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emu/riscv64.c

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// https://docs.riscv.org/reference/isa/_attachments/riscv-unprivileged.pdf
// https://riscv.org/wp-content/uploads/2024/12/riscv-calling.pdf
#include <gelf.h>
#include <libelf.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define PARSE_I_INS(ins) \
uint8_t rd = (ins >> 7) & 0b11111; \
uint8_t funct3 = (ins >> 12) & 0b111; \
uint8_t rs1 = (ins >> 15) & 0b11111; \
int32_t imm = ((int32_t)ins) >> 20
#define PARSE_R_INS(ins) \
uint8_t rd = (ins >> 7) & 0b11111; \
uint8_t funct3 = (ins >> 12) & 0b111; \
uint8_t rs1 = (ins >> 15) & 0b11111; \
uint8_t rs2 = (ins >> 20) & 0b11111; \
int32_t funct7 = ((int32_t)ins) >> 25
#define PARSE_B_INS(ins) \
uint8_t funct3 = (ins >> 12) & 0b111; \
uint8_t rs1 = (ins >> 15) & 0b11111; \
uint8_t rs2 = (ins >> 20) & 0b11111; \
int32_t imm_12 = (ins >> 31) & 0b1; \
int32_t imm_10_5 = (ins >> 25) & 0b111111; \
int32_t imm_4_1 = (ins >> 8) & 0b1111; \
int32_t imm_11 = (ins >> 7) & 0b1; \
int32_t imm = \
(imm_12 << 12) | (imm_11 << 11) | (imm_10_5 << 5) | (imm_4_1 << 1); \
imm = (imm << 19) >> 19
#define PARSE_S_INS(ins) \
uint8_t funct3 = (ins >> 12) & 0b111; \
uint8_t rs1 = (ins >> 15) & 0b11111; \
uint8_t rs2 = (ins >> 20) & 0b11111; \
int32_t imm_11_5 = (ins >> 25) & 0b1111111; \
int32_t imm_4_0 = (ins >> 7) & 0b11111; \
int32_t imm = (imm_11_5 << 5) | imm_4_0; \
imm = (imm << 20) >> 20
#define PARSE_J_INS(ins) \
uint8_t rd = (ins >> 7) & 0b11111; \
int32_t imm_20 = (ins >> 31) & 0b1; \
int32_t imm_10_1 = (ins >> 21) & 0b1111111111; \
int32_t imm_11 = (ins >> 20) & 0b1; \
int32_t imm_19_12 = (ins >> 12) & 0b11111111; \
int32_t imm = \
(imm_20 << 20) | (imm_19_12 << 12) | (imm_11 << 11) | (imm_10_1 << 1); \
imm = (imm << 11) >> 11
#define PARSE_U_INS(ins) \
uint8_t rd = (ins >> 7) & 0b11111; \
int32_t imm = ins >> 12
static const char *const REGS[32] = {
"zero", "ra", "sp", "gp", "tp", "t0", "t1", "t2", "fp", "s1", "a0",
"a1", "a2", "a3", "a4", "a5", "a6", "a7", "s2", "s3", "s4", "s5",
"s6", "s7", "s8", "s9", "s10", "s11", "t3", "t4", "t5", "t6"};
typedef struct {
size_t offset;
size_t size;
} Section;
typedef struct {
uint8_t *memory;
size_t pc;
int64_t regs[32];
Section code_section;
} RISCV64;
Section riscv64_get_code_section(uint8_t *exe_bytes, size_t exe_size) {
Elf *elf = elf_memory((char *)exe_bytes, exe_size);
if (!elf) {
fprintf(stderr, "elf_begin failed: %s\n", elf_errmsg(-1));
exit(1);
}
size_t str_table_index;
if (elf_getshdrstrndx(elf, &str_table_index) != 0) {
fprintf(stderr, "elf_getshdrstrndx failed: %s\n", elf_errmsg(-1));
exit(1);
}
Elf_Scn *section = NULL;
while ((section = elf_nextscn(elf, section)) != NULL) {
GElf_Shdr header;
if (gelf_getshdr(section, &header) != &header)
continue;
const char *name = elf_strptr(elf, str_table_index, header.sh_name);
if (name && strcmp(name, ".text") == 0) {
elf_end(elf);
return (Section){.offset = header.sh_offset, .size = header.sh_size};
}
}
fprintf(stderr, "Failed to locate .text\n");
exit(1);
}
RISCV64 riscv64_load(uint8_t *exe_bytes, size_t exe_size) {
Section section = riscv64_get_code_section(exe_bytes, exe_size);
return (RISCV64){
.memory = exe_bytes,
.pc = section.offset,
.regs = {0},
.code_section = section,
};
}
