Changelog

• 14 March 2018: Add more explicit note about what stalling a register means when discussing halt implementation.

Your Task

1. Download pipelab1_base.hcl (on Collab in the resources tab) to get a copy of the sequential simulator with only nop, halt, irmovq, and rrmovq implemented.

2. Rename this to pipelab1.hcl, and add pipelining to this simulator with two pipeline stages that uses forwarding to handle data hazards:
   • Stage 1: Fetch and Decode
   • Stage 2: Writeback

   (There is no execute or memory phase for these instructions.)

3. Test your simulator with make test-pipelab1.

4. Submit to archimedes.

Suggested Approach

We suggest the following steps:

1. Add a pipeline register bank. Identify where the two pipeline stages will split your processor, then identify all wires which cross that point and put them in a pipeline register
2. Replace wires with pipeline register inputs and outputs wires.
3. Look for hazards and solve them with forwarding (if possible) or stalling

Adding a pipeline register bank

Following the textbook’s tradition, we suggest calling the input side of the register d for decode and the output side W for writeback:

register dW {
	# todo: fill in the details here
}

Look through pipelab1_base.hcl; each value used as an input in writeback that is not also computed in that stage will need to be stored in a pipeline register. For example, the reg_inputE mux uses icode, reg_outputA, and valC as inputs, so we’ll need all three of those in our new register, as well as registers for the other signals used as inputs in writeback.

Always pick the default values in the pipeline register to be the values you’d expect for nop; in NOP in the icode, REG_NONE in any register spots, etc.

Replacing wires with register inputs/outputs

Recall that if we name our register bank dW then whatever signal we put into d_thing will come out of W_thing on the next cycle.

Go through each signal and, if it crosses the register bank, replace every use before the register bank with d_... and every use after the register bank with W_....

For example, consider icode:

• Remove the wire:4 icode declaration since we have it in dW.
• In fetch and decode, replace all occurrences of icode with d_icode
• In writeback replace all occurrences of icode with W_icode

Do the same thing with valC.

Handling built-in signals

The signals reg_outputA, reg_dstE, and Stat have to be treated specially because they interact with fixed functionality (like the register file). We will need make an explicit connection between the fixed functionality and the appropriate pipeline register since we can’t, for example, tell the register file to send its output directly to a pipeline register bank.

Thus, reg_outputA (an output set by fixed functionality during decode) will need to be saved into d_... during decode and used as W_... afterwards, as in

# in decode:
d_rvalA = reg_outputA;
# in execute and later phases, use W_rvalA instead of reg_outputA

Similarly, reg_dstE (an input used by fixed functionality during writeback) will be originally computed into d_dstE during Decode and then reg_dstE = W_dstE placed in writeback to pass the value from the pipeline register to HCLRS’s built-in register file.

Stat is an output like reg_dstE and will need the same treatment (set d_Stat before the pipeline register based on the icode, and Stat = W_Stat afterward). Stat needs to be set in the writeback stage and not the fetch/decode stage, so a halt doesn’t halt the processor early.

(You could also compute Stat from the icode in the writeback stage instead of computing it during fetch+decode, but computing the new Stat value early may make implementing the full processor easier and matches our textbook’s design.)

Testing without hazard handling

At this point, the rrmovq.yo we used for irrr.hcl

irmovq $5678, %rax
irmovq $34, %rcx
rrmovq %rax, %rdx
rrmovq %rcx, %rax

should take 6 (not 5) cycles to set three registers:

| RAX:               22   RCX:               22   RDX:             162e |

Once you handle halt according to the instructions below, it should leave the PC at address 0x18, like the single-cycle processor.

Handling data hazards

Consider

irmovq $1, %rax
rrmovq %rax, %rbx

In a pipeline diagram (given that we have no execute or memory phases), these will look like

Note that the immediate value won’t be written to the register file until the after of cycle 2, but it will be attempted to be read by the next instruction at the during of cycle 2. This is an example of a data dependency that exercises a hazard in our hardware design so far.

We can bypass this hazard in two ways. We can either stall, or we can forward data. Forwarding is always preferred to stalling if both are possible, so we’ll forward.

Forward

We want to grab the value that is being prepped for writing to the register file before it actually gets written if it is the register we are trying to read. Thus, d_rvalA (the value of register A being sent from the decode stage to writeback writeback stage) will probably be set by a MUX:

• if reg_dstE is the same as the decode phase’s reg_srcA and is not REG_NONE, it should come from reg_inputE (the value that is about to be written);
• otherwise, it should be reg_outputA, as usual

If correctly implemented, y86/irrr7.yo

irmovq $1, %rax
rrmovq %rax, %rbx

should take 4 cycles to put a 1 in both %rax and %rbx, while y86/rrmovq.yo

irmovq $5678, %rax
irmovq $34, %rcx
rrmovq %rax, %rdx
rrmovq %rcx, %rax

should still take 6 cycles and result in

| RAX:               22   RCX:               22   RDX:             162e |

like it did before.

I mention the number of cycles because the other solution (stalling) would increase them.

Handling halt

You can think of halt and invalid instructions as special kind of control hazard, since the instructions after a halt (or an invalid instruction) are not supposed to run.

We recommend stalling the register that feeds the fetch stage when you encounter a halt or invalid instruction to avoid starting to executing instructions that aren’t part of the program. However, in the two-stage pipeline in this lab, instructions do not change any state (memory, program registers, condition codes) until the last stage, so this is not strictly necessary.

(By stalling a register, we mean keeping it from changing. You can do this with the stall_F signal or by changing the f_pc assignment.)

Debugging and Miscellaneous Hints

Pay attention to your pipeline registers

Reading the values of the pipeline registers is a good way to tell what instruction is being run in each stage.

If you declare a register bank like:

register dW {
    icode : 4 = NOP;
    otherValue : 64 = 0;
}

Then part of the HCLRS output (when not using -q) will have a line like following for every cycle:

|  register dW(N) { icode=7 otherValue=00000000000000A4 }

indicating that during that cycle, the W_icode was 0x7 (the number for JXX, as you can lookup in the HCLRS manual or your textbook) and W_otherValue was 0xA4. With our usual pipeline register naming convention, this means that a JXX insturction is currently in the writeback stage, and the otherValue for that instruction was 0xA4.

Use debug mode

You can run it in debug mode: ./hclrs -d pipelab1.hcl somefile.yo. This will show you the value of all wires during each step.

You can also redirect the output from debug mode to a file, like:

./hclrs -d pipelab1.hcl somefile.yo >output.txt 2>&1

Then open output.txt in a text editor. (>output.txt sends normal output to the file output.txt instead of the normal output file. 2>&1 means to redirect error output where normal output is going.) This redirection support is a shell feature that works with any command, not just hclrs.

Use reference outputs

You can look at the outputs in hclrs/testdata/pipelab1-reference to see what our implementation of pipelab1.hcl did for each .yo file included with HCLRS.

Initialize what you declare

The simulator has to provide input to every wire and register in order to run. It does not know what those inputs should be unless you tell it. Thus, if you say

wire baz:4;
register qB { xyxxy:32 = 0; }

then you must also say

baz = something;
q_xyxxy = something_else;

or else you will get an error.

Test your code often

We’ve been telling you this for years now, test your code often! At least check that it compiles with ./hclrs --check pipelab1.hcl. Particularly when working with a language you don’t know well, frequent feedback is useful.