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## Rev 33 → Rev 36

/spec.tex
48,7 → 48,13
 \title{Specification} \author{Dan Gisselquist, Ph.D.} \email{dgisselq (at) opencores.org} \revision{Rev.~0.3} \revision{Rev.~0.4} \definecolor{webred}{rgb}{0.2,0,0} \definecolor{webgreen}{rgb}{0,0.2,0} \usepackage[dvips,ps2pdf,colorlinks=true,  anchorcolor=black,pagecolor=webgreen,pdfpagelabels,hypertexnames,  pdfauthor={Dan Gisselquist},  pdfsubject={Zip CPU}]{hyperref} \begin{document} \pagestyle{gqtekspecplain} \titlepage
70,6 → 76,7
 copy. \end{license} \begin{revisionhistory} 0.4 & 9/19/2015 & Gisselquist & Added DMA controller, improved stall information, and self--assessment info.\\\hline 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
89,13 → 96,16
   There's more to it, though. There's a lot that I would like to do with a processor, and I want to be able to do it in a vendor independent fashion. I would like to be able to generate Verilog code that can run equivalently on both Xilinx and Altera chips, and that can be easily ported from one manufacturer's chipsets to another. Even more, before purchasing a chip or a board, I would like to know that my soft core works. I would like to build a test bench to test components with, and Verilator is my chosen test bench. This forces me to use all Verilog, and it prevents me from using any proprietary cores. For this reason, Microblaze and Nios are out of the question. First, I would like to be able to place this processor inside an FPGA. Without paying royalties, ARM is out of the question. I would then like to be able to generate Verilog code, both for the processor and the system it sits within, that can run equivalently on both Xilinx and Altera chips, and that can be easily ported from one manufacturer's chipsets to another. Even more, before purchasing a chip or a board, I would like to know that my soft core works. I would like to build a test bench to test components with, and Verilator is my chosen test bench. This forces me to use all Verilog, and it prevents me from using any proprietary cores. For this reason, Microblaze and Nios are out of the question.   Why not OpenRISC? That's a hard question. The OpenRISC team has done some wonderful work on an amazing processor, and I'll have to admit that I am
122,7 → 132,7
 \setcounter{page}{1}     The original goal of the ZIP CPU was to be a very simple CPU. You might 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
297,7 → 307,7
 \section{Simplified Bus} The bus architecture of the Zip CPU is that of a simplified WISHBONE bus. It has been simplified in this fashion: all operations are 32--bit operations. The bus is neither little endian nor bit endian. For this reason, all words The bus is neither little endian nor big endian. For this reason, all words are 32--bits. All instructions are also 32--bits wide. Everything has been built around the 32--bit word. 
318,9 → 328,28
 The CPU can access both register sets via move instructions from the supervisor state, whereas the user state can only access the user registers.   The status register is special, and bears further mention. The lower 10 bits of the status register form a set of CPU state and condition codes.  Writes to other bits of this register are preserved. The status register is special, and bears further mention. As shown in  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 9 & R & Trap, or user interrupt, Flag. Cleared on return to userspace.\\\hline 8 & R & (Reserved for) Illegal Instruction Flag\\\hline 7 & R/W & Break--Enable\\\hline 6 & R/W & Step\\\hline 5 & R/W & Global Interrupt Enable (GIE)\\\hline 4 & R/W & Sleep. When GIE is also set, the CPU waits for an interrupt.\\\hline 3 & R/W & Overflow\\\hline 2 & R/W & Negative. The sign bit was set as a result of the last ALU instruction.\\\hline 1 & R/W & Carry\\\hline 0 & R/W & Zero. The last ALU operation produced a zero.\\\hline \end{bitlist} \caption{Condition Code Register Bit Assignment}\label{tbl:cc-register} \end{center}\end{table} the lower 11~bits of the status register form a set of CPU state and condition codes. Writes to other bits of this register are preserved.   Of the condition codes, the bottom four bits are the current flags:  Zero (Z),
371,13 → 400,11
 This functionality was added to enable an external debugger to  set and manage breakpoints.   The ninth bit is reserved for a floating point enable bit. When set, the arithmetic for the next instruction will be sent to a floating point unit. Such a unit may later be added as an extension to the Zip CPU. If the CPU does not support floating point instructions, this bit will never be set. The instruction set could also be simply extended to allow other data types in this fashion, such as two by 16--bit vector operations or four by 8--bit vector operations. The ninth bit is reserved for an illegal instruction bit. When the CPU tries to execute either a non-existant instruction, or an instruction from an address that produces a bus error, the CPU will (once implemented) switch to supervisor mode while setting this bit. The bit will automatically be cleared upon any return to user mode.   The tenth bit is a trap bit. It is set whenever the user requests a soft interrupt, and cleared on any return to userspace command. This allows the
404,7 → 431,7
 \end{center}\end{table}   \section{Conditional Instructions} Most, although not quite all, instructions are conditionally executed. From 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}
424,9 → 451,12
 \end{center} \end{table} There is no condition code for less than or equal, not C or not V. Sorry, I ran out of space in 3--bits. Using these conditions will take an extra instruction and a pipeline stall. (Ex: \hbox{\em (Stall)}; \hbox{\tt TST \$4,CC;} \hbox{\tt STO.NZ R0,(R1)}) 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)})   Conditionally executed ALU instructions will not further adjust the  condition codes.   \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,
448,7 → 478,7
 instruction possibilities.   \section{Address Modes} The ZIP CPU supports two addressing modes: register plus immediate, and 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. 
