two instructions that
cannot be executed in a single cycle: {\tt DIVS}, or signed divide, and
{\tt DIVU}, the unsigned divide. These are both 32--bit divide instructions,
dividing one 32--bit number by another. In this case, the Operand B field,
whether it be register or register plus immediate, constitutes the denominator,
whereas the numerator is given by the other register.
 
The first floating point operation hole in the instruction set involves
setting a proposed (but non-existent) floating point bit in the CC register.
The next instruction
would then 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 Divide is also a multi--clock instruction. While the divide is running,
the ALU, memory unit, and floating point unit (if installed) will be idle.
Once the divide completes, other units may continue.
 
The other possibility for floating point operations involves exploiting the
hole in the instruction set that the NOOP and BREAK instructions reside within.
These two instructions use 24--bits of address space, when only a single bit
is necessary. A simple adjustment 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.
Of course, divides can have errors: division by zero. In the case of division
by zero, an exception will be caused that will send the CPU either from
user mode to supervisor mode, or halt the CPU if it is already in supervisor
mode.
 
In both cases, the Zip CPU would support 32--bit single precision floats
only, since other choices would complicate the pipeline.
\section{NOOP, BREAK, and Bus Lock Instruction}
Three instructions are not listed in the opcode list in
Tbl.~\ref{tbl:iset-opcodes}, yet fit in the NOOP type instruction format of
Fig.~\ref{fig:iset-format}. These are the {\tt NOOP}, {\tt Break}, and
bus {\tt LOCK} instructions. These are encoded according to
Fig.~\ref{fig:iset-noop}, and have the following meanings:
\begin{figure}\begin{center}
\begin{bytefield}[endianness=big]{32}
\bitheader{0-31}\\
\begin{leftwordgroup}{NOOP}
\bitbox{1}{0}\bitbox{3}{3'h7}\bitbox{1}{}
\bitbox{2}{11}\bitbox{3}{001}\bitbox{22}{Ignored} \\
\bitbox{1}{1}\bitbox{3}{3'h7}\bitbox{1}{}
\bitbox{2}{11}\bitbox{3}{001}\bitbox{22}{---} \\
\bitbox{1}{1}\bitbox{9}{---}\bitbox{3}{---}\bitbox{5}{---}
\bitbox{3}{3'h7}\bitbox{1}{}\bitbox{2}{11}\bitbox{3}{001}
\bitbox{5}{Ignored}
\end{leftwordgroup} \\
\begin{leftwordgroup}{BREAK}
\bitbox{1}{0}\bitbox{3}{3'h7}
\bitbox{1}{}\bitbox{2}{11}\bitbox{3}{010}\bitbox{22}{Ignored}
\end{leftwordgroup} \\
\begin{leftwordgroup}{LOCK}
\bitbox{1}{0}\bitbox{3}{3'h7}
\bitbox{1}{}\bitbox{2}{11}\bitbox{3}{100}\bitbox{22}{Ignored}
\end{leftwordgroup} \\
\end{bytefield}
\caption{NOOP/Break/LOCK Instruction Format}\label{fig:iset-noop}
\end{center}\end{figure}
 
The current architecture does not support a floating point not-implemented
interrupt. Any soft floating point emulation must be done deliberately.
The {\tt NOOP} instruction is just that: an instruction that does not perform
any operation. While many other instructions, such as a move from a register to
itself, could also fit these roles, only the NOOP instruction guarantees that
it will not stall waiting for a register to be available. For this reason,
it gets its own place in the instruction set.
 
\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
\rowcolor[gray]{0.85}
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\hline
{\tt CMP(Sub)} & \multicolumn{4}{l|}{4'h0}
& \multicolumn{4}{l|}{D. Reg}
& \multicolumn{3}{l|}{Cond.}
& \multicolumn{21}{l|}{Operand B}
& Yes \\\hline
{\tt TST(And)} & \multicolumn{4}{l|}{4'h1}
& \multicolumn{4}{l|}{D. Reg}
& \multicolumn{3}{l|}{Cond.}
& \multicolumn{21}{l|}{Operand B}
& Yes \\\hline
{\tt 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
{\tt LODI} & \multicolumn{4}{l|}{4'h3}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{24}{l|}{24'bit Signed Immediate}
& \\\hline
{\tt NOOP} & \multicolumn{4}{l|}{4'h4}
& \multicolumn{4}{l|}{4'he}
& \multicolumn{24}{l|}{24'h00}
& \\\hline
{\tt BREAK} & \multicolumn{4}{l|}{4'h4}
& \multicolumn{4}{l|}{4'he}
& \multicolumn{24}{l|}{24'h01}
& \\\hline
{\em Reserved} & \multicolumn{4}{l|}{4'h4}
& \multicolumn{4}{l|}{4'he}
& \multicolumn{24}{l|}{24'bits, but not 0 or 1.}
& \\\hline
{\tt 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
{\tt 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 {\tt 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 {\tt 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 {\tt 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 {\tt 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
{\tt 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
{\tt LOD} & \multicolumn{4}{l|}{4'h6}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{3}{l|}{Cond.}
& \multicolumn{21}{l|}{Operand B address}
& \\\hline
{\tt STO} & \multicolumn{4}{l|}{4'h7}
& \multicolumn{4}{l|}{D. Reg}
& \multicolumn{3}{l|}{Cond.}
& \multicolumn{21}{l|}{Operand B address}
& \\\hline
{\tt SUB} & \multicolumn{4}{l|}{4'h8}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{3}{l|}{Cond.}
& \multicolumn{21}{l|}{Operand B}
& Yes \\\hline
{\tt AND} & \multicolumn{4}{l|}{4'h9}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{3}{l|}{Cond.}
& \multicolumn{21}{l|}{Operand B}
& Yes \\\hline
{\tt ADD} & \multicolumn{4}{l|}{4'ha}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{3}{l|}{Cond.}
& \multicolumn{21}{l|}{Operand B}
& Yes \\\hline
{\tt OR} & \multicolumn{4}{l|}{4'hb}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{3}{l|}{Cond.}
& \multicolumn{21}{l|}{Operand B}
& Yes \\\hline
{\tt XOR} & \multicolumn{4}{l|}{4'hc}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{3}{l|}{Cond.}
& \multicolumn{21}{l|}{Operand B}
& Yes \\\hline
{\tt 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
{\tt 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
{\tt 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}
The {\tt BREAK} instruction is useful for creating a debug instruction that
will halt the CPU without executing. If in user mode, depending upon the
setting of the break enable bit, it will either switch to supervisor mode or
halt the CPU--depending upon where the user wishes to do his debugging.
 
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. The rest of this space is reserved for future enhancements.
Finally, the {\tt LOCK} instruction was added in order to make a test and
set multi--CPU operation possible. Following a LOCK instruction, the next
two instructions, if they are memory LOD/STO instructions, will execute without
dropping the wishbone {\tt CYC} line between the instructions. Thus a
{\tt LOCK} followed by {\tt LOD (Rx),Ry} and a {\tt STO Rz,(Rx)}, where Rz
is initially set, can be used to set an address while guaranteeing that Ry
was the value before setting the address to Rz. This is a useful instruction
while trying to achieve concurrency among multiple CPU's.
 
