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5,7 → 5,7
%% Project: Zip CPU -- a small, lightweight, RISC CPU soft core
%% Purpose: This LaTeX file contains all of the documentation/description
%% currently provided with this Zip CPU soft core. It supercedes
%% currently provided with this Zip CPU soft core. It supersedes
%% any information about the instruction set or CPUs found
%% elsewhere. It's not nearly as interesting, though, as the PDF
%% file it creates, so I'd recommend reading that before diving
48,7 → 48,7
\author{Dan Gisselquist, Ph.D.}
\email{dgisselq (at)}
70,6 → 70,7
0.3 & 8/22/2015 & Gisselquist & First completed draft\\\hline
0.2 & 8/19/2015 & Gisselquist & Still Draft, more complete \\\hline
0.1 & 8/17/2015 & Gisselquist & Incomplete First Draft \\\hline
91,7 → 92,7
I would like to be able to generate Verilog code that can run equivalently
on both Xilinx and Altera chips, and that can be easily ported from one
manufacturer's chipsets to another. Even more, before purchasing a chip or a
board, I would like to know that my chip works. I would like to build a test
board, I would like to know that my soft core works. I would like to build a test
bench to test components with, and Verilator is my chosen test bench. This
forces me to use all Verilog, and it prevents me from using any proprietary
cores. For this reason, Microblaze and Nios are out of the question.
159,7 → 160,7
as simple as I originally hoped. Worse, I've had to adjust to create
capabilities that I was never expecting to need. These include:
\item {\bf Extenal Debug:} Once placed upon an FPGA, some external means is
\item {\bf External Debug:} Once placed upon an FPGA, some external means is
still necessary to debug this CPU. That means that there needs to be
an external register that can control the CPU: reset it, halt it, step
it, and tell whether it is running or not. My chosen interface
241,10 → 242,10
enters into either the ALU or memory unit, the instruction is
guaranteed to complete. If the logic recognizes a branch or a
condition that would render the instruction entering into this stage
possibly inappropriate (i.e. a conditional branch preceeding a store
possibly inappropriate (i.e. a conditional branch preceding a store
instruction for example), then the pipeline stalls for one cycle
until the conditional branch completes. Then, if it generates a new
PC address, the stages preceeding are all wiped clean.
PC address, the stages preceding are all wiped clean.
The discrete execution model allows such things as sleeping: if the
CPU is put to ``sleep,'' the ALU and memory stages stall and back up
321,7 → 322,7
10 bits of the status register form a set of CPU state and condition codes.
Writes to other bits of this register are preserved.
Of the eight condition codes, the bottom four are the current flags:
Of the condition codes, the bottom four bits are the current flags:
Zero (Z),
Carry (C),
Negative (N),
331,6 → 332,7
the CPU to sleep). Setting this bit will cause the CPU to
wait for an interrupt (if interrupts are enabled), or to
completely halt (if interrupts are disabled).
The sixth bit is a global interrupt enable bit (GIE). When this
sixth bit is a `1' interrupts will be enabled, else disabled. When
interrupts are disabled, the CPU will be in supervisor mode, otherwise
387,7 → 389,7
Bit & Meaning \\\hline
9 & Soft trap, set on a trap from user mode, cleared when returing to user mode\\\hline
9 & Soft trap, set on a trap from user mode, cleared when returning to user mode\\\hline
8 & (Reserved for) Floating point enable \\\hline
7 & Halt on break, to support an external debugger \\\hline
6 & Step, single step the CPU in user mode\\\hline
439,8 → 441,8
\caption{Bit allocation for Operand B}\label{tbl:opb}
Sixteen and twenty bit immediates don't make sense for all instructions. For
example, what is the point of a 20--bit immediate when executing a 16--bit
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.
