Lab 2: RISC-V 5 Stage Pipeline Simulator
In this Lab assignment, you will implement a cycle-accurate simulator for a 5-stage pipelined RISC-V processor in C++. The simulator supports a subset of the RISC-V instruction set and should model the execution of each instruction cycle by cycle.
A RISC-V program should be provided to the simulator as a text file “imem.txt” file that is used to initialize the Instruction Memory. Each line of the file corresponds to a Byte stored in the Instruction Memory in binary format, with the first line at address 0, the next line at address 1 and so on. Four contiguous lines correspond to a whole instruction. Note that the words stored in memory are in “Big-Endian” format, meaning that the most significant byte is stored first.
The Data Memory is initialized using the “dmem.txt” file. The format of the stored words is the same as the Instruction Memory. As with the instruction memory, the data memory addresses also begin at 0 and increment by one in each line.
The instructions to be supported and their encodings are shown in Table 1. Note that all instructions, except for “halt”, exist in the RISC-V ISA. The RISC-V Instruction Set Manual in Assignments defines the semantics of each instruction (page 130).
For the purposes of this lab only, we will assume that the beq (branch-if-qual) instruction operates like a bne (branch-if-not-equal) instruction. In other words, in your implementations you will assume that the beq jumps to the branch address if R[rs1] != R[rs2] and jumps to PC+4 , otherwise, i.e., if R[rs1] = R[rs2]
(Note that a real beq instruction would operate in the opposite fashion, that is, it will jump to the branch address if R[rs1] = R[rs2] and to PC+4 otherwise. The reason we had to make this modification for this lab is because to implement loops we actually need the bne instruction.
Name Format Type Opcode (Binary) Func 3(Binary) Func 7(Binary)
add R-Type 0110011 000 0000000
sub R-Type 0110011 000 0100000
addi I-Type 0010011 000
and R-Type 0110011 111 0000000
or R-Type 0110011 110 0000000
xor R-Type 0110011 100 0000000
beq SB-Type 1100011 000
jal UJ-Type 1101111
ld I-Type 0000011 011
sd S-Type 0100011 011
Table 1. Instruction encodings for a reduced RISC-V ISA
Pipeline Structure
Your RISC-V pipeline has the following 5 stages:
1.Fetch (IF): fetches an instruction from instruction memory. Updates PC.
2.Decode (ID/RF): reads from the register RF and generates control signals required in subsequent stages. In addition, branches are resolved in this stage by checking for the branch condition and computing the effective address.
3.Execute (EX): performs an ALU operation.
4.Memory (MEM): loads or stores a 64-bit word from data memory.
5.Writeback (WB): writes back data to the RF.
Your simulator can make use of the same RF, IMEM and DMEM classes that you used for Lab1. Complete implementations of these classes are provided for you in the skeleton code. Note that we have not defined an ALU class since, for this lab, the ALU is simple and only needs to perform adds and subtracts. Each pipeline stages takes inputs from flip-flops. The input flip-flops for each pipeline stage are described in the tables below.
IF Stage Input Flip-Flops
Flip-Flop Name Bit-width Functionality
PC 32 Current value of PC
nop 1 If set, IF stage performs a nop
IF/ID Stage Flip-Flops
Flip-Flop Name Bit-width Functionality
Instr 32 32b instruction read from Imem
nop 1 If set, ID stage performs a nop
ID/EXE Stage Flip-Flops
Flip-Flop Name Bit-width Functionality
Read_data1, Read_data2 64 32 64b data values read from RF
rs1, rs2 5 Addresses of source registers rs1, rs2. Note these are defined for R-type, I-type, S-type, and SB-type instructions
Wrt_reg_addr 5 Address of the instruction’s destination register. Don’t care if the instruction doesn’t update the RF
Alu_op 1 Set for add, addi, ld, sd; unset for sub, subi
Is_I_type 1 Set if the instruction is I-type
Wrt_enable 1 Set if instruction updates RF
Rd_mem, Wr_mem 1 Rd_mem set for ld and wrt_mem set for sd instructions
nop 1 If set, EXE stage performs a nop
EXE/MEM Stage Flip-Flops
Flip-Flop Name Bit-width Functionality
ALU_result 64 32 64b ALU result, don’t care for beq
Store_data 64 32 64b value to be stored in DMEM for sd instruction. Don’t care otherwise
rs1, rs2 5 Addresses of source registers rs1, rs2. Note these are defined for R-type, I-type, S-type, and SB-type instructions
Wrt_reg_addr 5 Address of the instruction’s destination register. Don’t care if the instruction doesn’t update the RF
Wrt_enable 1 Set if instruction updates RF
Rd_mem, Wr_mem 1 Rd_mem set for ld and wrt_mem set for sd instructions
nop 1 If set, EXE stage performs a nop
WB Stage Input Flip-Flops
Flip-Flop Name Bit-width Functionality
Wrt_data 64 32 64b value to be written back to RF. Don’t care for sd and beq
rs1, rs2 5 Addresses of source registers rs1, rs2. Note these are defined for R-type, S-type, and SB-type instructions
Wrt_reg_addr 5 Address of the instruction’s destination register. Don’t care if the instruction doesn’t update the RF
Wrt_enable 1 Set if instruction updates RF
nop 1 If set, EXE stage performs a nop
Dealing with Hazards
Your processor must deal with two types of hazards.