\section{Floating Point}
Although the Zip CPU does not (yet) have a floating point unit, the current
instruction set offers eight opcodes for floating point operations, and treats
floating point exceptions like divide by zero errors. Once this unit is built
and integrated together with the rest of the CPU, the Zip CPU will support
32--bit floating point instructions natively. Any 64--bit floating point
instructions will still need to be emulated in software.
 
\section{Derived Instructions}
The Zip CPU supports many other common instructions, but not all of them
are single cycle instructions. The derived instruction tables,
870,7 → 974,7
& Add with carry \\\hline
{\tt BRA.Cond +/-\$Addr}
& \hbox{\tt MOV.cond \$Addr+PC,PC}
& Branch or jump on condition. Works for 15--bit
& Branch or jump on condition. Works for 13--bit
signed address offsets.\\\hline
{\tt BRA.Cond +/-\$Addr}
& \parbox[t]{1.5in}{\tt LDI \$Addr,Rx \\ ADD.cond Rx,PC}
896,7 → 1000,7
& {\tt Or \$SLEEP,CC}
& This only works when issued in interrupt/supervisor mode. In user
mode this is simply a wait until interrupt instruction. \\\hline
{\tt INT } & {\tt LDI \$0,CC} & \\\hline
{\tt INT } & {\tt LDI \$0,CC} & This is also known as a trap instruction\\\hline
{\tt IRET}
& {\tt OR \$GIE,CC}
& Also known as an RTU instruction (Return to Userspace) \\\hline
903,40 → 1007,36
{\tt JMP R6+\$Addr}
& {\tt MOV \$Addr(R6),PC}
& \\\hline
{\tt JSR PC+\$Addr}
& \parbox[t]{1.5in}{\tt 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. This could easily be turned into a three instruction
operand, removing the preliminary stack instruction before and
the cleanup after, by adjusting how any stack frame was built for
this routine to include space at the top of the stack for the PC.
Note also that jumping to a subroutine costs a copy register, {\tt R0}
in this case.
\\\hline
{\tt LJMP \$Addr}
& \parbox[t]{1.5in}{\tt LOD (PC),PC \\ {\em Address }}
& Although this only works for an unconditional jump, and it only
works in a Von Neumann architecture, this instruction combination makes
for a nice combination that can be adjusted by a linker at a later
time.\\\hline
{\tt JSR PC+\$Addr }
& \parbox[t]{1.5in}{\tt 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
& \parbox[t]{1.5in}{\tt MOV \$1+PC,R0 \\ MOV \$addr+PC,PC}
& This is similar to the jump and link instructions from other
architectures, save only that it requires a specific link
instruction, also known as the {\tt MOV} instruction on the
left.\\\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
{\tt LDI.l \$val,Rx }
& \parbox[t]{1.8in}{\tt LDIHI (\$val$>>$16)\&0x0ffff, Rx \\
LDILO (\$val\&0x0ffff),Rx}
& Sadly, there's not enough instruction
& \parbox[t]{3.0in}{Sadly, there's not enough instruction
space to load a complete immediate value into any register.
Therefore, fully loading any register takes two cycles.
The LDIHI (load immediate high) and LDILO (load immediate low)
instructions have been created to facilitate this. \\\hline
\end{tabular}
\caption{Derived Instructions}\label{tbl:derived-1}
\end{center}\end{table}
\begin{table}\begin{center}
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
Mapped & Actual & Notes \\\hline
instructions have been created to facilitate this.
\\
This is also the appropriate means for setting a register value
to an arbitrary 32--bit value in a post--assembly link
operation.}\\\hline
{\tt LOD.b \$addr,Rx}
& \parbox[t]{1.5in}{\tt %
LDI \$addr,Ra \\
983,12 → 1083,8
instruction. \\\hline
{\tt NOT Rx } & {\tt XOR \$-1,Rx } & \\\hline
{\tt POP Rx }
& \parbox[t]{1.5in}{\tt LOD \$1(SP),Rx \\ ADD \$1,SP}
& Note
that for interrupt purposes, one can never depend upon the value at
(SP). Hence you read from it, then increment it, lest having
incremented it first something then comes along and writes to that
value before you can read the result. \\\hline
& \parbox[t]{1.5in}{\tt LOD \$(SP),Rx \\ ADD \$1,SP}
& \\\hline
\end{tabular}
\caption{Derived Instructions, continued}\label{tbl:derived-2}
\end{center}\end{table}
995,16 → 1091,18
\begin{table}\begin{center}
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
{\tt PUSH Rx}
& \parbox[t]{1.5in}{SUB \$1,SP \\
STO Rx,\$1(SP)}
& \parbox[t]{1.5in}{\hbox{\tt SUB \$1,SP}
\hbox{\tt STO Rx,\$(SP)}}
& Note that for pipelined operation, it helps to coalesce all the
{\tt SUB}'s into one command, and place the {\tt STO}'s right
after each other.\\\hline
after each other. Further, to avoid a pipeline stall, the
immediate value for the store must be zero.
\\\hline
{\tt PUSH Rx-Ry}
& \parbox[t]{1.5in}{\tt SUB \$n,SP \\
STO Rx,\$n(SP)
& \parbox[t]{1.5in}{\tt SUB \$$n$,SP \\
STO Rx,\$(SP)
\ldots \\
STO Ry,\$1(SP)}
STO Ry,\$$\left(n-1\right)$(SP)}
& Multiple pushes at once only need the single subtract from the
stack pointer. This derived instruction is analogous to a similar one
on the Motoroloa 68k architecture, although the Zip Assembler
1013,16 → 1111,13
{\tt RESET}
& \parbox[t]{1in}{\tt STO \$1,\$watchdog(R12)\\NOOP\\NOOP}
& This depends upon the peripheral base address being
in R12.
preloaded into R12.
 
Another opportunity might be to jump to the reset address from within
supervisor mode.\\\hline
{\tt RET} & \parbox[t]{1.5in}{\tt LOD \$1(SP),PC}
& Note that this depends upon the calling context to clean up the
stack, as outlined for the JSR instruction. \\\hline
{\tt RET} & {\tt 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.
{\tt RET} & {\tt MOV R0,PC}
& This depends upon the form of the {\tt JSR} given on the previous
page that stores the return address into R0.
\\\hline
{\tt STEP Rr,Rt}
& \parbox[t]{1.5in}{\tt LSR \$1,Rr \\ XOR.C Rt,Rr}
1051,12 → 1146,14
Further, this instruction implies a byte ordering,
such as big or little endian.} \\\hline
{\tt SWAP Rx,Ry }
& \parbox[t]{1.5in}{\tt
XOR Ry,Rx \\
XOR Rx,Ry \\
XOR Ry,Rx}
& \parbox[t]{1.5in}{\tt XOR Ry,Rx \\ XOR Rx,Ry \\ XOR Ry,Rx}
& While no extra registers are needed, this example
does take 3-clocks. \\\hline
\end{tabular}
\caption{Derived Instructions, continued}\label{tbl:derived-3}
\end{center}\end{table}
\begin{table}\begin{center}
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
{\tt TRAP \#X}
& \parbox[t]{1.5in}{\tt LDI \$x,R0 \\ AND \~\$GIE,CC }
& This works because whenever a user lowers the \$GIE flag, it sets
1066,17 → 1163,23
the LDI and the AND conditional. In that case, the assembler would
quietly turn the LDI instruction into an LDILO and LDIHI pair,
but the effect would be the same. \\\hline
\end{tabular}
\caption{Derived Instructions, continued}\label{tbl:derived-3}
\end{center}\end{table}
\begin{table}\begin{center}
\begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline
{\tt TS Rx,Ry,(Rz)}
& \hbox{\tt LDI 1,Rx}
\hbox{\tt LOCK}
\hbox{\tt LOD (Rz),Ry}
\hbox{\tt STO Rx,(Rz)}
& A test and set instruction. The {\tt LOCK} instruction insures
that the next two instructions lock the bus between the instructions,
so no one else can use it. Thus guarantees that the operation is
atomic.
\\\hline
{\tt TST Rx}
& {\tt 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
of Rx. (Future versions of the assembler may shorten this to a
{\tt TST Rx} instruction.)\\\hline
{\tt WAIT}
& {\tt Or \$GIE | \$SLEEP,CC}
& Wait until the next interrupt, then jump to supervisor/interrupt
1084,6 → 1187,17
\end{tabular}
\caption{Derived Instructions, continued}\label{tbl:derived-4}
\end{center}\end{table}
 