647,17 → 649,17
LSL/ASL & \multicolumn{4}{l|}{4'hd}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{3}{l|}{Cond.}
& \multicolumn{21}{l|}{Operand B, imm. trucated to 6 bits}
& \multicolumn{21}{l|}{Operand B, imm. truncated to 6 bits}
& Yes \\\hline
ASR & \multicolumn{4}{l|}{4'he}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{3}{l|}{Cond.}
& \multicolumn{21}{l|}{Operand B, imm. trucated to 6 bits}
& \multicolumn{21}{l|}{Operand B, imm. truncated to 6 bits}
& Yes \\\hline
LSR & \multicolumn{4}{l|}{4'hf}
& \multicolumn{4}{l|}{R. Reg}
& \multicolumn{3}{l|}{Cond.}
& \multicolumn{21}{l|}{Operand B, imm. trucated to 6 bits}
& \multicolumn{21}{l|}{Operand B, imm. truncated to 6 bits}
& Yes \\\hline
\caption{Zip CPU Instruction Set}\label{tbl:zip-instructions}
685,7 → 687,7
& \parbox[t]{1.5in}{Add Ra,Rx\\ADD.C \$1,Ry\\Add Rb,Ry}
& Add with carry \\\hline
BRA.Cond +/-\$Addr
& \hbox{Mov.cond \$Addr+PC,PC}
& \hbox{MOV.cond \$Addr+PC,PC}
& Branch or jump on condition. Works for 15--bit
signed address offsets.\\\hline
BRA.Cond +/-\$Addr
692,7 → 694,7
& \parbox[t]{1.5in}{LDI \$Addr,Rx \\ ADD.cond Rx,PC}
& Branch/jump on condition. Works for
23 bit address offsets, but costs a register, an extra instruction,
and setsthe flags. \\\hline
and sets the flags. \\\hline
BNC PC+\$Addr
& \parbox[t]{1.5in}{Test \$Carry,CC \\ MOV.Z PC+\$Addr,PC}
& Example of a branch on an unsupported
711,7 → 713,7
& Executed while in interrupt mode. In user mode this is simply a
wait until interrupt instructioon. \\\hline
wait until interrupt instruction. \\\hline
INT & LDI \$0,CC
& Since we're using the CC register as a trap vector as well, this
executes TRAP \#0. \\\hline
776,7 → 778,7
OR.C \$1,Ry}
& Logical shift left with carry. Note that the
instruction order is now backwards, to keep the conditions valid.
That is, LSL sets the carry flag, so if we did this the othe way
That is, LSL sets the carry flag, so if we did this the other way
with Rx before Ry, then the condition flag wouldn't have been right
for an OR correction at the end. \\\hline
\parbox[t]{1.5in}{LSR \$1,Rx \\ LSRC \$1,Ry}
798,10 → 800,10
& Note
that for interrupt purposes, one can never depend upon the value at
(SP). Hence you read from it, then increment it, lest having
incremented it firost something then comes along and writes to that
incremented it first something then comes along and writes to that
value before you can read the result. \\\hline
& \parbox[t]{1.5in}{SUB \$1,SPa \\
& \parbox[t]{1.5in}{SUB \$1,SP \\
STO Rx,\$1(SP)}
& \\\hline
917,6 → 919,8
unit stalls, every other instruction stalls. Memory stores, however, can take
place concurrently with ALU operations, although memory reads cannot.
\section{Pipeline Logic}
How the CPU handles some instruction combinations can be telling when
determining what happens in the pipeline. The following lists some examples:
923,36 → 927,49
\item {\bf Delayed Branching}
I had originally hoped to implement delayed branching. However, what
happens in debug mode?
That is, what happens when a debugger tries to single step an
instruction? While I can easily single step the computer in either
user or supervisor mode from externally, this processor does not appear
able to step the CPU in user mode from within user mode--gosh, not even
from within supervisor mode--such as if a process had a debugger
attached. As the processor exists, I would have one result stepping
the CPU from a debugger, and another stepping it externally.
I had originally hoped to implement delayed branching. My goal
was that the compiler would handle any pipeline stall conditions so
that the pipeline logic could be simpler within the CPU. I ran into
two problems with this.
This is unacceptable, and so this CPU does not support delayed
The first problem has to deal with debug mode. When the debugger
single steps an instruction, that instruction goes to completion.
This means that if the instruction moves a value to the PC register,
the PC register would now contain that value, indicating that the
next instruction would be on the other side of the branch. There's
just no easy way around this: the entire CPU state must be captured
by the registers, to include the program counter. What value should
the program counter be equal to? The branch? Fine. The address
you are branching to? Fine. The address of the delay slot? Problem.
The second problem with delayed branching is the idea of suspending
processing for an interrupt. Which address should the CPU return
to upon completing the interrupt processing? The branch? Good. The
address after the branch? Also good. The address of the delay slot?
Not so good.
If you then add into this mess the idea that, if the CPU is running
from a really slow memory such as the flash, the delay slot may never
be filled before the branch is determined, then this makes even less
For all of these reasons, this CPU does not support delayed branching.
