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cs3220 CS3220 Lab #1 : Pipeline Design Solution
100 pts in total, will be rescaled into 11.25% of your final score of the course.
Part 1: 50 pts, submission ddl: Sep 11th
Part 2: 50 pts, submission ddl: Sep 18th
Part 3 (Optional): 20 bonus pts, submission ddl: Sep 18th
Description: In this assignment, you will create a 5-stage RISC-V pipelined processor using Verilog, focusing on a subset of the RISC-V ISA. We will be using the Tiny RISC-V version from Cornell, which is provided in the Tiny RISC-V ISA file.
In part 0, you will familiarize yourself with the essential software tools required for the experiments on the PACE cluster. In part-1, you only need to implement addi, add, beq instructions to pass all 5 test cases in test/part1/test[1-5].mem. In part 2, you will expand your processor by adding more instructions to pass the test cases under test/part2/. Part 3 is optional for bonus pts, where you will complete the RISC-V processor.
Part 0: Experiment Setup
Please follow the instructions provided to run experiments on the PACE cluster.
What to submit:
No submission is required for Part 0. However, ensure that you can independently utilize GTKWave to visualize waveforms effectively.
Part 1: Minimal functionality
In this part, you'll implement a subset of RISC-V instructions and aim to pass 5 tests in the test/part1 directory. Refer to the test cases and the README file in test/part1 for detailed requirements.
1. [20pts] Complete the agex_stage.v file. No modifications to other files are necessary. Your implementation should pass test/part1/test[15].mem. If all test cases don't pass, you'll receive partial scores. To test all cases together, run run_tests.sh part1, and it will produce part1_results.log and part1_tests.log for you. You can also run each test case independently, see FAQ for part 1. Note: If you encounter latch size errors, modify the corresponding latch size definition in define.vh.
2. [10pts] Explain the actions in each pipeline stage while executing test/part1/test1.mem. Include waveform screenshots illustrating relevant signals. For example, in the Execute stage (EX stag), you should visualize input (regval1_AGEX, regval2_AGEX) and output (aluout_AGEX) signals of the ALU, and the opcode (op_I_AGEX).
3. [10pts]Explain how your RISC-V processor resolves Read-After-Write hazards in test/part1/test2.mem. Include waveform screenshots illustrating the discussed signals.
4. [10pts] Explain how your RISC-V processor handles branch misprediction in test/part1/test4.mem. Include waveform screenshots illustrating relevant signals. Note: In Lab 1, branches are always predicted as not-taken; in Lab 2, you will implement your own branch predictor.
What to submit:
Submit the following to Canvas:
Include a PDF file containing your explanations and corresponding screenshots.
Start part 2 as early as possible and do not wait untill the last week, as it involves heavier workload than part 1.
Part 2: Expanding instruction set
Test cases:
In part-2, all instructions in the test cases under test/part2/ such as add, addi, auipc, beq, bge, (all branch instructions) jal, jalr instructions will be tested. To test all test cases together, use run_tests.sh part2, which will generate part2_results.log and part2_tests.log. Tests [7-9] are handwritten assembly code, which are easier to debug, so start with those.
In part-2, we start to use modified RISC-V test cases. *.S is assembly code that takes RISC-V macro. Macros are defined at include/test_macros.h or include/riscv_test.h. It also uses ABI names and Pseudo Instructions. You can find a summary of information [here].
*.dump is an dump file output from gcc riscv compiler. *.mem file has the format for verilog code. *.dec file is useful when using [RISC-V emulator]
What to submit:
Submit the following to Canvas:
Avoid procrastination; start early to manage the workload effectively.
Part 3 (Optional) Complete the processor
1. [20pts] In this part, you will complete the processor to fully support the RISC-V ISA (except CSR instructions). Your goal is to ensure your program passes all the test cases in the test/part3/ directory. To receive full credits, your program must pass test/part3/testall.mem.
Partial scores will be awarded based on the coverage of the Part 3 test suites.