\section{Interrupt Handling}
The Zip CPU does not maintain any interrupt vector tables. If an interrupt
takes place, the CPU simply switches to interrupt mode. The supervisor code
continues in this interrupt mode from where it left off before, after
executing a return to userspace {\tt RTU} instruction.
 
At this point, the supervisor code needs to determine first whether an
interrupt has occurred, and then whether it is in interrupt mode due to
an exception and handle each case appropriately.
 
\section{Pipeline Stages}
As mentioned in the introduction, and highlighted in Fig.~\ref{fig:cpu},
the Zip CPU supports a five stage pipeline.
1094,13 → 1208,13
ever changes. Stalls are also created here if the instruction isn't
in the prefetch cache.
 
The Zip CPU supports one of two prefetch methods, depending upon a flag
set at build time within the {\tt zipcpu.v} file. The simplest is a
non--cached implementation of a prefetch. This implementation is
fairly small, and ideal for
users of the Zip CPU who need the extra space on the FPGA fabric.
However, because this non--cached version has no cache, the maximum
number of instructions per clock is limited to about one per five.
The Zip CPU supports one of three prefetch methods, depending upon a
flag set at build time within the {\tt cpudefs.v} file. The simplest
is a non--cached implementation of a prefetch. This implementation is
fairly small, and ideal for users of the Zip CPU who need the extra
space on the FPGA fabric. However, because this non--cached version
has no cache, the maximum number of instructions per clock is limited
to about one per five.
 
The second prefetch module is a pipelined prefetch with a cache. This
module tries to keep the instruction address within a window of valid
1110,33 → 1224,47
feature, though, was that it needs an extra internal pipeline stage
to be implemented.
 
\item {\bf Decode}: Decodes an 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.
The third prefetch and cache module implements a more traditional cache.
While the resulting code tends to be twice as fast as the pipelined
cache architecture, this implementation uses a large amount of
distributed FPGA RAM to be successful. This then inflates the Zip CPU's
FPGA usage statistics.
 
\item {\bf Decode}: Decodes an instruction into OpCode, 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 handles {\tt LOD} (load)
and {\tt STO} (store) instructions.
 
\item Split into one of four tracks: An {\bf ALU} track which will accomplish
a simple instruction, the {\bf MemOps} stage which handles {\tt LOD}
(load) and {\tt STO} (store) instructions, the {\bf divide} unit,
and the {\bf floating point} unit.
\begin{itemize}
\item Loads will stall the entire pipeline until complete.
\item Condition codes are available upon completion of the ALU stage
\item Issuing an instruction to the memory unit while the memory unit
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.
\item Loads will stall instructions in the decode stage until the
entire pipeline until complete, lest a register be read in
the read operands stage only to be updated unseen by the
Load.
\item Condition codes are available upon completion of the ALU,
divide, or FPU stage.
\item Issuing a non--pipelined memory instruction to the memory unit
while the memory unit is busy will stall the entire pipeline.
\end{itemize}
\item {\bf Write-Back}: Conditionally write back the result to the register
set, applying the condition. This routine is bi-entrant: either the
memory or the simple instruction may request a register write.
set, applying the condition. This routine is quad-entrant: either the
ALU, the memory, the divide, or the FPU may write back a register.
The only design rule is that no more than a single register may be
written back in any given clock.
\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 (loads) cannot.
unit stalls, every other instruction stalls. The same is true for divide or
floating point instructions--all other instructions will stall while waiting
for these to complete. Memory stores, however, can take place concurrently
with non--memory operations, although memory reads (loads) cannot.
 
\section{Pipeline Stalls}
The processing pipeline can and will stall for a variety of reasons. Some of
1145,37 → 1273,38
\item When the prefetch cache is exhausted
 
This reason 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.
instruction in memory, the entire pipeline must stall until an instruction
can be made ready. In the case of the {\tt pipefetch} windowed approach
to the prefetch cache, this means the pipeline will stall until enough of the
prefetch cache is loaded to support the next instruction. In the case
of the more traditional {\tt pfcache} approach, the entire cache line must
fill before instruction execution can continue.
 
\item While waiting for the pipeline to load following any taken branch, jump,
return from interrupt or switch to interrupt context (5 stall cycles)
return from interrupt or switch to interrupt context (4 stall cycles)
 
Fig.~\ref{fig:bcstalls}
\begin{figure}\begin{center}
\includegraphics[width=3.5in]{../gfx/bc.eps}
\caption{A conditional branch generates 5 stall cycles}\label{fig:bcstalls}
\caption{A conditional branch generates 4 stall cycles}\label{fig:bcstalls}
\end{center}\end{figure}
illustrates the situation for a conditional branch. In this case, the branch
instruction, {\tt BC}, is nominally followed by instructions {\tt I0} and so
instruction, {\tt BC}, is nominally followed by instructions {\tt I1} and so
forth. However, since the branch is taken, the next instruction must be
{\tt IA}. Therefore, the pipeline needs to be cleared and reloaded.
Given that there are five stages to the pipeline, that accounts
for four of the five stalls. The last stall cycle is lost in the pipelined
prefetch stage which needs at least one clock with a valid PC before it can
produce a new output. {\Large\bf Note: When I did this myself, I counted
six stall cycles, for a total of seven cycles for this instruction. Is five
really the right answer?}
for the four stalls. (Were the {\tt pipefetch} cache chosen, there would
be another stall internal to the {\tt pipefetch} cache.)
 