\item {\bf Register Result:} {\tt MOV R0,R1; MOV R1,R2 }
What value does
R2 get, the value of R1 before the first move or the value of R0?
Placing the value of R0 into R1 requires a pipeline stall, and possibly
two, as I have the pipeline designed.
What value does R2 get, the value of R1 before the first move or the
value of R0? The Zip CPU has been optimized so that neither of these
instructions require a pipeline stall--unless an immediate were to
be added to R1 in the second instruction.
The ZIP CPU architecture requires that R2 must equal R0 at the end of
this operation. Even better, such combinations do not (normally)
stall the pipeline.
\item {\bf Condition Codes Result:} {\tt CMP R0,R1;Mov.EQ \$x,PC}
\item {\bf Condition Codes Result:} {\tt CMP R0,R1;} {\tt MOV.EQ \$x,PC}
At issue is the same item as above, save that the CMP instruction
updates the flags that the MOV instruction depends
updates the flags that the MOV instruction depends upon.
The Zip CPU architecture requires that condition codes must be updated
and available immediately for the next instruction without stalling the
965,15 → 982,11
the logic supporting any other register.
The ZIP CPU will stall for a cycle cycle on this instruction.
\item {\bf Condition Codes Register Operand:} {\tt MOV R0,R1; MOV CC,R2}
\item {\bf Delayed Branching: } {\tt ADD \$x,PC; MOV R0,R1}
At issues is whether or not the instruction following the jump will
take place before the jump. In other words, is the MOV to the PC
register handled differently from an ADD to the PC register?
In the Zip architecture, MOV'es and ADD's use the same logic
(simplifies the logic).
Unlike the previous case, this move prior to reading the {\tt CC}
register does not impact the {\tt CC} register. Therefore, this
does not stall the bus, whereas the previous one would.
As I've studied this, I find several approaches to handling pipeline
980,7 → 993,7
issues. These approaches (and their consequences) are listed below.
\item {\bf All All issued instructions complete, Stages stall individually}
\item {\bf All issued instructions complete, stages stall individually}
What about a slow pre-fetch?
995,47 → 1008,32
or a full pipeline if reading from cache. Each of these approaches
would produce a different response.
For this reason, the Zip CPU works off of a different basis: All
instructions that enter either the ALU or the memory unit will
complete. Stages still stall individually.
\item {\bf Issued instructions may be canceled}
Stages stall individually
First problem:
Memory operations cannot be canceled, even reads may have side effects
The problem here is that
memory operations cannot be canceled: even reads may have side effects
on peripherals that cannot be canceled later. Further, in the case of
an interrupt, it's difficult to know what to cancel. What happens in
a \hbox{\tt MOV.C \$x,PC} followed by a \hbox{\tt MOV \$y,PC}
instruction? Which get
instruction? Which get canceled?
Because it isn't clear what would need to be canceled,
this instruction combination is not recommended.
Because it isn't clear what would need to be canceled, the Zip CPU
will not permit this combination. A MOV to the PC register will be
followed by a stall, and possibly many stalls, so that the second
move to PC will never be executed.
\item {\bf All issued instructions complete.}
All stages are filled, or the entire pipeline stalls.
In this example, we try all issued instructions complete, but the
entire pipeline stalls if one stage is not filled. In this approach,
though, we again struggle with the problems associated with
delayed branching. Upon attempting to restart the processor, where
do you restart it from?
What about debug control? What about
register writes taking an extra clock stage? MOV R0,R1; MOV R1,R2
should place the value of R0 into R2. How do you restart the pipeline
after an interrupt? What address do you use? The last issued
instruction? But the branch delay slots may make that invalid!
Reading from the CPU debug port in this case yields inconsistent
results: the CPU will halt or step with instructions stuck in the
pipeline. Reading registers will give no indication of what is going
on in the pipeline, just the results of completed operations, not of
operations that have been started and not yet completed.
Perhaps we should just report the state of the CPU based upon what
instructions (PC values) have successfully completed? Thus the
debug instruction is the one that will write registers on the next
Suggestion: Suppose we load extra information in the two
CC register(s) for debugging intermediate pipeline stages?
The next problem, though, is how to deal with the read operand
pipeline stage needing the result from the register pipeline.
\item {\bf Memory instructions must complete}
All instructions that enter into the memory module {\em must}
1056,6 → 1054,7
decode, and read-op stage will be invalidated.