void riscv64_dump(const RISCV64 *r) {
printf("REGS:");
for (size_t i = 0; i < 32; i++) {
printf(" %lu", r->regs[i]);
}
printf("\n");
}
void riscv64_disassemble_one(const RISCV64 *r) {
uint32_t ins;
memcpy(&ins, r->memory + r->pc, 4);
uint16_t opcode = ins & 0b1111111;
// https://stackoverflow.com/questions/62939410/how-can-i-find-out-the-instruction-format-of-a-risc-v-instruction
switch (opcode) {
case 0b1100011: {
PARSE_B_INS(ins);
if (funct3 == 0b000) {
printf("beq %s, %s, %d\n", REGS[rs1], REGS[rs2], imm);
} else if (funct3 == 0b001) {
printf("bne %s, %s, %d\n", REGS[rs1], REGS[rs2], imm);
} else if (funct3 == 0b100) {
printf("blt %s, %s, %d\n", REGS[rs1], REGS[rs2], imm);
} else if (funct3 == 0b101) {
printf("bge %s, %s, %d\n", REGS[rs1], REGS[rs2], imm);
break;
} else {
fprintf(stderr, "B-type: unrecognized funct3: %03b\n", funct3);
exit(1);
}
}; break;
case 0b0010011: {
PARSE_I_INS(ins);
if (funct3 == 0b000) {
printf("addi %s, %s, %d\n", REGS[rd], REGS[rs1], imm);
} else if (funct3 == 0b001) {
printf("slli %s, %s, %d\n", REGS[rd], REGS[rs1], imm);
} else {
fprintf(stderr, "I-type 1: unrecognized funct3: %03b\n", funct3);
exit(1);
}
}; break;
case 0b0000011: {
PARSE_I_INS(ins);
if (funct3 == 0b000) {
printf("lb %s, %d(%s)\n", REGS[rd], imm, REGS[rs1]);
} else if (funct3 == 0b011) {
printf("ld %s, %d(%s)\n", REGS[rd], imm, REGS[rs1]);
} else {
fprintf(stderr, "I-type 2: unrecognized funct3: %03b\n", funct3);
exit(1);
}
}; break;
case 0b1100111: {
PARSE_I_INS(ins);
printf("jalr %s, %s, %d\n", REGS[rd], REGS[rs1], imm);
}; break;
case 0b1110011: {
PARSE_I_INS(ins);
if (funct3 == 0b000) {
if (imm == 0) {
printf("ecall\n");
} else {
fprintf(stderr, "I-type 4: funct3=000 unrecognized imm: %b\n", imm);
exit(1);
}
break;
} else {
fprintf(stderr, "I-type 4: unrecognized funct3: %03b\n", funct3);
exit(1);
}
}; break;
case 0b1101111: {
PARSE_J_INS(ins);
printf("jal %s, %d\n", REGS[rd], imm);
}; break;
case 0b0110011: {
PARSE_R_INS(ins);
if (funct3 == 0b000) {
if (funct7 == 0b0000000) {
printf("add %s, %s, %s\n", REGS[rd], REGS[rs1], REGS[rs2]);
} else if (funct7 == 0b0100000) {
printf("sub %s, %s, %s\n", REGS[rd], REGS[rs1], REGS[rs2]);
} else if (funct7 == 0b0000001) {
printf("mul %s, %s, %s\n", REGS[rd], REGS[rs1], REGS[rs2]);
} else {
fprintf(stderr, "R-type 1: funct3=0b000: unrecognized funct7: %b\n",
funct7);
exit(1);
}
} else {
fprintf(stderr, "R-type 1: unrecognized funct3: %03b\n", funct3);
exit(1);
}
}; break;
case 0b0101111:
fprintf(stderr, "R-type 2: unimplemented\n");
exit(1);
case 0b0111011: {
PARSE_R_INS(ins);
if (funct3 == 0b101) {
if (funct7 == 0b0000001) {
printf("divuw %s, %s, %s\n", REGS[rd], REGS[rs1], REGS[rs2]);
} else {
fprintf(stderr, "R-type 3: funct3=101: unrecognized funct7: %b\n",
funct7);
exit(1);
}
} else if (funct3 == 0b111) {
printf("remuw %s, %s, %s\n", REGS[rd], REGS[rs1], REGS[rs2]);
} else {
fprintf(stderr, "R-type 3: unrecognized funct3: %03b\n", funct3);
exit(1);
}
}; break;
case 0b0011011: {
uint8_t funct3 = (ins >> 12) & 0b111;
if (funct3 == 0b000) {
PARSE_I_INS(ins);
printf("addiw %s, %s, %d\n", REGS[rd], REGS[rs1], imm);
} else {
fprintf(stderr, "R-type 4: unrecognized funct3: %03b\n", funct3);
exit(1);
}
}; break;
case 0b0100011: {
PARSE_S_INS(ins);
if (funct3 == 0b000) {
printf("sb %s, %d(%s)\n", REGS[rs2], imm, REGS[rs1]);