The Zip CPU handles {\tt MOV \$X(PC),PC}, {\tt ADD \$X,PC}, and
{\tt LDI \$X,PC} instructions specially, however. These instructions, when
not conditioned on the flags, can execute with only 2~stall cycles, such as
is shown in Fig.~\ref{fig:branch}.\footnote{Note that this behavior is
slated to be improved upon in subsequent releases. With a better prefetch,
it should be possible to drop this down to one or zero stall cycles.}
not conditioned on the flags, can execute with only a single stall cycle,
such as is shown in Fig.~\ref{fig:branch}.\footnote{Note that when using the
{\tt pipefetch} cache, this requires an additional stall cycle due to that
cache's implementation.}
\begin{figure}\begin{center}
\includegraphics[width=4in]{../gfx/bra.eps}
\caption{An expedited delay costs only 2~stall cycles}\label{fig:branch}
\includegraphics[width=4in]{../gfx/bra.eps} %0.4in per clock
\caption{An expedited branch costs a single stall cycle}\label{fig:branch}
\end{center}\end{figure}
In this example, {\tt BR} is a branch always taken, {\tt I1} is the instruction
following the branch in memory, while {\tt IA} is the first instruction at the
1197,33 → 1326,20
That is, any instruction that does not add an immediate to {\tt RA} may be
scheduled into the stall slot.
 
\item When any (conditional) write to either the CC or PC Register is followed
by a memory operation
This is also the reason why, when setting up a stack frame, the top of the
stack frame is used first: it eliminates this stall cycle. Hence, to save
registers at the top of a procedure, one would write:
\begin{enumerate}
\item\ {\tt OPCODE RA,PC} {\em Ex: a branch opcode}
\item\ {\em (stall, even if jump not taken)}
\item\ {\tt LOD \$X(RA),RB}
\item\ {\tt SUB 2,SP}
\item\ {\tt STO R1,(SP)}
\item\ {\tt STO R2,1(SP)}
\end{enumerate}
A timing diagram of this pipeline situation is shown in Fig.~\ref{fig:bcmem},
\begin{figure}\begin{center}
\includegraphics[width=2in]{../gfx/bcmem.eps}
\caption{A (not taken) conditional branch followed by a memory operation}\label{fig:bcmem}
\end{center}\end{figure}
for a conditional branch, {\tt BC}, a memory operation, {\tt Mem} (which
must be a load here), and ALU instructions {\tt I1} and so forth.
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--forcing the
memory operation to stay in the operand decode stage. This prevents
an instruction from executing a memory access after the jump but before the
jump is recognized.
Had {\tt R1} instead been stored at {\tt 1(SP)} as the top of the stack,
there would've been an extra stall in setting up the stack frame.
 
This stall may be mitigated by shuffling the operations immediately following
a potential branch so that an ALU operation follows the branch instead of a
memory operation.
 
\item When reading from the CC register after setting the flags
\begin{enumerate}
\item\ {\tt ALUOP RA,RB} {\em Ex: a compare opcode}
\item\ {\tt ALUOP RA,RB} {\em ; Ex: a compare opcode}
\item\ {\em (stall)}
\item\ {\tt TST sys.ccv,CC}
\item\ {\tt BZ somewhere}
1243,7 → 1359,7
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, while a {\tt LDI \$BREAKEN|\$STEP,CC} will not since
it doesn't read the condition codes.
it doesn't read the condition codes before executing.
 
\item When waiting for a memory read operation to complete
\begin{enumerate}
1257,17 → 1373,20
stall until the memory unit is cleared. This is illustrated in
Fig.~\ref{fig:memrd},
\begin{figure}\begin{center}
\includegraphics[width=5in]{../gfx/memrd.eps}
\includegraphics[width=5.6in]{../gfx/memrd.eps}
\caption{Pipeline handling of a load instruction}\label{fig:memrd}
\end{center}\end{figure}
since it is especially true of a load
instruction, which must still write its operand back to the register file.
Note that there is an extra stall at the end of the memory cycle, so that
the memory unit will be idle for one clock before an instruction will be
accepted into the ALU.
Store instructions are different, as shown in Fig.~\ref{fig:memwr},
instruction, which must still write its operand back to the register file.
Further, note that on a pipelined memory operation, the instruction must
stall in the decode operand stage, lest it try to read a result from the
register file before the load result has been written to it. Finally, note
that there is an extra stall at the end of the memory cycle, so that
the memory unit will be idle for two clocks before an instruction will be
accepted into the ALU. Store instructions are different, as shown in
Fig.~\ref{fig:memwr},
\begin{figure}\begin{center}
\includegraphics[width=5in]{../gfx/memwr.eps}
\includegraphics[width=4in]{../gfx/memwr.eps}
\caption{Pipeline handling of a store instruction}\label{fig:memwr}
\end{center}\end{figure}
since they can be busy with the bus without impacting later write back
1310,21 → 1429,6
Fig.~\ref{fig:memrd} and Fig.~\ref{fig:memwr} illustrated pipelined memory
accesses.
 
\item When waiting for a conditional memory read operation to complete
\begin{enumerate}
\item\ {\tt LOD.Z address,RA}
\item\ {\em (multiple stalls, bus dependent, 7 clocks best)}
\item\ {\tt OPCODE I+RA,RB}
\end{enumerate}
 
In this case, the Zip CPU doesn't warn the prefetch cache to get off the bus
two cycles before using the bus, so there's a potential for an extra three
cycle cost due to bus contention between the prefetch and the CPU.
 
This is true for both the LOD and the STO instructions, with the exception that
the STO instruction will continue in parallel with any ALU instructions that
follow it.
 
\end{itemize}
 
 
1373,12 → 1477,12
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 (perhaps
more if you configure it for more) 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.
secondary. The primary interrupt controller has one (or more) interrupt line(s)
which may come from an external interrupt source, 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}
 
1427,7 → 1531,8
then read from its port to find out which memory location created the problem.
 
Aside from its unusual configuration, the bus watchdog is just another
implementation of the fundamental timer described above.
implementation of the fundamental timer described above--stripped down
for simplicity.
 
\section{Jiffies}
 
1435,7 → 1540,8
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
sleep length to it, and write the result back to the peripheral. The
{\tt 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.
1457,8 → 1563,8
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.
should be resorted, and the next alarm in terms of Jiffies should be written
to the register--possibly for a second time.
 
\section{Direct Memory Access Controller}
 
1469,22 → 1575,19
any DMA memory move will by nature be faster than a corresponding program
accomplishing the same move. To put this to numbers, it may take a program
18~clocks per word transferred, whereas this DMA controller can move one
word in two clocks--provided it has bus access. (The CPU gets priority over the
bus.)
word in two clocks--provided it has bus access. (The CPU gets priority over
the bus.)
 
When copying memory from one location to another, the DMA controller will
copy in units of a given transfer length--up to 1024 words at a time. It will
read that transfer length into its internal buffer, and then write to the
destination address from that buffer. If the CPU interrupts a DMA transfer,
it will release the bus, let the CPU complete whatever it needs to do, and then
restart its transfer by writing the contents of its internal buffer and then
re-entering its read cycle again.
destination address from that buffer.
 
When coupled with a peripheral, the DMA controller can be configured to start
a memory copy on an interrupt line going high. Further, the controller can be
configured to issue reads from (or to) the same address instead of incrementing
the address at each clock. The DMA completes once the total number of items
specified (not the transfer length) have been transferred.
a memory copy when any interrupt line going high. Further, the controller can
be configured to issue reads from (or to) the same address instead of
incrementing the address at each clock. The DMA completes once the total
number of items specified (not the transfer length) have been transferred.
 