1.RAW Hazards: RAW hazards are dealt with using either only forwarding (if possible) or, if not, using stalling + forwarding. You must follow the mechanisms described in Lecture to deal RAW hazards.
2.Control Flow Hazards: You will assume that branch conditions are resolved in the ID/RF stage of the pipeline. Your processor deals with beq instructions as follows:
a) Branches are always assumed to be NOT TAKEN. That is, when a beq is fetched in the IF stage, the PC is speculatively updated as PC+4.
b) Branch conditions are resolved in the ID/RF stage. To make your life easier, will ensure that every beq instruction has no RAW dependency with its previous two instructions. In other words, you do NOT have to deal with RAW hazards for branches!
c)Two operations are performed in the ID/RF stage: (i) Read_data1 and Read_data2 are compared to determine the branch outcome; (ii) the effective branch address is computed.
d)If the branch is NOT TAKEN, execution proceeds normally. However, if the branch is TAKEN, the speculatively fetched instruction from PC+4 is quashed in its ID/RF stage using the nop bit and the next instruction is fetched from the effective branch address. Execution now proceeds normally.
The nop bit
The nop bit for any stage indicates whether it is performing a valid operation in the current clock cycle. The nop bit for the IF stage is initialized to 0 and for all other stages is initialized to 1. (This is because in the first clock cycle, only the IF stage performs a valid operation.)
In the absence of hazards, the value of the nop bit for a stage in the current clock cycle is
equal to the nop bit of the prior stage in the previous clock cycle.
However, the nop bit is also used to implement stall that result from a RAW hazard or to quash speculatively fetched instructions if the branch condition evaluates to TAKEN.
The HALT Instruction
The halt instruction is a “custom” instruction we introduced so you know when to stop the simulation. When a HALT instruction is fetched in IF stage at cycle N, the nop bit of the IF stage in the next clock cycle (cycle N+1) is set to 1 and subsequently stays at 1. The nop bit of the ID/RF stage is set to 1 in cycle N+1 and subsequently stays at 1. The nop bit of the EX stage is set to 1 in cycle N+2 and subsequently stays at 1. The nop bit of the MEM stage is set to 1 in cycle N+3 and subsequently stays at 1. The nop bit of the WB stage is set to 1 in cycle N+4 and subsequently stays at 1.
At the end of each clock cycle the simulator checks to see if the nop bit of each stage is 1. If so, the simulation halts. Note that this logic is already implemented in the skeleton code provided to you.
What to Output
Your simulator will output the values of all flip-flops at the end of each clock cycle. Further, when the simulation terminates, the simulator also outputs the state of the RF and Dmem. The skeleton code already prints out everything that your simulator needs to output.
Testbenches and Grading
Test inputs for your simulator are in the form of “imem.txt” and “dmem.txt” files, and the expected outputs are in the form of “RFresult.txt”, “dmemresult.txt” and “stateresult.txt” files.
You are encouraged to develop your own test cases. You code will be graded on our own test inputs.
You are encouraged to develop your code in steps. A rough time frame for how long each step would take is given below.
1.Step 1: your simulator should be able to handle add, sub, ld and sd instructions without any RAW hazards. (1 Week)
2.Step 2: in addition, your simulator should be able to handle add, sub, ld and sd instructions with RAW hazards. (1 Week)
3.Step 3: finally, enhance your simulator to handle beq instructions.
The grading scheme that we will use to test your code is:
• Code compiles and executes without crashing on all test inputs.
[10 Points]
• Code handles inputs with only add, sub, ld and sd instructions without any RAW hazards.
[30 Points]
• Code handles inputs with only add, sub, ld and sd instructions with RAW hazards.
[30 Points]
• Code handles all test inputs including those with beq instructions.
[30 Points]