} else if (funct3 == 0b011) {
printf("sd %s, %d(%s)\n", REGS[rs2], imm, REGS[rs1]);
} else {
fprintf(stderr, "S-type: unrecognized funct3: %03b\n", funct3);
exit(1);
}
}; break;
case 0b0110111: {
PARSE_U_INS(ins);
printf("lui %s, %d\n", REGS[rd], imm);
}; break;
case 0b0010111: {
PARSE_U_INS(ins);
printf("auipc %s, %d\n", REGS[rd], imm);
}; break;
default:
fprintf(stderr, "Unrecognized opcode: %07b\n", opcode);
exit(1);
}
}
void riscv64_execute(RISCV64 *r) {
r->pc = r->code_section.offset;
r->regs[2] = (20 * 1024 * 1024) - 1; // set the stack pointer
while (r->pc < r->code_section.offset + r->code_section.size) {
r->regs[0] = 0; // clear the zero register
uint32_t ins;
memcpy(&ins, r->memory + r->pc, 4);
uint16_t opcode = ins & 0b1111111;
// https://stackoverflow.com/questions/62939410/how-can-i-find-out-the-instruction-format-of-a-risc-v-instruction
switch (opcode) {
case 0b1100011: {
PARSE_B_INS(ins);
if (funct3 == 0b000) { // beq
if (r->regs[rs1] == r->regs[rs2]) {
r->pc += imm;
continue;
}
} else if (funct3 == 0b001) { // bne
if (r->regs[rs1] != r->regs[rs2]) {
r->pc += imm;
continue;
}
} else if (funct3 == 0b101) { // bge
if (r->regs[rs1] >= r->regs[rs2]) {
r->pc += imm;
continue;
}
} else if (funct3 == 0b100) { // blt
if (r->regs[rs1] < r->regs[rs2]) {
r->pc += imm;
continue;
}
} else {
fprintf(stderr, "B-type: unrecognized funct3: %03b\n", funct3);
exit(1);
}
}; break;
case 0b0010011: {
PARSE_I_INS(ins);
if (funct3 == 0b000) { // addi
r->regs[rd] = r->regs[rs1] + imm;
} else if (funct3 == 0b001) { // slli
r->regs[rd] = r->regs[rs1] << (imm & 0b111111);
} else {
fprintf(stderr, "I-type 1: unrecognized funct3: %03b\n", funct3);
exit(1);
}
}; break;
case 0b0000011: {
PARSE_I_INS(ins);
if (funct3 == 0b000) { // lb
r->regs[rd] = r->memory[r->regs[rs1] + imm];
} else if (funct3 == 0b011) { // ld
r->regs[rd] = *(uint64_t *)&r->memory[r->regs[rs1] + imm];
} else {
fprintf(stderr, "I-type 2: unrecognized funct3: %03b\n", funct3);
exit(1);
}
}; break;
case 0b1100111: { // jalr
PARSE_I_INS(ins);
uint32_t target = (r->regs[rs1] + imm) & ~1;
r->regs[rd] = r->pc + 4;
r->pc = target;
continue;
}; break;
case 0b1110011: {
PARSE_I_INS(ins);
if (funct3 == 0b000) {
if (imm == 0) { // ecall
// https://jborza.com/post/2021-05-11-riscv-linux-syscalls/
switch (r->regs[17]) {
case 63: { // read
if (r->regs[10] != 0) {
fprintf(stderr, "read syscall implemented only for stdin.\n");
exit(1);
}
size_t start = r->regs[11];
size_t count = r->regs[12];
size_t bytes_read = 0;
for (size_t i = 0; i < count; i++) {
int c = getchar();
if (c == EOF)
break;
r->memory[start + i] = (uint8_t)c;
bytes_read++;
if (c == '\n')
break;
}
r->regs[10] = bytes_read;
}; break;
case 64: { // write
if (r->regs[10] != 1) {
fprintf(stderr, "write syscall implemented only for stdout.\n");
exit(1);
}
size_t start = r->regs[11];
size_t end = r->regs[11] + r->regs[12];
for (size_t i = start; i < end; i++) {
putchar(r->memory[i]);
}
}; break;
case 93: { // exit
printf("Program exited with code %lu.\n", r->regs[10]);
return;
}; break;
case 169: { // gettimeofday
// TODO: actually return the time
int64_t tv_addr = r->regs[10];
int64_t tz_addr = r->regs[11];
uint8_t tv_buf[16] = {
0xD2, 0x02, 0x96, 0x49,
0x00, 0x00, 0x00, 0x00, // tv_sec = 1234567890
0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, // tv_usec = 0