In each case, once the transfer is complete and the DMA unit returns to
idle, the DMA will issue an interrupt.
1500,20 → 1603,29
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 To conserve logic, you'll want to set the {\tt ADDRESS\_WIDTH} parameter
to the number of address bits on your wishbone bus.
\item Likewise, the {\tt LGICACHE} parameter sets the number of bits in
the instruction cache address. This means that the instruction cache
will have $2^{\mbox{\tiny\tt LGICACHE}}$ locations within it.
\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.
one. This latter configuration is useful for a CPU that should be
idle (i.e. halted) until given an explicit instruction from somewhere
else to start.
\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.)
to sixteen, but it cannot be zero. (Set this to 1 and wire the single
interrupt 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.
CPU will immediately start executing your instructions, starting at the given
{\tt RESET\_ADDRESS}.
 
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
1521,8 → 1633,8
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.
The CPU then runs until either its halt condition or an exception occurrs in
supervisor mode, at which point its task is complete.
 
Eventually, I intend to place an operating system onto the ZipSystem, I'm
just not there yet.
1651,9 → 1763,9
\> {\tt MOV uR0,R0} \\
\> {\tt MOV uCC,R1} \\
\> {\tt MOV uPC,R2} \\
\> {\tt STO R0,1(R3)} {\em ; Exploit memory pipelining: }\\
\> {\tt STO R1,2(R3)} {\em ; All instructions write to stack }\\
\> {\tt STO R2,3(R3)} {\em ; All offsets increment by one }\\
\> {\tt STO R0,(R3)} {\em ; Exploit memory pipelining: }\\
\> {\tt STO R1,1(R3)} {\em ; All instructions write to stack }\\
\> {\tt STO R2,2(R3)} {\em ; All offsets increment by one }\\
\> {\tt MOV R3,uSP} {\em ; Return the updated stack pointer } \\
\end{tabbing}
\caption{Example Saving Minimal User Context}\label{tbl:save-partial}
1694,9 → 1806,9
RESTORE\_PARTIAL\_CONTEXT: \\
\hbox to 0.25in{}\= {\em ; We retore R0, CC, and PC only} \\
\> {\tt MOV uSP,R3} {\em ; Return the updated stack pointer } \\
\> {\tt LOD R0,1(R3),R0} {\em ; Exploit memory pipelining: }\\
\> {\tt LOD R1,2(R3),R1} {\em ; All instructions write to stack }\\
\> {\tt LOD R2,3(R3),R2} {\em ; All offsets increment by one }\\
\> {\tt LOD R0,(R3),R0} {\em ; Exploit memory pipelining: }\\
\> {\tt LOD R1,1(R3),R1} {\em ; All instructions write to stack }\\
\> {\tt LOD R2,2(R3),R2} {\em ; All offsets increment by one }\\
\> {\tt MOV R0,uR0} \\
\> {\tt MOV R1,uCC} \\
\> {\tt MOV R2,uPC} \\
1756,19 → 1868,29
\begin{table}\begin{center}
\begin{tabular}{ll}
memcp: \\
& {\em ; R0 = *dest, R1 = *src, R2 = LEN} \\
& {\em ; The following will operate in 17 clocks per word minus one clock} \\
& {\tt CMP 0,R2} \\
& {\tt LOD.Z -1(SP),PC} {\em ; A conditional return }\\
& {\em ; (One stall on potentially writing to PC)} \\
& {\tt LOD (R1),R3} \\
& {\em ; R0 = *dest, R1 = *src, R2 = LEN, R3 = return addr} \\
& {\em ; The following will operate in $12N+19$ clocks.} \\
& {\tt CMP 0,R2} \\ % 8 clocks per setup
& {\tt MOV.Z R3,PC} {\em ; A conditional return }\\
& {\tt SUB 1,SP} {\em ; Create a stack frame}\\
& {\tt STO R4,(SP)} {\em ; and a local variable}\\
& {\em ; (4 stalls, cannot be further scheduled away)} \\
loop: \\ % 12 clocks per loop
& {\tt LOD (R1),R4} \\
& {\em ; (4 stalls, cannot be scheduled away)} \\
& {\tt STO R3,(R2)} {\em ; (4 schedulable stalls, has no impact now)} \\
& {\tt STO R4,(R0)} {\em ; (4 schedulable stalls, has no impact now)} \\
& {\tt SUB 1,R2} \\
& {\tt BZ memcpend} \\
& {\tt ADD 1,R0} \\
& {\tt ADD 1,R1} \\
& {\tt SUB 1,R2} \\
& {\tt BNZ loop} \\
& {\em ; (5 stalls, if branch taken, to clear and refill the pipeline)} \\
& {\tt RET} \\
& {\tt BRA loop} \\
& {\em ; (1 stall on a BRA instruction)} \\
memcpend: % 11 clocks
& {\tt LOD (SP),R4} \\
& {\em ; (4 stalls, cannot be further scheduled away)} \\
& {\tt ADD 1,SP} \\
& {\tt JMP R3} \\
& {\em ; (4 stalls)} \\
\end{tabular}
\caption{Example Memory Copy code in Zip Assembly}\label{tbl:memcp-asm}
\end{center}\end{table}
1788,9 → 1910,11
ways. The first step is that an interrupt happens. Anytime an interrupt