\section{Pipeline Stalls}
The processing pipeline can and will stall for a variety of reasons. Some of
1101,6 → 1100,8
an instruction from executing a memory access after the jump but before the
jump is recognized.
This stall cannot be mitigated through scheduling.
\item When reading from the CC register after setting the flags
\item\ {\tt ALUOP RA,RB}
1116,6 → 1117,13
clock cycle. (The time delay of the multiply within the ALU wasn't helping
either \ldots).
This stall may be eliminated via proper scheduling, by placing an instruction
that does not set flags in between the ALU operation and the instruction
that references the CC register. For example, {\tt MOV \$addr+PC,uPC}
followed by an {\tt RTU} ({\tt OR \$GIE,CC}) instruction will not incur
this stall, whereas an {\tt OR \$BREAKEN,CC} followed by an {\tt OR \$STEP,CC}
will incur the stall.
\item When waiting for a memory read operation to complete
\item\ {\tt LOD address,RA}
1126,13 → 1134,13
Remember, the ZIP CPU does not support out of order execution. Therefore,
anytime the memory unit becomes busy both the memory unit and the ALU must
stall until the memory unit is cleared. This is especially true of a load
instruction, which will write its operand back to the register file. Store
instructions are different, since they can be busy with no impact on later
ALU write back operations. Hence, only loads stall the pipeline.
instruction, which must still write its operand back to the register file.
Store instructions are different, since they can be busy with no impact on
later ALU write back operations. Hence, only loads stall the pipeline.
This also assumes that the memory being accessed is a single cycle memory.
Slower memories, such as the Quad SPI flash, will take longer--perhaps even
as long as fourty clocks. During this time the CPU and the external bus
as long as forty clocks. During this time the CPU and the external bus
will be busy, and unable to do anything else.
\item Memory operation followed by a memory operation
1290,6 → 1298,42
The Zip CPU, and even the Zip System, is not a System on a Chip (SoC). It
needs to be connected to its operational environment in order to be used.
Specifically, some per system adjustments need to be made:
\item The Zip System depends upon an external 32-bit Wishbone bus. This
must exist, and must be connected to the Zip CPU for it to work.
\item The Zip System needs to be told of its {\tt RESET\_ADDRESS}. This is
the program counter of the first instruction following a reset.
\item If you want the Zip System to start up on its own, you will need to
set the {\tt START\_HALTED} parameter to zero. Otherwise, if you
wish to manually start the CPU, that is if upon reset you want the
CPU start start in its halted, reset state, then set this parameter to
\item The third parameter to set is the number of interrupts you will be
providing from external to the CPU. This can be anything from one
to nine, but it cannot be zero. (Wire this line to a 1'b0 if you
do not wish to support any external interrupts.)
\item Finally, you need to place into some wishbone accessible address, whether
RAM or (more likely) ROM, the initial instructions for the CPU.
If you have enabled your CPU to start automatically, then upon power up the
CPU will immediately start executing your instructions.
This is, however, not how I have used the Zip CPU. I have instead used the
ZIP CPU in a more controlled environment. For me, the CPU starts in a
halted state, and waits to be told to start. Further, the RESET address is a
location in RAM. After bringing up the board I am using, and further the
bus that is on it, the RAM memory is then loaded externally with the program
I wish the Zip System to run. Once the RAM is loaded, I release the CPU.
The CPU then runs until its halt condition, at which point its task is
Eventually, I intend to place an operating system onto the ZipSystem, I'm
just not there yet.
The ZipSystem registers fall into two categories, ZipSystem internal registers
1316,7 → 1360,7
\caption{Zip System Internal/Peripheral Registers}\label{tbl:zpregs}
and the two debug registers showin in Tbl.~\ref{tbl:dbgregs}.
and the two debug registers shown in Tbl.~\ref{tbl:dbgregs}.
ZIPCTRL & 0 & 32 & R/W & Debug Control Register \\\hline
1325,8 → 1369,214
\caption{Zip System Debug Registers}\label{tbl:dbgregs}
\section{Peripheral Registers}
The peripheral registers, listed in Tbl.~\ref{tbl:zpregs}, are shown in the
CPU's address space. These may be accessed by the CPU at these addresses,
and when so accessed will respond as described in Chapt.~\ref{chap:periph}.
These registers will be discussed briefly again here.
