RISC-V RV32I Implementation

Implements the base integer instruction set for the RISC-V 32 bit CPU.

Linting

Run the following to run verilator and check that the language rules are followed correctly.

make lint instmem
make lint reg_file

This can also be executed as a single command

make lint instmem reg_file

Verify modules

Run the following to have verilator create the module cpp files, build and run the executable generated.

make verify instmem
make verify reg_file

This can also be executed as a single command like the lint command above

make verify instmem reg_file

---

Implementation Details

Instruction memory [instmem.sv]

For the ease of implementation this CPU will have a separate code and data memory, i.e to say it is based on the Harvard Architecture. The size of instruction memory is 128 Bytes.

The instruction memory takes in a 7 Bit wide address and outputs 32 Bit instruction which is then consumed by rest of the system.

Note: The instruction memory module has a configurable parameter - code_file which is a text file that can be provided during instantiation. This file would contain the instructions in Hex format and be used to init the instruction memory content during synthesis.

Register File [reg_file.sv]

RISC-V specification dictates that the base implmentation would need to have 32 general purpose registers, each 32 Bits wide. These are denoted by x0 to x1. x0 is hardwired to 0. The module takes input as the source 1 (rs1), source 2 (rs2) and destination register (rd) addresses which are 5 Bits wide each. The destination register is written to only if the write enable line is asserted. Further, not all instructions use the source 2 register, hence there is an extra input line that tracks if source 2 register is to be considered for the reads.

Note: It is assumed that x0 won't be written to. This needs to be corrected in future improvements in the module.

---

Resources and References

1. RISC-V Specification
2. Youtube Channel: RISC-V International
3. Youtube Channel: Robert Baruch
4. E85: Digital Electronics & Computer Architecture (David Harris, Joshua Brake)
5. CSCI 463 Computer Architecture and Systems Organization Fall 2020 (John Winans)
   • Youtube Playlist: Fall 2020 NIU CSCI 463
6. Verilator Documentation
   • Verilator Manual