What to submit:
Submit the following to Canvas:
Useful Information
References summary of RISC-V Assembly coding
RISC-V emulator (tiny RV2)
Verilator manual
GTKWave manual
Tutorial about RISC-V TEST SUITE
FAQ for part 1
(Q) How do I run a specific test file? (A) Please see "define.vh": you need to change line 21 to change which test file to read: `define
IDMEMINITFILE "/home/zhifan/workspace/cs3220-23fall/lab1/test/part1/test4.mem". You need to change "test4.mem" into
(Q) Debugging takes so much time. Any tips to reduce the debugging time? (A) Some suggestions: 1. Review code carefully and understand the ISA behavior correctly. 2. If make command fails to compile, read the error messages carefully. 3. make command generates vcd file. Please use GTKWave to see important signals and check whether the signals works as expected according to *.asm files or RISC-V enumlators. When debugging, it is always helpful to visualize clk signal and pc values along with other important signals.
(Q) How do I know whether my implementation is correct or not? (A) If you run make, you would see "Pass" message.
(Q) Can I add new files? (A) Yes, but please make sure they are added in the zip file.
(Q) Do we need to implement a branch predictor? (A) It's not required for lab 1.
(Q) Do we need to create a stack for nested JAL instructions? (A) The hardware does not know any nested calls, so you do not need to implement it.
(Q) BEQ t1, t1, imm : if a branch is taken, is the new PC = PC + imm or new PC = PC + 4+ imm? (A) The answer is PC = PC + offset. Please be careful with converting imm to offset.
(Q) Do we need to worry about whether we should prevent all writes to the zero register and treat it as always zero, or if that is solely up to us dependent on our design? (A) This is purely S/W job. The H/W doesn't have to check whether x0 is writable or not. The Hardware also doesn't have explicitly insert 0 in hardware.
(Q) Is the immediate field inside assembly code decimal? (A) If the number starts with 0x, it's hexadecimal.
imem[PC_FE_latch[`IMEMADDRBITS-1:`IMEMWORDBITS]]; dmem[memaddr_MEM[`DMEMADDRBITS-1:`DMEMWORDBITS]];
(Q) What does assign inst_FE = imem[PC_FE_latch[`IMEMADDRBITS-1:`IMEMWORDBITS]]; mean? (A) PC_FE_latch contains PC value. Again imem and dmem are word addressable, so we don't need LSB 2 bits. Since imem and dmem has only 2^14 size, we just use addr [15:2] bits to index imem/dmem.
(Q) I'm not sure how to understand part 2 test code. (A) The test in test/part2 is modified code from RISC-V test suite. It uses macro function to generate test code.
(Q) How do I know what is the correct instruction/code behavior? (A) You can probably use RISC-V enumlators or other RISC-V machine to execute the code. One example is here .
(Q) How do I know whether I pass the code or not? (A) For part 1, we provide test code. Your code should print out "Pass" message if you run make.
(Q) My code does not load any instructions. Do I need to change anything? (A) Carefully check if you encountered any error messages and make sure you have set IDMEMINITFILE to the right path.
FAQ for part 2
(Q) what is li instructions in add.dump? (A) li instruction is one of the pseudo instructions. It is the same as addi x0, imm
(Q) I passed test[1-5].mem. why do I fail addi.mem? (A) It contains bne, auipc, jal instructions. So in order to pass part 2 test cases, you need to complete those instructions.
(Q) I'd like to use RISC-V emulator for testing the test code, but it won't take dump file. what should I do? (A) Unfortunately RISC-V emulator only takes assembly instructions. Hence, we recommend to use another emulator . You can use *.dec file in this simulator.
(Q) Behavior of lui. The documentation says that - Semantics : R[rd] = imm << 12. But U-immediate already shifted the immediate by 12 bits. Do I need to shift the sxt_imm_DE. Do I need to shift immediate value again? (A) No. if you have already shift immediage bits in instruction into sxt_imm_DE, you don't have to shift sxt_imm_DE again.