\chapter{Wishbone Datasheet}\label{chap:wishbone}
The Zip CPU Interrupt controller has four different types of bits, as shown in
31 & R/W & Master Interrupt Enable\\\hline
30\ldots 16 & R/W & Interrupt Enables, write '1' to change\\\hline
15 & R & Current Master Interrupt State\\\hline
15\ldots 0 & R/W & Input Interrupt states, write '1' to clear\\\hline
\caption{Interrupt Controller Register Bits}\label{tbl:picbits}
The high order bit, or bit--31, is the master interrupt enable bit. When this
bit is set, then any time an interrupt occurs the CPU will be interrupted and
will switch to supervisor mode, etc.
Bits 30~\ldots 16 are interrupt enable bits. Should the interrupt line go
ghile while enabled, an interrupt will be generated. To set an interrupt enable
bit, one needs to write the master interrupt enable while writing a `1' to this
the bit. To clear, one need only write a `0' to the master interrupt enable,
while leaving this line high.
Bits 15\ldots 0 are the current state of the interrupt vector. Interrupt lines
trip when they go high, and remain tripped until they are acknowledged. If
the interrupt goes high for longer than one pulse, it may be high when a clear
is requested. If so, the interrupt will not clear. The line must go low
again before the status bit can be cleared.
As an example, consider the following scenario where the Zip CPU supports four
interrupts, 3\ldots0.
\item The Supervisor will first, while in the interrupts disabled mode,
write a {\tt 32'h800f000f} to the controller. The supervisor may then
switch to the user state with interrupts enabled.
\item When an interrupt occurs, the supervisor will switch to the interrupt
state. It will then cycle through the interrupt bits to learn which
interrupt handler to call.
\item If the interrupt handler expects more interrupts, it will clear its
current interrupt when it is done handling the interrupt in question.
To do this, it will write a '1' to the low order interrupt mask,
such as writing a {\tt 32'h80000001}.
\item If the interrupt handler does not expect any more interrupts, it will
instead clear the interrupt from the controller by writing a
{\tt 32'h00010001} to the controller.
\item Once all interrupts have been handled, the supervisor will write a
{\tt 32'h80000000} to the interrupt register to re-enable interrupt
\item The supervisor should also check the user trap bit, and possible soft
interrupt bits here, but this action has nothing to do with the
interrupt control register.
\item The supervisor will then leave interrupt mode, possibly adjusting
whichever task is running, by executing a return from interrupt
Leaving the interrupt controller, we show the timer registers bit definitions
in Tbl.~\ref{tbl:tmrbits}.
31 & R/W & Auto-Reload\\\hline
30\ldots 0 & R/W & Current timer value\\\hline
\caption{Timer Register Bits}\label{tbl:tmrbits}
As you may recall, the timer just counts down to zero and then trips an
interrupt. Writing to the current timer value sets that value, and reading
from it returns that value. Writing to the current timer value while also
setting the auto--reload bit will send the timer into an auto--reload mode.
In this mode, upon setting its interrupt bit for one cycle, the timer will
also reset itself back to the value of the timer that was written to it when
the auto--reload option was written to it. To clear and stop the timer,
just simply write a `32'h00' to this register.
The Jiffies register is somewhat similar in that the register always changes.
In this case, the register counts up, whereas the timer always counted down.
Reads from this register, as shown in Tbl.~\ref{tbl:jiffybits},
31\ldots 0 & R & Current jiffy value\\\hline
31\ldots 0 & W & Value/time of next interrupt\\\hline
\caption{Jiffies Register Bits}\label{tbl:jiffybits}
always return the time value contained in the register. Writes greater than
the current Jiffy value, that is where the new value minus the old value is
greater than zero while ignoring truncation, will set a new Jiffy interrupt
time. At that time, the Jiffy vector will clear, and another interrupt time
may either be written to it, or it will just continue counting without
activating any more interrupts.
The Zip CPU also supports several counter peripherals, mostly in the way of
process accounting. This peripherals have a single register associated with
them, shown in Tbl.~\ref{tbl:ctrbits}.
31\ldots 0 & R/W & Current counter value\\\hline
\caption{Counter Register Bits}\label{tbl:ctrbits}
Writes to this register set the new counter value. Reads read the current
counter value.
The current design operation of these counters is that of performance counting.
Two sets of four registers are available for keeping track of performance.
The first is a task counter. This just counts clock ticks. The second
counter is a prefetch stall counter, then an master stall counter. These
allow the CPU to be evaluated as to how efficient it is. The fourth and
final counter is an instruction counter, which counts how many instructions the
CPU has issued.
