Penn State University School of Electrical Engineering and Computer Science Page 1 of 5 CMPEN 331 – Computer Organization and Design,
Lab 3
This lab introduces the idea of the pipelining technique for building a fast CPU. The students will obtain experience with the design implementation and testing of the first two stages (Instruction Fetch, Instruction Decode) of the five-stage pipelined CPU using the Xilinx design package for FPGAs. It is assumed that students are familiar with the operation of the Xilinx design package for Field Programmable Gate Arrays (FPGAs) through the Xilinix tutorial available in the class website.
1. Pipelining
Pipelining is an implementation technique in which multiple instructions are overlapped in execution. The five- stage pipelined CPU allows overlapping execution of multiple instructions. Although an instruction takes five clock cycle to pass through the pipeline, a new instruction can enter the pipeline during every clock cycle. Under ideal circumstances, the pipelined CPU can produce a result in every clock cycle. Because in a pipelined CPU there are multiple operations in each clock cycle, we must save the temporary results in each pipeline stage into pipeline registers for use in the follow-up stages. We have five stages: IF, ID, EXE, MEM, and WB. The PC can be considered as the first pipeline register at the beginning of the first stage. We name the other pipeline registers as IF/ID, ID/EXE, EXE/MEM, and MEM/WB in sequence. In order to understand in depth how the pipelined CPU works, we will show the circuits that are required in each pipeline stage of a baseline CPU.
2. Circuits of the Instruction Fetch Stage
The circuit in the IF stage are shown in Figure 2. Also, looking at the first clock cycle in Figure 1(b), the first lw instruction is being fetched. In the IF stage, there is an instruction memory module and an adder between two pipeline registers. The left most pipeline register is the PC; it holds 100. In the end of the first cycle (at the rising edge of clk), the instruction fetched from instruction memory is written into the IF/ID register. Meanwhile, the output of the adder (PC + 4, the next PC) is written into PC.
3. Circuits of the Instruction Decode Stage
Referring to Figure 3, in the second cycle, the first instruction entered the ID stage. There are two jobs in the second cycle: to decode the first instruction in the ID stage, and to fetch the second instruction in the IF stage. The two instructions are shown on the top of the figures: the first instruction is in the ID stage, and the second instruction is in the IF stage. The first instruction in the ID stage comes from the IF/ID register. Two operands are read from the register file (Regfile in the figure) based on rs and rt, although the lw instruction does not use the operand in the register rt. The immediate (imm) is sign- extended into 32 bits. The regrt signal is used in the ID stage that selects the destination register number; all others must be written into the ID/EXE register for later use. At the end of the second cycle, all the data and control signals, except for regrt, in the ID stage are written into the ID/EXE register. At the same time, the PC and the IF/ID register are also updated.
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Figure 1 Timing chart comparison between two types of CPUs
Figure 2 Pipeline instruction fetch (IF) stage
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Figure 3 Pipeline instruction decode (ID) stage
4. Table 1 lists the names and usages of the 32 registers in the register file. Table 1 MIPS general purpose register
$zero
0
Constant 0
$at
1
Reserved for assembler
$v0, $v1
2, 3
Function return values
$a0 – $a3
4–7
Function argument values
$t0 – $t7
8 – 15
Temporary (caller saved)
$s0 – $s7
16 – 23
Temporary (callee saved)
$t8, $t9
24, 25
Temporary (caller saved)
$k0, $k1
26, 27
Reserved for OS Kernel
$gp
28
Pointer to Global Area
$sp
29
Stack Pointer
$fp
30
Frame Pointer
$ra
31
Return Address
5. Table 2 lists some MIPS instructions that will be implemented in our CPU Table 2 MIPS integration instruction
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6. Initialize the first 10 words of the Data memory with the following HEX values: A00000AA
10000011 20000022 30000033 40000044 50000055 60000066 70000077 80000088 90000099
7. Write a Verilog code that implement the following instructions using the design shown in Figure 2 and Figure 3. Write a Verilog test bench to verify your code: (You have to show all the signals written into the IF/ID register and the ID/EXE register in your simulation outputs)
# address instruction 100: lw $v0, 00($at) 104: lw $v1, 04($at)
comment
# $2 ß memory[$1+00]; load x[0] # $3 ß memory[$1+04]; load x[1]
Assume that the register $at has the value of 0
8. Write a report that contains the following: a. Your Verilog design code. Use:
i. Device: XC7Z010- CLG400 -1 or choose any other FPGA type. You can use Arria II if you are using Quartus II software.
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b. Your Verilog® Test Bench design code. Add “`timescale 1ns/1ps” as the first line of your test bench file.
c. The waveforms resulting from the verification of your design with simulation showing all the signals
written into the IF/ID register and the ID/EXE register.
d. The design schematics from the Xilinx synthesis of your design. Do not use any area constraints.
e. Snapshot of the I/O Planning and
f. Snapshot of the floor planning
9. REPORT FORMAT: Free form, but it must be:
g. One report per student.
h. Have a cover sheet with identification: Title, Class, Your Name, etc.
i. Using Microsoft word and it should be uploaded in word format not PDF. If you know LaTex, you should
upload the Tex file in addition to the PDF file.
j. Double spaced
10. You have to upload the whole project design file zipped with the word file.
Please find below some hints that may help you in the implementation of this lab.
• You should have a module for each of the pieces of the CPU, 9 in total for this lab. I would recommend having all 9 modules within the same Verilog design source, instead of having each module in a separate design source. Since future labs will build off of this one, I think it will be easier to access each module if they’re in the same design source.
• Initialize all the registers in RegFile to 0.
• For the Sign Extension module, Review & Verilog_1 slide 60 concatenation will help. The last example
essentially gives you “aaaaab”
• For the Control Unit, there are good tables in Zybooks 4.4 to help you. The “aluc” output is going to be
the 4-bit output that would come from the ALU control shown in Zybooks. Essentially, we’re getting rid of the ALU control and incorporating it into the Control Unit. Use the op and func to help determine what this output should be.
• You will not need the Data Memory portion for this first lab; feel free to ignore this until we implement it in a later lab.
• You will almost always need an always begin block in each of your modules. Having statements outside of these blocks will generally cause errors.
• Make sure your inputs are wires and your outputs are registers. Remember, registers are the only things that can be assigned value (placed on the left-hand side of an = statement).
• You will do a lot of debugging in this lab. My advice is to debug one thing at a time, starting with as early in the stages as possible and continuing. This way, you can identify if your waveform is producing correct results.
E.g. Make sure your program counter is incrementing correctly. Then make sure the instructions you’re retrieving from the IM are correct. etc. etc.
If you haven’t started yet, I highly recommend starting as soon as possible. Too many students always put this off until right before it’s due and end up having major problems / not being able to finish. Future labs build upon this one; if you don’t finish this lab this time around, you’re going to have to finish in the future, anyway.
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