484,12 → 514,11
 all MOV registers are supervisor registers, and display them as normal.  Anything with the user bit set will be treated as a user register. The CPU will quietly ignore the supervisor bits while in user mode, and anything marked as a user register will always be valid. (Did I just say that in the last paragraph?) marked as a user register will always be valid.   \section{Multiply Operations} The Zip CPU supports two Multiply operations, a 16x16 bit signed multiply (MPYS) and the same but unsigned (MPYU). In both 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
496,13 → 525,14
 CC register field is reserved.   \section{Floating Point} The ZIP CPU does not support floating point operations. However, the The Zip CPU does not (yet) support floating point operations. However, the instruction set reserves two possibilities for future floating point operations.   The first floating point operation hole in the instruction set involves setting the floating point bit in the CC register. The next instruction will simply interpret its operands as floating point instructions. 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.
513,15 → 543,15
   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. 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. 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.   In both cases, the Zip CPU would support 32--bit single precision floats only. only, since other choices would complicate the pipeline.   The current architecture does not support a floating point not-implemented interrupt. Any soft floating point emulation must be done deliberately.
531,9 → 561,10
 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  & Sets CC? \\\hline\hline CMP(Sub) & \multicolumn{4}{l|}{4'h0}  & \multicolumn{4}{l|}{D. Reg}  & \multicolumn{3}{l|}{Cond.}
564,7 → 595,7
  & \multicolumn{4}{l|}{4'he}  & \multicolumn{24}{l|}{24'h01}  & \\\hline {\em Rsrd} & \multicolumn{4}{l|}{4'h4} {\em Reserved} & \multicolumn{4}{l|}{4'h4}  & \multicolumn{4}{l|}{4'he}  & \multicolumn{24}{l|}{24'bits, but not 0 or 1.}  & \\\hline
667,22 → 698,22
   As you can see, there's lots of room for instruction set expansion. The NOOP and BREAK instructions are the only instructions within one particular 24--bit hole. This spaces are reserved for future enhancements. For example, floating point operations, consisting of a 3-bit floating point operation, two 4-bit registers, no immediate offset, and a 3-bit condition would fit nicely into 14--bits of this address space--making it so that the floating point bit in the CC register need not be used. 24--bit hole. The rest of this space is reserved for future enhancements.    \section{Derived Instructions} The ZIP CPU supports many other common instructions, but not all of them The Zip CPU supports many other common instructions, but not all of them are single cycle instructions. The derived instruction tables, Tbls.~\ref{tbl:derived-1}, \ref{tbl:derived-2}, and~\ref{tbl:derived-3}, Tbls.~\ref{tbl:derived-1}, \ref{tbl:derived-2}, \ref{tbl:derived-3}  and~\ref{tbl:derived-4}, help to capture some of how these other instructions may be implemented on the ZIP CPU. Many of these instructions will have assembly equivalents, the Zip CPU. Many of these instructions will have assembly equivalents, such as the branch instructions, to facilitate working with the CPU. \begin{table}\begin{center} \begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline Mapped & Actual & Notes \\\hline ABS Rx  & \parbox[t]{1.5in}{TST -1,Rx\\NEG.LT Rx}  & Absolute value, depends upon derived NEG.\\\hline \parbox[t]{1.4in}{ADD Ra,Rx\\ADDC Rb,Ry}  & \parbox[t]{1.5in}{Add Ra,Rx\\ADD.C \$1,Ry\\Add Rb,Ry}  & Add with carry \\\hline 730,7 → 761,11   MOV \$Addr+PC,PC \\  ADD \$1,SP}  & Jump to Subroutine. Note the required cleanup instruction after  returning. \\\hline  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.  \\\hline JSR PC+\$Addr   & \parbox[t]{1.5in}{MOV \$3+PC,R12 \\ MOV \$addr+PC,PC}  &This is the high speed
789,6 → 824,7
  OR Rz,Rx}  & Logical shift right with carry \\\hline NEG Rx & \parbox[t]{1.5in}{XOR \$-1,Rx \\ ADD \$1,Rx} & \\\hline NEG.C Rx & \parbox[t]{1.5in}{MOV.C \$-1+Rx,Rx\\XOR.C \$-1,Rx} & \\\hline NOOP & NOOP & While there are many  operations that do nothing, such as MOV Rx,Rx, or OR \$0,Rx, these  operations have consequences in that they might stall the bus if 802,10 → 838,24   (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 \end{tabular} \caption{Derived Instructions, continued}\label{tbl:derived-2} \end{center}\end{table} \begin{table}\begin{center} \begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline PUSH Rx  & \parbox[t]{1.5in}{SUB \$1,SP \\  STO Rx,\$1(SP)}  & \\\hline PUSH Rx-Ry  & \parbox[t]{1.5in}{SUB \$n,SP \\  STO Rx,\$n(SP)  \ldots \\  STO Ry,\$1(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  does not support this instruction (yet).\\\hline RESET  & \parbox[t]{1in}{STO \$1,\$watchdog(R12)\\NOOP\\NOOP}  & \parbox[t]{3in}{This depends upon the peripheral base address being
813,14 → 863,9
    Another opportunity might be to jump to the reset address from within  supervisor mode.}\\\hline RET & \parbox[t]{1.5in}{LOD \$-1(SP),PC} RET & \parbox[t]{1.5in}{LOD \$1(SP),PC}  & Note that this depends upon the calling context to clean up the  stack, as outlined for the JSR instruction. \\\hline \end{tabular} \caption{Derived Instructions, continued}\label{tbl:derived-2} \end{center}\end{table} \begin{table}\begin{center} \begin{tabular}{p{1.4in}p{1.5in}p{3in}}\\\hline RET & MOV R12,PC  & This is the high(er) speed version, that doesn't touch the stack.   As such, it doesn't suffer a stall on memory read/write to the stack.