It is envisioned that these counters will be used as follows: First, every time
a master counter rolls over, the supervisor (Operating System) will record
the fact. Second, whenever activating a user task, the Operating System will
set the four user counters to zero. When the user task has completed, the
Operating System will read the timers back off, to determine how much of the
CPU the process had consumed.
\section{Debug Port Registers}
Accessing the Zip System via the debug port isn't as straight forward as
accessing the system via the wishbone bus. The debug port itself has been
reduced to two addresses, as outlined earlier in Tbl.~\ref{tbl:dbgregs}.
Access to the Zip System begins with the Debug Control register, shown in
31\ldots 14 & R & Reserved\\\hline
13 & R & CPU GIE setting\\\hline
12 & R & CPU is sleeping\\\hline
11 & W & Command clear PF cache\\\hline
10 & R/W & Command HALT, Set to '1' to halt the CPU\\\hline
9 & R & Stall Status, '1' if CPU is busy\\\hline
8 & R/W & Step Command, set to '1' to step the CPU\\\hline
7 & R & Interrupt Request \\\hline
6 & R/W & Command RESET \\\hline
5\ldots 0 & R/W & Debug Register Address \\\hline
\caption{Debug Control Register Bits}\label{tbl:dbgctrl}
The first step in debugging access is to determine whether or not the CPU
is halted, and to halt it if not. To do this, first write a '1' to the
Command HALT bit. This will halt the CPU and place it into debug mode.
Once the CPU is halted, the stall status bit will drop to zero. Thus,
if bit 10 is high and bit 9 low, the debug port is open to examine the
internal state of the CPU.
At this point, the external debugger may examine internal state information
from within the CPU. To do this, first write again to the command register
a value (with command halt still high) containing the address of an internal
register of interest in the bottom 6~bits. Internal registers that may be
accessed this way are listed in Tbl.~\ref{tbl:dbgaddrs}.
sR0 & 0 & 32 & R/W & Supervisor Register R0 \\\hline
sR1 & 0 & 32 & R/W & Supervisor Register R1 \\\hline
sSP & 13 & 32 & R/W & Supervisor Stack Pointer\\\hline
sCC & 14 & 32 & R/W & Supervisor Condition Code Register \\\hline
sPC & 15 & 32 & R/W & Supervisor Program Counter\\\hline
uR0 & 16 & 32 & R/W & User Register R0 \\\hline
uR1 & 17 & 32 & R/W & User Register R1 \\\hline
uSP & 29 & 32 & R/W & User Stack Pointer\\\hline
uCC & 30 & 32 & R/W & User Condition Code Register \\\hline
uPC & 31 & 32 & R/W & User Program Counter\\\hline
PIC & 32 & 32 & R/W & Primary Interrupt Controller \\\hline
WDT & 33 & 32 & R/W & Watchdog Timer\\\hline
CCHE & 34 & 32 & R/W & Manual Cache Controller\\\hline
CTRIC & 35 & 32 & R/W & Secondary Interrupt Controller\\\hline
TMRA & 36 & 32 & R/W & Timer A\\\hline
TMRB & 37 & 32 & R/W & Timer B\\\hline
TMRC & 38 & 32 & R/W & Timer C\\\hline
JIFF & 39 & 32 & R/W & Jiffies peripheral\\\hline
MTASK & 40 & 32 & R/W & Master task clock counter\\\hline
MMSTL & 41 & 32 & R/W & Master memory stall counter\\\hline
MPSTL & 42 & 32 & R/W & Master Pre-Fetch Stall counter\\\hline
MICNT & 43 & 32 & R/W & Master instruction counter\\\hline
UTASK & 44 & 32 & R/W & User task clock counter\\\hline
UMSTL & 45 & 32 & R/W & User memory stall counter\\\hline
UPSTL & 46 & 32 & R/W & User Pre-Fetch Stall counter\\\hline
UICNT & 47 & 32 & R/W & User instruction counter\\\hline
\caption{Debug Register Addresses}\label{tbl:dbgaddrs}
Primarily, these ``registers'' include access to the entire CPU register
set, as well as the 16~internal peripherals. To read one of these registers
once the address is set, simply issue a read from the data port. To write
one of these registers or peripheral ports, simply write to the data port
after setting the proper address.
In this manner, all of the CPU's internal state may be read and adjusted.