(Q) bge is signed comparison and bgeu is unsigned comparison. What does it mean and what should I do? (A) by default, in verilog all operations are unsigned. However, you can use signed comparisons in verilog by defining wires as signed variables. Here is an example for signed comparisons and unsigned comparisons
`` wire signed [DBITS-1:0] s_regval1_AGEX; // note signed wire signed [`DBITS-1:0] s_regval2_AGEX; //note signed assign s_regval1_AGEX = regval1_AGEX; assign s_regval2_AGEX = regval2_AGEX;
// signed comparison wire s_less; assign s_less = (s_regval1_AGEX < s_regval2_AGEX);
// unsigned comparison wire less; assign less = (regval1_AGEX < regval2_AGEX);
```
(Q) bgeu and bltu use unsigned comparisons. Does it mean I shouldn't sign extend immediage values at the decode stage and keep both unsiged and signed extension versions? (A) No, in RISC-V, all immediate values are sign-extended. begu and bltu are unsigned comparisons with singextended values (e.g. sxt_imm_DE)
(Q) I'm still confused with signed keyword in verilog. Does it perform any sign conversion when I put signed keyword in the above example?
(A) In Verilog, values are just binaries. s_regval1_AGEX and regval1_AGEX have the same value. Signed unsigned are just a matter of interpretation. When arithmetic operations are used such as comparator, signed/unsigned decide how to interpret the value. e.g.) In the above example, let's assume that reval1_AGEX is 0x0000 and regval2_AGEX is 0xFFFF. In that case, s_regval1_AGEX is 0x0000 and s_regval2_AGEX is still 0xFFFF. However, s_regval2_AGEX is interpreted as -1 whereas regval2_AGEX is interpreted as 65535. Hence, if (regval1_AGEX < regval2_AGEX) returns false but if (s_regval1_AGEX < s_regval2_AGEX) returns true.
(Q) Do I need to put the signed keyword for immediate values? (A) Yes, even though immediate values are sign-extended, if we want to treat the immediate value as 2's complement value such as in SLTI_I instruction case, you need to put signed keyword.
FAQ for part 3
(Q) Can you explain the behavior of slti and sltiu. Does it store the outcome of shift value? (A) The outcome of both instructions should be either 0 or 1. It checks whether (R[rs1] < sext(imm)) (signed comparisons for SLTI and unsigned comparisons for SLTIU) and if the condition is true, it sets 1 for the destination.
Part 1: 50 pts, submission ddl: Sep 11th
Part 2: 50 pts, submission ddl: Sep 18th
Part 3 (Optional): 20 bonus pts, submission ddl: Sep 18th
Description: In this assignment, you will create a 5-stage RISC-V pipelined processor using Verilog, focusing on a subset of the RISC-V ISA. We will be using the Tiny RISC-V version from Cornell, which is provided in the Tiny RISC-V ISA file.
In part 0, you will familiarize yourself with the essential software tools required for the experiments on the PACE cluster. In part-1, you only need to implement addi, add, beq instructions to pass all 5 test cases in test/part1/test[1-5].mem. In part 2, you will expand your processor by adding more instructions to pass the test cases under test/part2/. Part 3 is optional for bonus pts, where you will complete the RISC-V processor.
Part 0: Experiment Setup
Please follow the instructions provided to run experiments on the PACE cluster.
What to submit:
No submission is required for Part 0. However, ensure that you can independently utilize GTKWave to visualize waveforms effectively.
Part 1: Minimal functionality
In this part, you'll implement a subset of RISC-V instructions and aim to pass 5 tests in the test/part1 directory. Refer to the test cases and the README file in test/part1 for detailed requirements.
1. [20pts] Complete the agex_stage.v file. No modifications to other files are necessary. Your implementation should pass test/part1/test[15].mem. If all test cases don't pass, you'll receive partial scores. To test all cases together, run run_tests.sh part1, and it will produce part1_results.log and part1_tests.log for you. You can also run each test case independently, see FAQ for part 1. Note: If you encounter latch size errors, modify the corresponding latch size definition in define.vh.
2. [10pts] Explain the actions in each pipeline stage while executing test/part1/test1.mem. Include waveform screenshots illustrating relevant signals. For example, in the Execute stage (EX stag), you should visualize input (regval1_AGEX, regval2_AGEX) and output (aluout_AGEX) signals of the ALU, and the opcode (op_I_AGEX).