859,14 → 904,19
  & While no extra registers are needed, this example  does take 3-clocks. \\\hline TRAP \#X  & LDILO \$x,CC  & This approach uses the unused bits of the CC register as a TRAP  address. The user will need to make certain  that the SLEEP and GIE bits are not set in \$x. LDI would also work,  however using LDILO permits the use of conditional traps. (i.e.,  trap if the zero flag is set.) Should you wish to trap off of a  register value, you could equivalently load \$x into the register and  then MOV it into the CC register. \\\hline  & \parbox[t]{1.5in}{LDI \$x,R0 \\ AND ~\$GIE,CC }  & This works because whenever a user lowers the \$GIE flag, it sets  a TRAP bit within the CC register. Therefore, upon entering the   supervisor state, the CPU only need check this bit to know that it  got there via a TRAP. The trap could be made conditional by making  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 effectt 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 TST Rx  & TST \$-1,Rx  & Set the condition codes based upon Rx. Could also do a CMP \$0,Rx,
878,20 → 928,36
  & Wait 'til interrupt. In an interrupts disabled context, this  becomes a HALT instruction. \end{tabular} \caption{Derived Instructions, continued}\label{tbl:derived-3} \caption{Derived Instructions, continued}\label{tbl:derived-4} \end{center}\end{table} \iffalse \fi \section{Pipeline Stages} As mentioned in the introduction, and highlighted in Fig.~\ref{fig:cpu}, the Zip CPU supports a five stage pipeline. \begin{enumerate} \item {\bf Prefetch}: Read instruction from memory (cache if possible). This \item {\bf Prefetch}: Reads instruction from memory and into a cache, if so   configured. This  stage is actually pipelined itself, and so it will stall if the PC  ever changes. Stalls are also created here if the instruction isn't  in the prefetch cache. \item {\bf Decode}: Decode instruction into op code, register(s) to read, and  immediate offset. This stage also determines whether the flags will    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 second prefetch module is a pipelined prefetch with a cache. This  module tries to keep the instruction address within a window of valid  instruction addresses. While effective, it is not a traditional  cache implementation. One unique feature of this cache implementation,  however, is that it can be cleared in a single clock. A disappointing  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. \item {\bf Read Operands}: Read registers and apply any immediate values to  them. There is no means of detecting or flagging arithmetic overflow
898,164 → 964,26
  or carry when adding the immediate to the operand. This stage will  stall if any source operand is pending. \item Split into two tracks: An {\bf ALU} which will accomplish a simple  instruction, and the {\bf MemOps} stage which accomplishes memory  read/write.  instruction, and the {\bf MemOps} stage which handles {\tt LOD} (load)  and {\tt STO} (store) instructions.  \begin{itemize}  \item Loads stall instructions that access the register until it is  written to the register set.  \item Condition codes are available upon completion  \item Issuing an instruction to the memory while the memory is busy will  stall the entire pipeline. If the bus deadlocks, only a reset  will release the CPU. (Watchdog timer, anyone?)  \item 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.  \end{itemize} \item {\bf Write-Back}: Conditionally write back the result to the register  set, applying the condition. This routine is bi-re-entrant: either the  set, applying the condition. This routine is bi-entrant: either the  memory or the simple instruction may request a register write.  \end{enumerate}   The Zip CPU does not support out of order execution. Therefore, if the memory unit stalls, every other instruction stalls. Memory stores, however, can take place concurrently with ALU operations, although memory reads cannot. place concurrently with ALU operations, although memory reads (loads) cannot.   \iffalse   \section{Pipeline Logic} How the CPU handles some instruction combinations can be telling when determining what happens in the pipeline. The following lists some examples: \begin{itemize} \item {\bf Delayed Branching}    I had originally hoped to implement delayed branching. My goal  was that the compiler would handle any pipeline stall conditions so  that the pipeline logic could be simpler within the CPU. I ran into  two problems with this.    The first problem has to deal with debug mode. When the debugger  single steps an instruction, that instruction goes to completion.  This means that if the instruction moves a value to the PC register,  the PC register would now contain that value, indicating that the  next instruction would be on the other side of the branch. There's  just no easy way around this: the entire CPU state must be captured  by the registers, to include the program counter. What value should  the program counter be equal to? The branch? Fine. The address  you are branching to? Fine. The address of the delay slot? Problem.    The second problem with delayed branching is the idea of suspending  processing for an interrupt. Which address should the CPU return  to upon completing the interrupt processing? The branch? Good. The  address after the branch? Also good. The address of the delay slot?  Not so good.    If you then add into this mess the idea that, if the CPU is running  from a really slow memory such as the flash, the delay slot may never  be filled before the branch is determined, then this makes even less  sense.    For all of these reasons, this CPU does not support delayed branching.   \item {\bf Register Result:} {\tt MOV R0,R1; MOV R1,R2 }    What value does R2 get, the value of R1 before the first move or the  value of R0? The Zip CPU has been optimized so that neither of these  instructions require a pipeline stall--unless an immediate were to  be added to R1 in the second instruction.    The ZIP CPU architecture requires that R2 must equal R0 at the end of  this operation. Even better, such combinations do not (normally)  stall the pipeline.   \item {\bf Condition Codes Result:} {\tt CMP R0,R1;} {\tt MOV.EQ \$x,PC}    At issue is the same item as above, save that the CMP instruction  updates the flags that the MOV instruction depends upon.    The Zip CPU architecture requires that condition codes must be updated  and available immediately for the next instruction without stalling the  pipeline.   \item {\bf Condition Codes Register Result:} {\tt CMP R0,R1; MOV CC,R2}    At issue is the  fact that the logic supporting the CC register is more complicated than  the logic supporting any other register.    The ZIP CPU will stall for a cycle cycle on this instruction. \item {\bf Condition Codes Register Operand:} {\tt MOV R0,R1; MOV CC,R2}    Unlike the previous case, this move prior to reading the {\tt CC}  register does not impact the {\tt CC} register. Therefore, this  does not stall the bus, whereas the previous one would. \end{itemize}   As I've studied this, I find several approaches to handling pipeline   issues. These approaches (and their consequences) are listed below.   \begin{itemize} \item {\bf All issued instructions complete, stages stall individually}    What about a slow pre-fetch?     Nominally, this works well: any issued instruction  just runs to completion. If there are four issued instructions in the  pipeline, with the writeback instruction being a write-to-PC  instruction, the other three instructions naturally finish.    This approach fails when reading instructions from the flash,  since such reads require N clocks to clocks to complete. Thus   there may be only one instruction in the pipeline if reading from flash,  or a full pipeline if reading from cache. Each of these approaches  would produce a different response.    For this reason, the Zip CPU works off of a different basis: All  instructions that enter either the ALU or the memory unit will  complete. Stages still stall individually.   \item {\bf Issued instructions may be canceled}    The problem here is that  memory operations cannot be canceled: even reads may have side effects  on peripherals that cannot be canceled later. Further, in the case of  an interrupt, it's difficult to know what to cancel. What happens in  a \hbox{\tt MOV.C \$x,PC} followed by a \hbox{\tt MOV \$y,PC}  instruction? Which get canceled?    Because it isn't clear what would need to be canceled, the Zip CPU  will not permit this combination. A MOV to the PC register will be  followed by a stall, and possibly many stalls, so that the second  move to PC will never be executed.   \item {\bf All issued instructions complete.}    In this example, we try all issued instructions complete, but the  entire pipeline stalls if one stage is not filled. In this approach,  though, we again struggle with the problems associated with  delayed branching. Upon attempting to restart the processor, where  do you restart it from?   \item {\bf Memory instructions must complete}    All instructions that enter into the memory module {\em must}  complete. Issued instructions from the prefetch, decode, or operand  read stages may or may not complete. Jumps into code must be valid,  so that interrupt returns may be valid. All instructions entering the  ALU complete.    This looks to be the simplest approach.  While the logic may be difficult, this appears to be the only  re-entrant approach.    A {\tt new\_pc} flag will be high anytime the PC changes in an  unpredictable way (i.e., it doesn't increment). This includes jumps  as well as interrupts and interrupt returns. Whenever this flag may  go high, memory operations and ALU operations will stall until the  result is known. When the flag does go high, anything in the prefetch,  decode, and read-op stage will be invalidated.   \end{itemize} \fi   \section{Pipeline Stalls} The processing pipeline can and will stall for a variety of reasons. Some of these are obvious, some less so. These reasons are listed below: 1062,20 → 990,23  \begin{itemize} \item When the prefetch cache is exhausted   This should be obvious. If the prefetch cache doesn't have the instruction in memory, the entire pipeline must stall until enough of the prefetch cache is loaded to support the next instruction. 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.   \item While waiting for the pipeline to load following any taken branch, jump,  return from interrupt or switch to interrupt context (6 clocks)  return from interrupt or switch to interrupt context (5 stall cycles)   If the PC suddenly changes, the pipeline is subsequently cleared and needs to be reloaded. Given that there are five stages to the pipeline, that accounts for five of the six delay clocks. The last clock is lost in the prefetch for four of the five stalls. The 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. Hence, six clocks will always be lost anytime the pipeline needs to be cleared. a new output.   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 3~stall cycles.   \item When reading from a prior register while also adding an immediate offset \begin{enumerate} \item\ {\tt OPCODE ?,RA}
1088,12 → 1019,15
 any instruction that will write back an operand must be separated from the opcode that will read and apply an immediate offset by one instruction. The good news is that this stall can easily be mitigated by proper scheduling. That is, any instruction that does not add an immediate to {\tt RA} may be scheduled into the stall slot.   \item When writing to the CC or PC Register \item When any write to either the CC or PC Register is followed by a memory  operation \begin{enumerate} \item\ {\tt OPCODE RA,PC} {\em Ex: a branch opcode} \item\ {\em (stall, even if jump not taken)} \item\ {\tt OPCODE RA,RB} \item\ {\tt LOD \$X(RA),RB} \end{enumerate} Since branches take place in the writeback stage, the Zip CPU will stall the pipeline for one clock anytime there may be a possible jump. This prevents 1100,12 → 1034,14  an instruction from executing a memory access after the jump but before the jump is recognized.   This stall cannot be mitigated through scheduling. 