happens, the CPU needs to execute the following tasks in supervisor mode:
\begin{enumerate}
\item Check for a trap instruction. That is, if the user task requested a
trap, we may not wish to adjust the context, check interrupts, or call
the scheduler. Tbl.~\ref{tbl:trap-check}
\item Check for a trap instruction, or other user exception such as a break,
bus error, division by zero error, or floating point exception. That
is, if the user process needs attending then we may not wish to adjust
the context, check interrupts, or call the scheduler.
Tbl.~\ref{tbl:trap-check}
\begin{table}\begin{center}
\begin{tabular}{ll}
{\tt return\_to\_user:} \\
1800,13 → 1924,13
& {\tt RTU} \\
{\tt trap\_check:} \\
& {\tt MOV uCC,R0} \\
& {\tt TST \$TRAP,R0} \\
& {\tt TST \$TRAP \textbar \$BUSERR \textbar \$DIVE \textbar \$FPE,R0} \\
& {\tt BNZ swap\_out} \\
& {; \em Do something here to execute the trap} \\
& {; \em Don't need to call the scheduler, so we can just return} \\
& {\tt BRA return\_to\_user} \\
\end{tabular}
\caption{Checking for whether the user issued a TRAP instruction}\label{tbl:trap-check}
\caption{Checking for whether the user task needs our attention}\label{tbl:trap-check}
\end{center}\end{table}
shows the rudiments of this code, while showing nothing of how the
actual trap would be implemented.
1836,11 → 1960,11
& {\tt MOV uR2,R2} \\
& {\tt MOV uR3,R3} \\
& {\tt MOV uR4,R4} \\
& {\tt STO R0,1(R5)} {\em ; Exploit memory pipelining: }\\
& {\tt STO R1,2(R5)} {\em ; All instructions write to stack }\\
& {\tt STO R2,3(R5)} {\em ; All offsets increment by one }\\
& {\tt STO R3,4(R5)} {\em ; Longest pipeline is 5 cycles.}\\
& {\tt STO R4,5(R5)} \\
& {\tt STO R0,(R5)} {\em ; Exploit memory pipelining: }\\
& {\tt STO R1,1(R5)} {\em ; All instructions write to stack }\\
& {\tt STO R2,2(R5)} {\em ; All offsets increment by one }\\
& {\tt STO R3,3(R5)} {\em ; Longest pipeline is 5 cycles.}\\
& {\tt STO R4,4(R5)} \\
& \ldots {\em ; Need to repeat for all user registers} \\
\iffalse
& {\tt MOV uR5,R0} \\
1848,11 → 1972,11
& {\tt MOV uR7,R2} \\
& {\tt MOV uR8,R3} \\
& {\tt MOV uR9,R4} \\
& {\tt STO R0,6(R5) }\\
& {\tt STO R1,7(R5) }\\
& {\tt STO R2,8(R5) }\\
& {\tt STO R3,9(R5) }\\
& {\tt STO R4,10(R5)} \\
& {\tt STO R0,5(R5) }\\
& {\tt STO R1,6(R5) }\\
& {\tt STO R2,7(R5) }\\
& {\tt STO R3,8(R5) }\\
& {\tt STO R4,9(R5)} \\
\fi
& {\tt MOV uR10,R0} \\
& {\tt MOV uR11,R1} \\
1859,11 → 1983,11
& {\tt MOV uR12,R2} \\
& {\tt MOV uCC,R3} \\
& {\tt MOV uPC,R4} \\
& {\tt STO R0,11(R5)}\\
& {\tt STO R1,12(R5)}\\
& {\tt STO R2,13(R5)}\\
& {\tt STO R3,14(R5)}\\
& {\tt STO R4,15(R5)} \\
& {\tt STO R0,10(R5)}\\
& {\tt STO R1,11(R5)}\\
& {\tt STO R2,12(R5)}\\
& {\tt STO R3,13(R5)}\\
& {\tt STO R4,14(R5)} \\
& {\em ; We can skip storing the stack, uSP, since it'll be stored}\\
& {\em ; elsewhere (in the task structure) }\\
\end{tabular}
1963,11 → 2087,11
& {\tt LOD stack(R12),R5} \\
& {\tt MOV 15(R1),uSP} \\
& {\em ; Be sure to exploit the memory pipelining capability} \\
& {\tt LOD 1(R5),R0} \\
& {\tt LOD 2(R5),R1} \\
& {\tt LOD 3(R5),R2} \\
& {\tt LOD 4(R5),R3} \\
& {\tt LOD 5(R5),R4} \\
& {\tt LOD (R5),R0} \\
& {\tt LOD 1(R5),R1} \\
& {\tt LOD 2(R5),R2} \\
& {\tt LOD 3(R5),R3} \\
& {\tt LOD 4(R5),R4} \\
& {\tt MOV R0,uR0} \\
& {\tt MOV R1,uR1} \\
& {\tt MOV R2,uR2} \\
1974,11 → 2098,11
& {\tt MOV R3,uR3} \\
& {\tt MOV R4,uR4} \\
& \ldots {\em ; Need to repeat for all user registers} \\
& {\tt LOD 11(R5),R0} \\
& {\tt LOD 12(R5),R1} \\
& {\tt LOD 13(R5),R2} \\
& {\tt LOD 14(R5),R3} \\
& {\tt LOD 15(R5),R4} \\
& {\tt LOD 10(R5),R0} \\
& {\tt LOD 11(R5),R1} \\
& {\tt LOD 12(R5),R2} \\
& {\tt LOD 13(R5),R3} \\
& {\tt LOD 14(R5),R4} \\
& {\tt MOV R0,uR10} \\
& {\tt MOV R1,uR11} \\
& {\tt MOV R2,uR12} \\
2016,7 → 2140,7
\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
& \scalebox{0.8}{\tt 0xc0000002} & 32 & R/W & {\em (Reserved for future use)} \\\hline
& \scalebox{0.8}{\tt 0xc0000002} & 32 & R & Address of last bus error \\\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
2053,14 → 2177,15
and when so accessed will respond as described in Chapt.~\ref{chap:periph}.
These registers will be discussed briefly again here.
 