3. [10pts]Explain how your RISC-V processor resolves Read-After-Write hazards in test/part1/test2.mem. Include waveform screenshots illustrating the discussed signals.
4. [10pts] Explain how your RISC-V processor handles branch misprediction in test/part1/test4.mem. Include waveform screenshots illustrating relevant signals. Note: In Lab 1, branches are always predicted as not-taken; in Lab 2, you will implement your own branch predictor.
What to submit:
Submit the following to Canvas:
Include a PDF file containing your explanations and corresponding screenshots.
Start part 2 as early as possible and do not wait untill the last week, as it involves heavier workload than part 1.
Part 2: Expanding instruction set
Test cases:
In part-2, all instructions in the test cases under test/part2/ such as add, addi, auipc, beq, bge, (all branch instructions) jal, jalr instructions will be tested. To test all test cases together, use run_tests.sh part2, which will generate part2_results.log and part2_tests.log. Tests [7-9] are handwritten assembly code, which are easier to debug, so start with those.
In part-2, we start to use modified RISC-V test cases. *.S is assembly code that takes RISC-V macro. Macros are defined at include/test_macros.h or include/riscv_test.h. It also uses ABI names and Pseudo Instructions. You can find a summary of information [here].
*.dump is an dump file output from gcc riscv compiler. *.mem file has the format for verilog code. *.dec file is useful when using [RISC-V emulator]
What to submit:
Submit the following to Canvas:
Avoid procrastination; start early to manage the workload effectively.
Part 3 (Optional) Complete the processor
1. [20pts] In this part, you will complete the processor to fully support the RISC-V ISA (except CSR instructions). Your goal is to ensure your program passes all the test cases in the test/part3/ directory. To receive full credits, your program must pass test/part3/testall.mem.
Partial scores will be awarded based on the coverage of the Part 3 test suites.
What to submit:
Submit the following to Canvas:
Useful Information
References summary of RISC-V Assembly coding
RISC-V emulator (tiny RV2)
Verilator manual
GTKWave manual
Tutorial about RISC-V TEST SUITE
FAQ for part 1
(Q) How do I run a specific test file? (A) Please see "define.vh": you need to change line 21 to change which test file to read: `define
IDMEMINITFILE "/home/zhifan/workspace/cs3220-23fall/lab1/test/part1/test4.mem". You need to change "test4.mem" into
(Q) Debugging takes so much time. Any tips to reduce the debugging time? (A) Some suggestions: 1. Review code carefully and understand the ISA behavior correctly. 2. If make command fails to compile, read the error messages carefully. 3. make command generates vcd file. Please use GTKWave to see important signals and check whether the signals works as expected according to *.asm files or RISC-V enumlators. When debugging, it is always helpful to visualize clk signal and pc values along with other important signals.
(Q) How do I know whether my implementation is correct or not? (A) If you run make, you would see "Pass" message.
(Q) Can I add new files? (A) Yes, but please make sure they are added in the zip file.
(Q) Do we need to implement a branch predictor? (A) It's not required for lab 1.
(Q) Do we need to create a stack for nested JAL instructions? (A) The hardware does not know any nested calls, so you do not need to implement it.
(Q) BEQ t1, t1, imm : if a branch is taken, is the new PC = PC + imm or new PC = PC + 4+ imm? (A) The answer is PC = PC + offset. Please be careful with converting imm to offset.
(Q) Do we need to worry about whether we should prevent all writes to the zero register and treat it as always zero, or if that is solely up to us dependent on our design? (A) This is purely S/W job. The H/W doesn't have to check whether x0 is writable or not. The Hardware also doesn't have explicitly insert 0 in hardware.
(Q) Is the immediate field inside assembly code decimal? (A) If the number starts with 0x, it's hexadecimal.
imem[PC_FE_latch[`IMEMADDRBITS-1:`IMEMWORDBITS]]; dmem[memaddr_MEM[`DMEMADDRBITS-1:`DMEMWORDBITS]];
(Q) What does assign inst_FE = imem[PC_FE_latch[`IMEMADDRBITS-1:`IMEMWORDBITS]]; mean? (A) PC_FE_latch contains PC value. Again imem and dmem are word addressable, so we don't need LSB 2 bits. Since imem and dmem has only 2^14 size, we just use addr [15:2] bits to index imem/dmem.