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} \item\ {\em (stall} \item\ {\tt ALUOP RA,RB} {\em Ex: a compare opcode} \item\ {\em (stall)} \item\ {\tt TST sys.ccv,CC} \item\ {\tt BZ somewhere} \end{enumerate} 1122,16 → 1058,16  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. will incur the stall, while a {\tt LDI \$BREAKEN|\$STEP,CC} will not.   \item When waiting for a memory read operation to complete \begin{enumerate} \item\ {\tt LOD address,RA} \item\ {\em (multiple stalls, bus dependent, 7 clocks best)} \item\ {\em (multiple stalls, bus dependent, 4 clocks best)} \item\ {\tt OPCODE I+RA,RB} \end{enumerate}   Remember, the ZIP CPU does not support out of order execution. Therefore, Remember, the Zip CPU does not support out of order execution. Therefore, anytime the memory unit becomes busy both the memory unit and the ALU must stall until the memory unit is cleared. This is especially true of a load instruction, which must still write its operand back to the register file.  1146,18 → 1082,35  \item Memory operation followed by a memory operation \begin{enumerate} \item\ {\tt STO address,RA} \item\ {\em (multiple stalls, bus dependent, 7 clocks best)} \item\ {\em (multiple stalls, bus dependent, 4 clocks best)} \item\ {\tt LOD address,RB} \item\ {\em (multiple stalls, bus dependent, 7 clocks best)} \item\ {\em (multiple stalls, bus dependent, 4 clocks best)} \end{enumerate}   In this case, the LOD instruction cannot start until the STALL is finished. In this case, the LOD instruction cannot start until the STO is finished. With proper scheduling, it is possible to do something in the ALU while the STO is busy, but otherwise this pipeline will stall waiting for it to complete. memory unit is busy with the STO instruction, but otherwise this pipeline will stall waiting for it to complete.   Note that even though the Wishbone bus can support pipelined accesses at one access per clock, only the prefetch stage can take advantage of this. Load and Store instructions are stuck at one wishbone cycle per instruction.   \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}     1279,23 → 1232,36  should be sorted, and the next alarm in terms of Jiffies should be written to the register.   \section{Manual Cache} \section{Direct Memory Access Controller}   The manual cache is an experimental setting that may not remain with the Zip CPU for very long. It is designed to facilitate running from FLASH or ROM memory, although the pipeline prefetch cache really makes this need obsolete.  The manual cache works by copying data from a wishbone address (range) into the cache register, and then by making that memory available as memory to the Zip System. It is a {\em manual cache} because the processor must first specify what memory to copy, and then once copied the processor can only access the cache memory by the cache memory location. There is no transparency. It is perhaps best described as a combination DMA controller and local memory. The Direct Memory Access (DMA) controller can be used to either move memory from one location to another, to read from a peripheral into memory, or to write from a peripheral into memory all without CPU intervention. Further, since the DMA controller can issue (and does issue) pipeline wishbone accesses, 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.)   Worse, this cache is likely going to be removed from the ZipSystem. Having used the ZipSystem now for some time, I have yet to find a need or use for the manual cache. I will likely replace this peripheral with a proper DMA controller. 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.   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 two) 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.     \chapter{Operation}\label{chap:ops}   The Zip CPU, and even the Zip System, is not a System on a Chip (SoC). It  1322,7 → 1288,7  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  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 1333,7 → 1299,269  Eventually, I intend to place an operating system onto the ZipSystem, I'm  just not there yet.   The rest of this chapter examines some common programming constructs, and how they might be applied to the Zip System.   \section{Example: Idle Task} One task every operating system needs is the idle task, the task that takes place when nothing else can run. On the Zip CPU, this task is quite simple, and it is shown in assemble in Tbl.~\ref{tbl:idle-asm}. \begin{table}\begin{center} \begin{tabular}{ll} {\tt idle\_task:} \\ & {\em ; Wait for the next interrupt, then switch to supervisor task} \\ & {\tt WAIT} \\ & {\em ; When we come back, it's because the supervisor wishes to} \\ & {\em ; wait for an interrupt again, so go back to the top.} \\ & {\tt BRA idle\_task} \\ \end{tabular} \caption{Example Idle Loop}\label{tbl:idle-asm} \end{center}\end{table} When this task runs, the CPU will fill up all of the pipeline stages up the ALU. The {\tt WAIT} instruction, upon leaving the ALU, places the CPU into a sleep state where nothing more moves. Sure, there may be some more settling, the pipe cache continue to read until full, other instructions may issue until the pipeline fills, but then everything will stall. Then, once an interrupt takes place, control passes to the supervisor task to handle the interrupt.  When control passes back to this task, it will be on the next instruction. Since that next instruction sends us back to the top of the task, the idle task thus does nothing but wait for an interrupt.   This should be the lowest priority task, the task that runs when nothing else can. It will help lower the FPGA power usage overall---at least its dynamic power usage.   \section{Example: Memory Copy} One common operation is that of a memory move or copy. Consider the C code shown in Tbl.~\ref{tbl:memcp-c}. \begin{table}\begin{center} \parbox{4in}{\begin{tabbing} {\tt void} \= {\tt memcp(void *dest, void *src, int len) \{} \\  \> {\tt for(int i=0; i \hspace{0.2in} {\tt *dest++ = *src++;} \\ \} \end{tabbing}} \caption{Example Memory Copy code in C}\label{tbl:memcp-c} \end{center}\end{table} This same code can be translated in Zip Assembly as shown in Tbl.