\subsection{Interrupt Controller(s)}
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
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
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}
2069,10 → 2194,11
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.
hi while enabled, an interrupt will be generated. (All interrupts are positive
edge triggered.) 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
2091,13 → 2217,13
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}.
To do this, it will write a `1' to the low order interrupt mask,
such as writing a {\tt 32'h0000\_0001}.
\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.
{\tt 32'h0001\_0001} 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
{\tt 32'h8000\_0000} 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
2107,6 → 2233,8
command.
\end{enumerate}
 
\subsection{Timer Register}
 
Leaving the interrupt controller, we show the timer registers bit definitions
in Tbl.~\ref{tbl:tmrbits}.
\begin{table}\begin{center}
2125,6 → 2253,8
the auto--reload option was written to it. To clear and stop the timer,
just simply write a `32'h00' to this register.
 
\subsection{Jiffies}
 
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},
2142,6 → 2272,8
may either be written to it, or it will just continue counting without
activating any more interrupts.
 
\subsection{Performance Counters}
 
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}.
2167,8 → 2299,12
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
CPU the process had consumed.
CPU the process had consumed. To keep this accurate, the user counters will
only increment when the GIE bit is set to indicate that the processor is
in user mode.
 
\subsection{DMA Controller}
 
The final peripheral to discuss is the DMA controller. This controller
has four registers. Of these four, the length, source and destination address
registers should need no further explanation. They are full 32--bit registers
2183,15 → 2319,15
31 & R & DMA Active\\\hline
30 & R & Wishbone error, transaction aborted. This bit is cleared the next time
this register is written to.\\\hline
29 & R/W & Set to '1' to prevent the controller from incrementing the source address, '0' for normal memory copy. \\\hline
28 & R/W & Set to '1' to prevent the controller from incrementing the
destination address, '0' for normal memory copy. \\\hline
29 & R/W & Set to `1' to prevent the controller from incrementing the source address, `0' for normal memory copy. \\\hline
28 & R/W & Set to `1' to prevent the controller from incrementing the
destination address, `0' for normal memory copy. \\\hline
27 \ldots 16 & W & The DMA Key. Write a 12'hfed to these bits to start the
activate any DMA transfer. \\\hline
27 & R & Always reads '0', to force the deliberate writing of the key. \\\hline
27 & R & Always reads `0', to force the deliberate writing of the key. \\\hline
26 \ldots 16 & R & Indicates the number of items in the transfer buffer that
have yet to be written. \\\hline
15 & R/W & Set to '1' to trigger on an interrupt, or '0' to start immediately
15 & R/W & Set to `1' to trigger on an interrupt, or `0' to start immediately
upon receiving a valid key.\\\hline
14\ldots 10 & R/W & Select among one of 32~possible interrupt lines.\\\hline
9\ldots 0 & R/W & Intermediate transfer length minus one. Thus, to transfer
2221,14 → 2357,15
Tbl.~\ref{tbl:dbgctrl}.
\begin{table}\begin{center}
\begin{bitlist}
31\ldots 14 & R & Reserved\\\hline
31\ldots 14 & R & External interrupt state. Bit 14 is valid for one
interrupt only, bit 15 for two, etc.\\\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, also sets the halt bit\\\hline
7 & R & Interrupt Request \\\hline
10 & R/W & Command HALT, Set to `1' to halt the CPU\\\hline
9 & R & Stall Status, `1' if CPU is busy (i.e., not halted yet)\\\hline
8 & R/W & Step Command, set to `1' to step the CPU, also sets the halt bit\\\hline
7 & R & Interrupt Request Pending\\\hline
6 & R/W & Command RESET \\\hline
5\ldots 0 & R/W & Debug Register Address \\\hline
\end{bitlist}
2236,7 → 2373,7
\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
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
2261,6 → 2398,7
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
BUS & 34 & 32 & R & Last Bus Error\\\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
2314,7 → 2452,8
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
Clock constraints & Works at 100~MHz on a Basys--3 board, and 80~MHz on a
XuLA2--LX25\\\hline
Signal Names & \begin{tabular}{ll}
Signal Name & Wishbone Equivalent \\\hline
{\tt i\_clk} & {\tt CLK\_I} \\
2336,12 → 2475,13
\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
Address Width & (Zip System parameter, can be up to 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
Clock constraints & Works at 100~MHz on a Basys--3 board, and 80~MHz on a
XuLA2--LX25\\\hline
Signal Names & \begin{tabular}{ll}
Signal Name & Wishbone Equivalent \\\hline
{\tt i\_clk} & {\tt CLK\_O} \\
2352,7 → 2492,8
{\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}
{\tt i\_wb\_data} & {\tt DAT\_I} \\
{\tt i\_wb\_err} & {\tt ERR\_I}
\end{tabular}\\\hline
\end{wishboneds}
\caption{Wishbone Datasheet for the CPU as Master}\label{tbl:wishbone-master}
2361,11 → 2502,12
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.
You may wish to notice that neither the {\tt LOCK} nor the {\tt RTY} (retry)
wires have been connected to the CPU's master interface. If necessary, a
rudimentary {\tt LOCK} may be created by tying the wire to the {\tt wb\_cyc}
line. As for the {\tt RTY}, all the CPU recognizes at this point are bus
errors---it cannot tell the difference between a temporary and a permanent bus
error.
 
\chapter{Clocks}\label{chap:clocks}
 
2383,6 → 2525,7
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.
 
On a SPARTAN 6, the clock can run successfully at 80~MHz.
 
\chapter{I/O Ports}\label{chap:ioports}
The I/O ports to the Zip CPU may be grouped into three categories. The first
2400,6 → 2543,7
{\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
{\tt i\_wb\_err} & 1 & Input & Bus Error indication\\\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}
2421,21 → 2565,21
\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 i\_ext\_int} & 1\ldots 16 & Input & Incoming external interrupts, actual
value set by implementation parameter \\\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
typically run it at 100~MHz, although we've needed to slow it down to 80~MHz
for some implementations. 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.
memory. Further, depending upon how the CPU is configured and specifically
based upon how the {\tt START\_HALTED} parameter is set, the CPU may or may
not start running automatically following a reset. The {\tt i\_ext\_int}
line is for an external interrupt. This line may actually be as wide as
16~external interrupts, depending upon the setting of
the {\tt EXTERNAL\_INTERRUPTS} parameter. Finally, the Zip System produces one
external interrupt whenever the entire CPU halts to wait for the debugger.
 
\chapter{Initial Assessment}\label{chap:assessment}
 
2446,11 → 2590,15
 
\section{The Good}
\begin{itemize}
\item The Zip CPU is light weight and fully featured as it exists today. For
anyone who wishes to build a general purpose CPU and then to
experiment with building and adding particular features, the Zip CPU
makes a good starting point--it is fairly simple. Modifications should
be simple enough.
\item The Zip CPU can be configured to be relatively light weight and fully
featured as it exists today. For anyone who wishes to build a general
purpose CPU and then to experiment with building and adding particular
features, the Zip CPU makes a good starting point--it is fairly simple.
Modifications should be simple enough. Indeed, a non--pipelined
version of the bare ZipBones (with no peripherals) has been built that
only uses 1.1k~LUTs. When using pipelining, the full cache, and all
of the peripherals, the ZipSystem can top 5~k LUTs. Where it fits
in between is a function of your needs.
\item The Zip CPU was designed to be an implementable soft core that could be
placed within an FPGA, controlling actions internal to the FPGA. It
fits this role rather nicely. It does not fit the role of a system on
2488,39 → 2636,6
 
\section{The Not so Good}
\begin{itemize}
\item While one of the stated goals was to use a small amount of logic,
3k~LUTs isn't that impressively small. Indeed, it's really much
too expensive when compared against other 8 and 16-bit CPUs that have
less than 1k LUTs.
 
Still, \ldots it's not bad, it's just not astonishingly good.
 
\item The fact that the instruction width equals the bus width means that the
instruction fetch cycle will always be interfering with any load or
store memory operation, with the only exception being if the
instruction is already in the cache.
 
This could be fixed in one of three ways: the instruction set
architecture could be modified to handle Very Long Instruction Words
(VLIW) so that each 32--bit word would encode two or more instructions,
the instruction fetch bus width could be increased from 32--bits to
64--bits or more, or the instruction bus could be separated from the
data bus. Any and all of these approaches would increase the overall
LUT count.
 
\item The (non-existant) floating point unit was an after-thought, isn't even
built as a potential option, and most likely won't support the full
IEEE standard set of FPU instructions--even for single point precision.
This (non-existant) capability would benefit the most from an
out-of-order execution capability, which the Zip CPU does not have.
 
Still, sharing FPU registers with the main register set was a good
idea and worth preserving, as it simplifies context swapping.
 
Perhaps this really isn't a problem, but rather a feature. By not
implementing FPU instructions, the Zip CPU maintains a lower LUT count
than it would have if it did implement these instructions.
 
\item The CPU has no character support. This is both good and bad.
Realistically, the CPU works just fine without it. Characters can be
supported as subsets of 32-bit words without any problem. Practically,
2553,11 → 2668,6
flexibility in its immediate operand mode, although that increased
flexibility isn't necessarily as valuable as one might like.
 
\item The Zip CPU does not currently detect and trap on either illegal
instructions or bus errors. Attempts to access non--existent
memory quietly return erroneous results, rather than halting the
process (user mode) or halting or resetting the CPU (supervisor mode).
 
\item The Zip CPU doesn't support out of order execution. I suppose it could
be modified to do so, but then it would no longer be the ``simple''
and low LUT count CPU it was designed to be. The two primary results
2583,190 → 2693,18
shot. My dream of having binutils and gcc support has not been
realized and at this rate may not be realized. (I've been intimidated
by the challenge everytime I've looked through those codes.)
 
\iffalse
\item While the Wishbone Bus (B4) supports a pipelined mode with single cycle
execution, the Zip CPU is unable to exploit this parallelism. Instead,
apart from the DMA and the pipelined prefetch, all loads and stores
are single wishbone bus operations requiring a minimum of 3 clocks.
(In practice, this has turned into 7-clocks.)
% Addressed, 20150929
 
\item There is no control over whether or not an instruction sets the
condition codes--certain instructions always set the condition codes,
other instructions never set them. This effectively limits conditional
instructions to a single instruction only (with two or more
instructions as an exception), as the first instruction that sets
condition codes will break the condition code chain.
 
