spacepirate / risk-v

32-bit RISC-V CPU in ~600 lines of C89

rv

RISC-V CPU core written in ANSI C.

Features:

• RV32IMC user-level implementation
• Passes all supported tests in riscv-tests
• ~600 lines of code
• Doesn't use any integer types larger than 32 bits, even for multiplication
• Simple API (two functions, plus two memory callback functions that you provide)
• No memory allocations

API

/* Memory access callbacks: data is input/output, return RV_BAD on fault, 0 otherwise */
typedef rv_res (*rv_store_cb)(void *user, rv_u32 addr, rv_u8 data);
typedef rv_res (*rv_load_cb)(void *user, rv_u32 addr, rv_u8 *data);

/* Initialize CPU. */
void rv_init(rv *cpu, void *user, rv_load_cb load_cb, rv_store_cb store_cb);

/* Single-step CPU. Returns 0 on success, one of RV_E* on exception. */
rv_u32 rv_step(rv *cpu);

Usage

#include <stdio.h>
#include <string.h>

#include "rv.h"

rv_res load_cb(void *user, rv_u32 addr, rv_u8 *data) {
  if (addr - 0x80000000 > 0x10000) /* Reset vector is 0x80000000 */
    return RV_BAD;
  *data = ((rv_u8 *)(user))[addr - 0x80000000];
  return RV_OK;
}

rv_res store_cb(void *user, rv_u32 addr, rv_u8 data) {
  if (addr - 0x80000000 > 0x10000)
    return RV_BAD;
  ((rv_u8 *)(user))[addr - 0x80000000] = data;
  return RV_OK;
}

rv_u32 program[2] = {
    /* _start: */
    0x02A88893, /* add a7, a7, 42 */
    0x00000073  /* ecall */
};

int main(void) {
  rv_u8 mem[0x10000];
  rv cpu;
  rv_init(&cpu, (void *)mem, &load_cb, &store_cb);
  memcpy((void *)mem, (void *)program, sizeof(program));
  while (rv_step(&cpu) != RV_EECALL) {
  }
  printf("Environment call @ %08X: %u\n", cpu.pc, cpu.r[17]);
  return 0;
}

Targeting rv

Use riscv-gnu-toolchain with tools/link.ld.

Suggested GCC commandline:

riscv64-unknown-elf-gcc example.S -nostdlib -nostartfiles -Tlink.ld -march=rv32imc -mabi=ilp32 -o example.o -e _start -g -no-pie

To dump a binary starting at 0x80000000 that can be directly loaded by rv as in the above example:

riscv64-unknown-elf-objcopy -g -O binary example.o example.bin

Instruction List

Click an instruction to see its implementation in rv.c.

• add addi and andi auipc beq bge bgtu
• blt bltu bne c.add c.addi c.addi16spc.and c.andi
• c.beqz c.bnez c.j c.jal c.jalr c.jr c.li c.lui
• c.lw c.lwsp c.mv c.or c.slli c.srai c.srli c.sub
• c.sw c.swsp c.xor csrrc csrrci csrrs csrrsi csrrw
• csrrwi div divu ebreak ecall fence fence.i jal
• jalr lb lbu lh lhu lui lw mret
• mul mulh mulhsu mulhu or ori rem remu
• sb sh sll slli slt slti sltiu sltu
• sra srai srl srli sub sw xor xori

FAQ

Spaghetti code!

• rv tries to strike a good balance between conciseness and readability. Of course, being able to read this code at all requires intimate prior knowledge of the ISA encoding.

No switch statements!

• C only allows constant expressions in switch statements. In addition to an abundance of break statements using these would result in more bloated code in the author's opinion. You are free to reimplement this code with switch statements. See LICENSE.txt.

Not useful!

• Ok

Caveats

• Written in C89.
• Not actually written in C89, since it uses external names longer than 6 characters.
• Doesn't use any integer types larger than 32 bits, even for multiplication, because it's written in C89.
• Assumes width of integer types in a way that's not completely compliant with C89/99. Fix for this is coming soon, I'm working on a watertight <stdint.h> for C89.
• Written in C89.