~\ref{tbl:memcp-asm}. \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 ; (4 stalls, cannot be scheduled away)} \\ & {\tt STO R3,(R2)} {\em ; (4 schedulable stalls, has no impact now)} \\ & {\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} \\ \end{tabular} \caption{Example Memory Copy code in Zip Assembly}\label{tbl:memcp-asm} \end{center}\end{table} This example points out several things associated with the Zip CPU. First, a straightforward implementation of a for loop is not the fastest loop structure. For this reason, we have placed the test to continue at the end. Second, all pointers are {\tt void} pointers to arbitrary 32--bit data types. The Zip CPU does not have explicit support for smaller or larger data types, and so this memory copy cannot be applied at a byte level. Third, we've optimized the conditional jump to a return instruction into a conditional return instruction.   \section{Context Switch}   Fundamental to any multiprocessing system is the ability to switch from one task to the next. In the ZipSystem, this is accomplished in one of a couple 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} \begin{table}\begin{center} \begin{tabular}{ll} {\tt return\_to\_user:} \\ & {\em; The instruction before the context switch processing must} \\ & {\em; be the RTU instruction that enacted user mode in the first} \\ & {\em; place. We show it here just for reference.} \\ & {\tt RTU} \\ {\tt trap\_check:} \\ & {\tt MOV uCC,R0} \\ & {\tt TST \$TRAP,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} \end{center}\end{table}  shows the rudiments of this code, while showing nothing of how the  actual trap would be implemented.   You may also wish to note that the instruction before the first instruction in our context swap {\em must be} a return to userspace instruction.  Remember, the supervisor process is re--entered where it left off. This is different from many other processors that enter interrupt mode at some vector or other. In this case, we always enter supervisor mode right where we last left.\footnote{The one exception to this rule is upon reset where supervisor mode is entered at a pre--programmed wishbone memory address.}   \item Capture user counters. If the operating system is keeping track of  system usage via the accounting counters, those counters need to be  copied and accumulated into some master counter at this point.   \item Preserve the old context. This involves pushing all the user registers  onto the user stack and then copying the resulting stack address  into the tasks task structure, as shown in Tbl.~\ref{tbl:context-out}. \begin{table}\begin{center} \begin{tabular}{ll} {\tt swap\_out:} \\ & {\tt MOV -15(uSP),R1} \\ & {\tt STO R1,stack(R12)} \\ & {\tt MOV uPC,R0} \\ & {\tt STO R0,15(R1)} \\ & {\tt MOV uCC,R0} \\ & {\tt STO R0,14(R1)} \\ & {\em ; We can skip storing the stack, uSP, since it'll be stored}\\ & {\em ; elsewhere (in the task structure) }\\ & {\tt MOV uR13,R0} \\ & {\tt STO R0,13(R1)} \\  & \ldots {\em ; Need to repeat for all user registers} \\ & {\tt MOV uR0,R0} \\ & {\tt STO R0,1(R1)} \\ \end{tabular} \caption{Example Storing User Task Context}\label{tbl:context-out} \end{center}\end{table} For the sake of discussion, we assume the supervisor maintains a pointer to the current task's structure in supervisor register {\tt R12}, and that {\tt stack} is an offset to the beginning of this structure indicating where the stack pointer is to be kept within it.    For those who are still interested, the full code for this context  save can be found as an assembler macro within the assembler  include file, {\tt sys.i}.   \item Reset the watchdog timer. If you are using the watchdog timer, it should  be reset on a context swap, to know that things are still working.  Example code for this is shown in Tbl.~\ref{tbl:reset-watchdog}. \begin{table}\begin{center} \begin{tabular}{ll} \multicolumn{2}{l}{{\tt define WATCHDOG\_ADDRESS 32'hc000\_0002}}\\ \multicolumn{2}{l}{{\tt define WATCHDOG\_TICKS 32'd1\_000\_000} {; \em = 10 ms}}\\ & {\tt LDI WATCHDOG\_ADDRESS,R0} \\ & {\tt LDI WATCHDOG\_TICKS,R1} \\ & {\tt STO R1,(R0)} \end{tabular} \caption{Example Watchdog Reset}\label{tbl:reset-watchdog} \end{center}\end{table}   \item Interrupt handling. An interrupt handler within the Zip System is nothing  more than a task. At context swap time, the supervisor needs to  disable all of the interrupts that have tripped, and then enable  all of the tasks that would deal with each of these interrupts.  These can be user tasks, run at higher priority than any other user  tasks. Either way, they will need to re--enable their own interrupt  themselves, if the interrupt is still relevant.    An example of this master interrut handling is shown in   Tbl.~\ref{tbl:pre-handler}. \begin{table}\begin{center} \begin{tabular}{ll} {\tt pre\_handler:} \\ & {\tt LDI PIC\_ADDRESS,R0 } \\ & {\em ; Start by grabbing the interrupt state from the interrupt}\\ & {\em ; controller. We'll store this into the register R7 so that }\\ & {\em ; we can keep and preserve this information for the scheduler}\\ & {\em ; to use later. }\\ & {\tt LOD (R0),R1} \\ & {\tt MOV R1,R7 } \\ & {\em ; As a next step, we need to acknowledge and disable all active}\\ & {\em ; interrupts. We'll start by calculating all of our active}\\ & {\em ; interrupts.}\\ & {\tt AND 0x07fff,R1 } \\ & {\em ; Put the active interrupts into the upper half of R1} \\ & {\tt ROL 16,R1 } \\ & {\tt LDILO 0x0ffff,R1 } \\ & {\tt AND R7,R1}\\ & {\em ; Acknowledge and disable active interrupts}\\ & {\em ; This also disables all interrupts from the controller, so}\\ & {\em ; we'll need to re-enable interrupts in general shortly } \\ & {\tt STO R1,(R0) } \\ & {\em ; We leave our active interrupt mask in R7 so the scheduler can}\\ & {\em ; release any tasks that depended upon them. } \\ \end{tabular} \caption{Example checking for active interrupts}\label{tbl:pre-handler} \end{center}\end{table}   \item Calling the scheduler. This needs to be done to pick the next task  to switch to. It may be an interrupt handler, or it may be a normal  user task. From a priority standpoint, it would make sense that the  interrupt handlers all have a higher priority than the user tasks,  and that once they have been called the user tasks may then be called  again. If no task is ready to run, run the idle task to wait for an  interrupt.    