As an example of how to use this, consider what would happen in the case
of an external break point. If and when the CPU hits a break point that
causes it to halt, the Command HALT bit will activate on its own, the CPU
will then raise an external interrupt line and wait for a debugger to examine
its state. After examining the state, the debugger will need to remove
the breakpoint by writing a different instruction into memory and by writing
to the command register while holding the clear cache, command halt, and
step CPU bits high, (32'hd00). The debugger may then replace the breakpoint
now that the CPU has gone beyond it, and clear the cache again (32'h500).
To leave this debug mode, simply write a `32'h0' value to the command register.
\chapter{Wishbone Datasheets}\label{chap:wishbone}
The Zip System supports two wishbone ports, a slave debug port and a master
port for the system itself. These are shown in Tbl.~\ref{tbl:wishbone-slave}
1410,7 → 1660,58
\chapter{I/O Ports}\label{chap:ioports}
The I/O ports to the Zip CPU may be grouped into three categories. The first
is that of the master wishbone used by the CPU, then the slave wishbone used
to command the CPU via a debugger, and then the rest. The first two of these
were already discussed in the wishbone chapter. They are listed here
for completeness in Tbl.~\ref{tbl:iowb-master}
{\tt o\_wb\_cyc} & 1 & Output & Indicates an active Wishbone cycle\\\hline
{\tt o\_wb\_stb} & 1 & Output & WB Strobe signal\\\hline
{\tt o\_wb\_we} & 1 & Output & Write enable\\\hline
{\tt o\_wb\_addr} & 32 & Output & Bus address \\\hline
{\tt o\_wb\_data} & 32 & Output & Data on WB write\\\hline
{\tt i\_wb\_ack} & 1 & Input & Slave has completed a R/W cycle\\\hline
{\tt i\_wb\_stall} & 1 & Input & WB bus slave not ready\\\hline
{\tt i\_wb\_data} & 32 & Input & Incoming bus data\\\hline
\end{portlist}\caption{CPU Master Wishbone I/O Ports}\label{tbl:iowb-master}\end{center}\end{table}
and~\ref{tbl:iowb-slave} respectively.
{\tt i\_wb\_cyc} & 1 & Input & Indicates an active Wishbone cycle\\\hline
{\tt i\_wb\_stb} & 1 & Input & WB Strobe signal\\\hline
{\tt i\_wb\_we} & 1 & Input & Write enable\\\hline
{\tt i\_wb\_addr} & 1 & Input & Bus address, command or data port \\\hline
{\tt i\_wb\_data} & 32 & Input & Data on WB write\\\hline
{\tt o\_wb\_ack} & 1 & Output & Slave has completed a R/W cycle\\\hline
{\tt o\_wb\_stall} & 1 & Output & WB bus slave not ready\\\hline
{\tt o\_wb\_data} & 32 & Output & Incoming bus data\\\hline
\end{portlist}\caption{CPU Debug Wishbone I/O Ports}\label{tbl:iowb-slave}\end{center}\end{table}
There are only four other lines to the CPU: the external clock, external
reset, incoming external interrupt line(s), and the outgoing debug interrupt
line. These are shown in Tbl.~\ref{tbl:ioports}.
{\tt i\_clk} & 1 & Input & The master CPU clock \\\hline
{\tt i\_rst} & 1 & Input & Active high reset line \\\hline
{\tt i\_ext\_int} & 1\ldots 6 & Input & Incoming external interrupts \\\hline
{\tt o\_ext\_int} & 1 & Output & CPU Halted interrupt \\\hline
\end{portlist}\caption{I/O Ports}\label{tbl:ioports}\end{center}\end{table}
The clock line was discussed briefly in Chapt.~\ref{chap:clocks}. We
typically run it at 100~MHz. The reset line is an active high reset. When
asserted, the CPU will start running again from its reset address in
memory. Further, depending upon how the CPU is configured and specifically on
the {\tt START\_HALTED} parameter, it may or may not start running
automatically. The {\tt i\_ext\_int} line is for an external interrupt. This
line may be as wide as 6~external interrupts, depending upon the setting of
the {\tt EXTERNAL\_INTERRUPTS} line. As currently configured, the ZipSystem
only supports one such interrupt line by default. For us, this line is the
output of another interrupt controller, but that's a board specific setup
detail. Finally, the Zip System produces one external interrupt whenever
the CPU halts to wait for the debugger.
% Appendices
% Index

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