{\em (A proposed change below address this.)}
 
\item Using the CC register as a trap address was a bad idea--it limits the CC
registers ability to be used in future expansion, such as by adding
exception indication flags: bus error, floating point exception, etc.
 
{\em (This can be changed by a different O/S implementation of the trap
instruction.)}
\item The current implementation suffers from too many stalls on any
branch--even if the branch is known early on.
 
{\em (This is addressed in proposals below.)}
% Addressed, 20150918
 
\item In a similar fashion, a switch to interrupt context forces the pipeline
to be cleared, whereas it might make more sense to just continue
executing the instructions already in the pipeline while the prefetch
stage is working on switching to the interrupt context.
 
{\em (Also addressed in proposals below.)}
% This should happen so rarely that it is not really a problem
\fi
 
\end{itemize}
 
\section{The Next Generation}
This section could also be labeled as my ``To do'' list.
This section could also be labeled as my ``To do'' list. Today's list is
much different than it was for the last version of this document, as much of
the prior to do list (such as VLIW instructions, and a more traditional
instruction cache) has now been implemented. The only things really and
truly waiting on my list today are assembler support for the VLIW instruction
set, linker and compiler support.
 
Given the feedback listed above, perhaps its time to consider what changes could be made to improve the Zip CPU in the future. I offer the following as proposals:
Stay tuned, these are likely to be coming next.
 
\begin{itemize}
\item {\bf Remove the low LUT goal.} It wasn't really achieved, and the
proposals below will only increase the amount of logic the Zip CPU
requires. While I expect that the Zip CPU will always be somewhat
of a light weight, it will never be the smallest kid on the block.
 
I'm actually struggling with this idea. The whole goal of the Zip
CPU was to be light weight. Wouldn't it make more sense to create and
maintain options whereby it would remain lightweight? For example, if
the process accounting registers are anything but light weight, why
keep them? Why not instead make some compile flags that just turn them
off, keeping the CPU lightweight? The same holds for the prefetch
cache.
 
\item The `{\tt .V}' condition was never used in any code other than my test
code. Suggest changing it to a `{\tt .LE}' condition, which seems
to be more useful.
 
\item {\bf Consider a more traditional Instruction Cache.} The current
pipelined instruction cache just reads a window of memory into
its cache. If the CPU leaves that window, the entire cache is
invalidated. A more traditional cache, however, might allow
common subroutines to stay within the cache without invalidating the
entire cache structure.
 
\iffalse
\item {\bf Adjust the Zip CPU so that conditional instructions do not set
flags}, although they may explicitly set condition codes if writing
to the CC register.
 
This is a simple change to the core, and may show up in new releases.
% Fixed, 20150918
 
\item Add in an {\bf unpredictable branch delay slot}, so that on any branch
the delay slot may or may not be executed before the branch.
Instructions that do not depend upon the branch, and that should be
executed were the branch not taken, could be placed into the delay
slot. Thus, if the branch isn't taken, we wouldn't suffer the stall,
whereas it wouldn't affect the timing of the branch if taken. It would
just do something irrelevant.
 
% Changes made, 20150918, make this option no longer relevant
 
\item {\bf Re-engineer Branch Processing.} There's no reason why a {\tt BRA}
instruction should create five stall cycles. The decode stage, plus
the prefetch engine, should be able to drop this number of stalls via
better branch handling.
 
Indeed, this could turn into a simple means of branch prediction:
if {\tt BRA} suffered a single stall only, whereas {\tt BRA.C}
suffered five stalls, then {\tt BRA.!C} followed by {\tt BRA} would
be faster than a {\tt BRA.C} instruction. This would then allow a
compiler to do explicit branch optimizations.
 
Of course, longer branches using {\tt ADD X,PC} would still not be
optimized.
 
% DONE: 20150918 -- to include the ADD X,PC instructions
 
\item {\bf Request bus access for Load/Store two cycles earlier.} The problem
here is the contention for the bus between the memory unit and the
prefetch unit. Currently, the memory unit must ask the prefetch
unit to release the bus if it is in the middle of a bus cycle. At this
point, the prefetch drops the {\tt STB} line on the next clock and must
then wait for the last {\tt ACK} before releasing the bus. If the
request takes one clock, dropping the strobe line the next, waiting
for an acknowledgement takes another, and then the bus must be idle
for one cycle before starting again, these extra cycles add up.
It should be possible to tell the prefetch stage to give up the bus
as soon as the decoder knows the instruction will need the bus.
Indeed, if done in the decode stage, this might drop the seven cycle
access down by two cycles.
% FIXED: 20150918
 
\item {\bf Very Long Instruction Word (VLIW).} Now, to speed up operation, I
propose that the Zip CPU instruction set be modified towards a Very
Long Instruction Word (VLIW) implementation. In this implementation,
an instruction word may contain either one or two separate
instructions. The first instruction would take up the high order bits,
the second optional instruction the lower 16-bits. Further, I propose
that any of the ALU instructions (SUB through LSR) automatically have
a second instruction whenever their operand `B' is a register, and use
the full 20-bit immediate if not. This will effectively eliminate the
register plus immediate mode for all of these instructions.
 
This is the minimal required change to increase the number of
instructions per clock cycle. Other changes would need to take place
as well to support this. These include:
\begin{itemize}
\item Instruction words containing two instructions would take two
clocks to complete, while requiring only a single cycle
instruction fetch.
\item Instructions preceded by a label in the asseembler must always
start in the high order word.
\item VLIW's, once started, must always execute to completion. The
upper word may set the PC, the lower word may not. Regardless
of whether the upper word sets the PC, the lower word must
still be guaranteed to complete before the PC changes. On any
switch to (or from) interrupt context, both instructions must
complete or none of the instructions in the word shall
complete prior to the switch.
\item STEP commands and BREAK instructions will only take place after
the entire word is executed.
\end{itemize}
 
If done well, the assembler should be able to handle these changes
with the biggest impacts to the user being increased performance and
a loss of the register plus immediate ALU modes. (These weren't really
relevant for the XOR, OR, AND, etc. operations anyway.) Machine code
compatibility will not be maintained.
 
A proposed secondary instruction set might consist of: a four bit
operand (any of the prior instructions would be supported, with some
exceptions such as moves to and from user registers while in
supervisor mode not being supported), a 4-bit register result (PC not
allowed), a 3-bit conditional (identical to the conditional for the
upper word), a single bit for whether or not an immediate is present
or not, followed by either a 4-bit register or a 4-bit signed
immediate. The multiply instruction would steal the immediate flag to
be used as a sign indication, forcing both operands to be registers
without any immediate offsets.
 
{\em Initial conversion of several library functions to this secondary
instruction set has demonstrated little to no gain. The problem was
that the new instruction set was made by joining a rarely used
instruction (ALU with register and not immediate) with a more common
instruction. The utility was then limited by the utility of the rare
instrction, which limited the impact of the entire approach. }
\else
\item {\bf Very Long Instruction Word (VLIW).} The goal here would be to
create a new instruction set whereby two instructions would be encoded
in each 32--bit word. While this may speed up
CPU operation, it would necessitate an instruction redesign.
\fi
 
\end{itemize}
 
% Appendices
% Index
\end{document}

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