This suggests a minimum of four task priorities:  \begin{enumerate}  \item Interrupt handlers, executed with their interrupts disabled  \item Device drivers, executed with interrupts re-enabled  \item User tasks  \item The idle task, executed when nothing else is able to execute  \end{enumerate}    For our purposes here, we'll just assume that a pointer to the current  task is maintained in {\tt R12}, that a {\tt JSR scheduler} is   called, and that the next current task is likewise placed into  {\tt R12}.   \item Restore the new tasks context. Given that the scheduler has returned a  task that can be run at this time, the stack pointer needs to be   pulled out of the tasks task structure, placed into the user  register, and then the rest of the user registers need to be popped  back off of the stack to run this task. An example of this is  shown in Tbl.~\ref{tbl:context-in}, \begin{table}\begin{center} \begin{tabular}{ll} {\tt swap\_in:} \\ & {\tt LOD stack(R12),R1} \\ & {\tt MOV 15(R1),uSP} \\ & {\tt LOD 15(R1),R0} \\ & {\tt MOV R0,uPC} \\ & {\tt LOD 14(R1),R0} \\ & {\tt MOV R0,uCC} \\ & {\tt LOD 13(R1),R0} \\ & {\tt MOV R0,uR12} \\  & \ldots {\em ; Need to repeat for all user registers} \\ & {\tt LOD 1(R1),R0} \\ & {\tt MOV R0,uR0} \\ & {\tt BRA return\_to\_user} \\ \end{tabular} \caption{Example Restoring User Task Context}\label{tbl:context-in} \end{center}\end{table}  assuming as before that the task  pointer is found in supervisor register {\tt R12}.  As with storing the user context, the full code associated with  restoring the user context can be found in the assembler include  file, {\tt sys.i}.   \item Clear the userspace accounting registers. In order to keep track of  per process system usage, these registers need to be cleared before  reactivating the userspace process. That way, upon the next  interrupt, we'll know how many clocks the userspace program has  encountered, and how many instructions it was able to issue in  those many clocks.    \item Jump back to the instruction just before saving the last tasks context,  because that location in memory contains the return from interrupt  command that we are going to need to execute, in order to guarantee  that we return back here again. \end{enumerate}   \chapter{Registers}\label{chap:regs}   The ZipSystem registers fall into two categories, ZipSystem internal registers
1342,7 → 1570,7
 \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 CCHE & \scalebox{0.8}{\tt 0xc0000002} & 32 & R/W & Manual Cache Controller \\\hline  & \scalebox{0.8}{\tt 0xc0000002} & 32 & R/W & {\em (Reserved for future use)} \\\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
1356,6 → 1584,10
 UMSTL & \scalebox{0.8}{\tt 0xc000000d} & 32 & R/W & User Stall Counter \\\hline UPSTL & \scalebox{0.8}{\tt 0xc000000e} & 32 & R/W & User Pre--Fetch Stall Counter \\\hline UICNT & \scalebox{0.8}{\tt 0xc000000f} & 32 & R/W & User Instruction Counter\\\hline DMACTRL & \scalebox{0.8}{\tt 0xc0000010} & 32 & R/W & DMA Control Register\\\hline DMALEN & \scalebox{0.8}{\tt 0xc0000011} & 32 & R/W & DMA total transfer length\\\hline DMASRC & \scalebox{0.8}{\tt 0xc0000012} & 32 & R/W & DMA source address\\\hline DMADST & \scalebox{0.8}{\tt 0xc0000013} & 32 & R/W & DMA destination address\\\hline % Cache & \scalebox{0.8}{\tt 0xc0100000} & & & Base address of the Cache memory\\\hline \end{reglist} \caption{Zip System Internal/Peripheral Registers}\label{tbl:zpregs}
1491,6 → 1723,49
 Operating System will read the timers back off, to determine how much of the CPU the process had consumed.   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 specifying the entire transfer length, the starting address to read from, and the starting address to write to. The registers can be written to when the DMA is idle, and read at any time. The control register, however, will need some more explanation.   The bit allocation of the control register is shown in Tbl.~\ref{tbl:dmacbits}. \begin{table}\begin{center} \begin{bitlist} 31 & R & DMA Active\\\hline 30 & R & Wishbone error, transaction aborted (cleared on any write)\\\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 '0' 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 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  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  one item at a time set this value to 0. To transfer 1024 at a time,  set it to 1024.\\\hline \end{bitlist} \caption{DMA Control Register Bits}\label{tbl:dmacbits} \end{center}\end{table} This control register has been designed so that the common case of memory access need only set the key and the transfer length. Hence, writing a \hbox{32'h0fed03ff} to the control register will start any memory transfer.  On the other hand, if you wished to read from a serial port (constant address) and put the result into a buffer every time a word was available, you  might wish to write \hbox{32'h2fed8000}--this assumes, of course, that you have a serial port wired to the zero bit of this interrupt control. (The DMA controller does not use the interrupt controller, and cannot clear interrupts.) As a third example, if you wished to write to an external FIFO anytime it was less than half full (had fewer than 512 items), and interrupt line 2 indicated this condition, you might wish to issue a \hbox{32'h1fed8dff} to this port.   \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
1505,7 → 1780,7
 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 8 & R/W & Step Command, set to '1' to step the CPU, also sets the halt bit\\\hline 7 & R & Interrupt Request \\\hline 6 & R/W & Command RESET \\\hline 5\ldots 0 & R/W & Debug Register Address \\\hline
1557,7 → 1832,7
 \caption{Debug Register Addresses}\label{tbl:dbgaddrs} \end{center}\end{table} Primarily, these registers'' include access to the entire CPU register set, as well as the 16~internal peripherals. To read one of these registers set, as well as the 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.