(Q) I'm not sure how to understand part 2 test code. (A) The test in test/part2 is modified code from RISC-V test suite. It uses macro function to generate test code.
(Q) How do I know what is the correct instruction/code behavior? (A) You can probably use RISC-V enumlators or other RISC-V machine to execute the code. One example is here .
(Q) How do I know whether I pass the code or not? (A) For part 1, we provide test code. Your code should print out "Pass" message if you run make.
(Q) My code does not load any instructions. Do I need to change anything? (A) Carefully check if you encountered any error messages and make sure you have set IDMEMINITFILE to the right path.
FAQ for part 2
(Q) what is li instructions in add.dump? (A) li instruction is one of the pseudo instructions. It is the same as addi x0, imm
(Q) I passed test[1-5].mem. why do I fail addi.mem? (A) It contains bne, auipc, jal instructions. So in order to pass part 2 test cases, you need to complete those instructions.
(Q) I'd like to use RISC-V emulator for testing the test code, but it won't take dump file. what should I do? (A) Unfortunately RISC-V emulator only takes assembly instructions. Hence, we recommend to use another emulator . You can use *.dec file in this simulator.
(Q) Behavior of lui. The documentation says that - Semantics : R[rd] = imm << 12. But U-immediate already shifted the immediate by 12 bits. Do I need to shift the sxt_imm_DE. Do I need to shift immediate value again? (A) No. if you have already shift immediage bits in instruction into sxt_imm_DE, you don't have to shift sxt_imm_DE again.
(Q) bge is signed comparison and bgeu is unsigned comparison. What does it mean and what should I do? (A) by default, in verilog all operations are unsigned. However, you can use signed comparisons in verilog by defining wires as signed variables. Here is an example for signed comparisons and unsigned comparisons
`` wire signed [DBITS-1:0] s_regval1_AGEX; // note signed wire signed [`DBITS-1:0] s_regval2_AGEX; //note signed assign s_regval1_AGEX = regval1_AGEX; assign s_regval2_AGEX = regval2_AGEX;
// signed comparison wire s_less; assign s_less = (s_regval1_AGEX < s_regval2_AGEX);
// unsigned comparison wire less; assign less = (regval1_AGEX < regval2_AGEX);
```
(Q) bgeu and bltu use unsigned comparisons. Does it mean I shouldn't sign extend immediage values at the decode stage and keep both unsiged and signed extension versions? (A) No, in RISC-V, all immediate values are sign-extended. begu and bltu are unsigned comparisons with singextended values (e.g. sxt_imm_DE)
(Q) I'm still confused with signed keyword in verilog. Does it perform any sign conversion when I put signed keyword in the above example?
(A) In Verilog, values are just binaries. s_regval1_AGEX and regval1_AGEX have the same value. Signed unsigned are just a matter of interpretation. When arithmetic operations are used such as comparator, signed/unsigned decide how to interpret the value. e.g.) In the above example, let's assume that reval1_AGEX is 0x0000 and regval2_AGEX is 0xFFFF. In that case, s_regval1_AGEX is 0x0000 and s_regval2_AGEX is still 0xFFFF. However, s_regval2_AGEX is interpreted as -1 whereas regval2_AGEX is interpreted as 65535. Hence, if (regval1_AGEX < regval2_AGEX) returns false but if (s_regval1_AGEX < s_regval2_AGEX) returns true.
(Q) Do I need to put the signed keyword for immediate values? (A) Yes, even though immediate values are sign-extended, if we want to treat the immediate value as 2's complement value such as in SLTI_I instruction case, you need to put signed keyword.
FAQ for part 3
(Q) Can you explain the behavior of slti and sltiu. Does it store the outcome of shift value? (A) The outcome of both instructions should be either 0 or 1. It checks whether (R[rs1] < sext(imm)) (signed comparisons for SLTI and unsigned comparisons for SLTIU) and if the condition is true, it sets 1 for the destination.
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