};
memcpy(&r->memory[tv_addr], tv_buf, 16);
memset(&r->memory[tz_addr], 0, 8);
r->regs[10] = 0;
}; break;
default:
fprintf(stderr, "Unimplemented syscall: %lu\n", r->regs[17]);
exit(1);
}
break;
} else {
fprintf(stderr, "I-type 4: funct3=000 unrecognized imm: %b\n", imm);
exit(1);
};
break;
} else {
fprintf(stderr, "I-type 4: unrecognized funct3: %03b\n", funct3);
exit(1);
}
}; break;
case 0b1101111: { // jal
PARSE_J_INS(ins);
r->regs[rd] = r->pc + 4;
r->pc += imm;
continue;
}; break;
case 0b0110011: {
PARSE_R_INS(ins);
if (funct3 == 0b000) {
if (funct7 == 0b0000000) { // add
r->regs[rd] = r->regs[rs1] + r->regs[rs2];
break;
} else if (funct7 == 0b0100000) { // sub
r->regs[rd] = r->regs[rs1] - r->regs[rs2];
break;
} else if (funct7 == 0b0000001) { // mul
r->regs[rd] = r->regs[rs1] * r->regs[rs2];
break;
} else {
fprintf(stderr, "R-type 1: funct3=0b000: unrecognized funct7: %b\n",
funct7);
exit(1);
}
} else {
fprintf(stderr, "R-type 1: unrecognized funct3: %03b\n", funct3);
exit(1);
}
}; break;
case 0b0111011: {
PARSE_R_INS(ins);
if (funct3 == 0b101) {
if (funct7 == 0b0000001) {
if (r->regs[rs2] == 0) {
r->regs[rd] = -1LL;
} else {
r->regs[rd] = (int32_t)(r->regs[rs1] / r->regs[rs2]);
}
} else {
fprintf(stderr, "R-type 3: funct3=101: unrecognized funct7: %b\n",
funct7);
exit(1);
}
} else if (funct3 == 0b111) {
if (r->regs[rs2] == 0) {
r->regs[rd] = (int32_t)r->regs[rs1];
} else {
r->regs[rd] = (int32_t)(r->regs[rs1] % r->regs[rs2]);
}
} else {
fprintf(stderr, "R-type 3: unrecognized funct3: %03b\n", funct3);
exit(1);
}
}; break;
case 0b0011011: {
uint8_t funct3 = (ins >> 12) & 0b111;
if (funct3 == 0b000) {
PARSE_I_INS(ins);
int32_t result = r->regs[rs1] + imm;
r->regs[rd] = result;
} else {
fprintf(stderr, "R-type 4: unrecognized funct3: %03b\n", funct3);
exit(1);
}
}; break;
case 0b0100011: {
PARSE_S_INS(ins);
if (funct3 == 0b000) { // sb
size_t addr = r->regs[rs1] + imm;
r->memory[addr] = r->regs[rs2];
} else if (funct3 == 0b011) { // sd
size_t addr = r->regs[rs1] + imm;
// take this rust
*(uint64_t *)(&r->memory[addr]) = r->regs[rs2];
} else {
fprintf(stderr, "S-type: unrecognized funct3: %03b\n", funct3);
exit(1);
}
}; break;
case 0b0110111: { // lui
PARSE_U_INS(ins);
r->regs[rd] = imm << 12;
}; break;
case 0b0010111: { // auipc
PARSE_U_INS(ins);
r->regs[rd] = r->pc + (imm << 12);
}; break;
default:
fprintf(stderr, "Unrecognized opcode: %07b\n", opcode);
exit(1);
}
r->pc += 4;
}
}
int main(int argc, char *argv[]) {
if (elf_version(EV_CURRENT) == EV_NONE) {
fprintf(stderr, "Failed to initialize libelf: %s\n", elf_errmsg(-1));
return 1;
}
const char *path = NULL;
for (int i = 1; i < argc; i++) {
path = argv[i];
}
if (path == NULL) {
fprintf(stderr, "Usage: %s <path>\n", argv[0]);
return 1;
}
FILE *file = fopen(path, "rb");
if (!file) {
fprintf(stderr, "Failed to open %s\n", path);
return 1;
}
// probably enough for anything we can handle
uint8_t *exe_bytes = malloc(20 * 1024 * 1024);
size_t exe_size = fread(exe_bytes, 1, 20 * 1024 * 1024, file);
fclose(file);
RISCV64 r = riscv64_load(exe_bytes, exe_size);
for (r.pc = r.code_section.offset;
r.pc < r.code_section.offset + r.code_section.size; r.pc += 4) {
riscv64_disassemble_one(&r);
}
printf("END DISASSEMBLY\n");
riscv64_execute(&r);
}
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