1646,7 → 1921,7
   This core is based upon the Basys--3 development board sold by Digilent.  The Basys--3 development board contains one external 100~MHz clock, which is sufficient to run the ZIP CPU core. sufficient to run the Zip CPU core. \begin{table}[htbp] \begin{center} \begin{clocklist}
1712,6 → 1987,340
 detail. Finally, the Zip System produces one external interrupt whenever the CPU halts to wait for the debugger.   \chapter{Initial Assessment}\label{chap:assessment}   Having now worked with the Zip CPU for a while, it is worth offering an honest assessment of how well it works and how well it was designed. At the end of this assessment, I will propose some changes that may take place in a later version of this Zip CPU to make it better.   \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 As an estimate of the weight'' of this implementation, the CPU has  cost me less than 150 hours to implement from its inception. \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  a chip very well, but then it was never intended to be a system on a  chip but rather a system within a chip. \item The extremely simplified instruction set of the Zip CPU was a good  choice. Although it does not have many of the commonly used  instructions, PUSH, POP, JSR, and RET among them, the simplified  instruction set has demonstrated an amazing versatility. I will contend  therefore and for anyone who will listen, that this instruction set  offers a full and complete capability for whatever a user might wish  to do with two exceptions: bytewise character access and accelerated  floating-point support. \item This simplified instruction set is easy to decode. \item The simplified bus transactions (32-bit words only) were also very easy  to implement. \item The novel approach of having a single interrupt vector, which just  brings the CPU back to the instruction it left off at within the last  interrupt context doesn't appear to have been that much of a problem.  If most modern systems handle interrupt vectoring in software anyway,  why maintain hardware support for it? \item My goal of a high rate of instructions per clock may not be the proper  measure. For example, if instructions are being read from a SPI flash  device, such as is common among FPGA implementations, these same  instructions may suffer stalls of between 64 and 128 cycles per  instruction just to read the instruction from the flash. Executing the  instruction in a single clock cycle is no longer the appropriate  measure. At the same time, it should be possible to use the DMA  peripheral to copy instructions from the FLASH to a temporary memory  location, after which they may be executed at a single instruction  cycle per access again.  \end{itemize}   \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. {\em This has become the  fundamental limit on the speed and performance of the CPU!}  Those familiar with the Von--Neumann approach of sharing a bus  between data and instructions will not be surprised by this assessment.    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,  though, it will make compiling non-Zip CPU code difficult--especially  anything that assumes sizeof(int)=4*sizeof(char), or that tries to  create unions with characters and integers and then attempts to  reference the address of the characters within that union.   \item The Zip CPU does not support a data cache. One can still be built  externally, but this is a limitation of the CPU proper as built.  Further, under the theory of the Zip CPU design (that of an embedded  soft-core processor within an FPGA, where any address'' may reference  either memory or a peripheral that may have side-effects), any data  cache would need to be based upon an initial knowledge of whether or  not it is supporting memory (cachable) or peripherals. This knowledge  must exist somewhere, and that somewhere is currently (and by design)  external to the CPU.    This may also be written off as a feature'' of the Zip CPU, since  the addition of a data cache can greatly increase the LUT count of  a soft core.   \item Many other instruction sets offer three operand instructions, whereas  the Zip CPU only offers two operand instructions. This means that it  takes the Zip CPU more instructions to do many of the same operations.  The good part of this is that it gives the Zip CPU a greater amount of  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  are that 1) loads may unnecessarily stall the CPU, even if other  things could be done while waiting for the load to complete, 2)  bus errors on stores will never be caught at the point of the error,  and 3) branch prediction becomes more difficult.   \item Although switching to an interrupt context in the Zip CPU design doesn't  require a tremendous swapping of registers, in reality it still  does--since any task swap still requires saving and restoring all  16~user registers. That's a lot of memory movement just to service  an interrupt.   \item The Zip CPU is by no means generic: it will never handle addresses  larger than 32-bits (16GB) without a complete and total redesign.  This may limit its utility as a generic CPU in the future, although  as an embedded CPU within an FPGA this isn't really much of a limit  or restriction.   \item While the Zip CPU has its own assembler, it has no linker and does not  (yet) support a compiler. The standard C library is an even longer  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.)   \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.)    \iffalse \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.   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:   \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.   \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 \fi   \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.   \iffalse \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 \fi   \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 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}`