5.1.2.1 Freestanding environment

2 The effect of program termination in a freestanding environment is
implementation-defined.

static void ebreak(long arg0, long arg1) {
  asm volatile("addi a0, x0, %0;"
               "addi a1, x0, %1;"
               "ebreak" : : "i"(arg0), "i"(arg1));
}
static void putch(char ch) { ebreak(0, ch); }
static void halt(int code) { ebreak(1, code); while (1); }

void _start() {
  putch('A');
  halt(0);
}

rv32gcc -ffreestanding -nostdlib -static -Wl,-Ttext=0 -O2 -o prog a.c
rvobjdump -M no-aliases -d prog

#include <stdint.h>
uint32_t R[32], PC; // according to the RISC-V manual
uint8_t M[64];      // 64-Byte memory

 31           20 19 15 14 12 11  7 6       0
+---------------+-----+-----+-----+---------+
|   imm[11:0]   | rs1 | 000 | rd  | 0010011 |    ADDI
+---------------+-----+-----+-----+---------+
+---------------+-----+-----+-----+---------+
| 000000000001  |00000| 000 |00000| 1110011 |   EBREAK
+---------------+-----+-----+-----+---------+

void inst_cycle() {
  uint32_t inst = *(uint32_t *)&M[PC];
  if (((inst & 0x7f) == 0x13) && ((inst >> 12) & 0x7) == 0) { // addi
    if (((inst >> 7) & 0x1f) != 0) {
      R[(inst >> 7) & 0x1f] = R[(inst >> 15) & 0x1f] +
        (((inst >> 20) & 0x7ff) - ((inst & 0x80000000) ? 4096 : 0));
    }
  } else if (inst == 0x00100073) { // ebreak
    if (R[10] == 0) { putchar(R[11] & 0xff); }
    else if (R[10] == 1) { halt = true; }
    else { printf("Unsupported ebreak command\n"); }
  } else { printf("Unsupported instuction\n"); }
  PC += 4;
}

3.4 Reset

The pc is set to an implementation-defined reset vector... All other hart state is
unspecified.

// ...
uint8_t M[1024];
int main(int argc, char *argv[]) {
  PC = 0; R[0] = 0;
  FILE *fp = fopen(argv[1], "r");
  fread(M, 1, 1024, fp);
  fclose(fp);
  while (!halt) { inst_cycle(); }
  return 0;
}

# 将可执行文件prog中的指令序列抽取到prog.bin
riscv64-linux-gnu-objcopy -j .text -O binary prog prog.bin
gcc -o yemu yemu.c && ./yemu prog.bin

./yemu not-exist.bin
segmentation fault

#include <assert.h>
// ...
int main(int argc, char *argv[]) {
  PC = 0; R[0] = 0;
  assert(argc >= 2);  // 要求至少包含一个参数
  FILE *fp = fopen(argv[1], "r");
  assert(fp != NULL); // 要求argv[1]是一个可以成功打开的文件
  int ret = fseek(fp, 0, SEEK_END);
  assert(ret != -1); // 要求fseek()成功
  long fsize = ftell(fp);
  assert(fsize != -1); // 要求ftell()成功
  rewind(fp);
  assert(fsize < 1024); // 要求程序大小不超过1024字节
  ret = fread(M, 1, 1024, fp);
  assert(ret == fsize); // 要求完全读出程序的内容
  fclose(fp);
  while (!halt) { inst_cycle(); }
  return 0;
}

./yemu not-exist.bin
yemu: yemu.c:27: main: Assertion `fp != NULL' failed.

#define Assert(cond, format, ...) \
  do { \
    if (!(cond)) { \
      fprintf(stderr, format "\n", ## __VA_ARGS__); \
      assert(cond); \
    } \
  } while (0)

int main(int argc, char *argv[]) {
  PC = 0; R[0] = 0;
  Assert(argc >= 2, "Program is not given");  // 要求至少包含一个参数
  FILE *fp = fopen(argv[1], "r");
  Assert(fp != NULL, "Fail to open %s", argv[1]); // 要求argv[1]是一个可以成功打开的文件
  int ret = fseek(fp, 0, SEEK_END);
  Assert(ret != -1, "Fail to seek the end of the file"); // 要求fseek()成功
  long fsize = ftell(fp);
  Assert(fsize != -1, "Fail to return the file position"); // 要求ftell()成功
  rewind(fp);
  Assert(fsize < 1024, "Program size exceeds 1024 Bytes"); // 要求程序大小不超过1024字节
  ret = fread(M, 1, 1024, fp);
  Assert(ret == fsize, "Fail to load the whole program"); // 要求完全读出程序的内容
  fclose(fp);
  while (!halt) { inst_cycle(); }
  return 0;
}

#include <string.h>
#include <errno.h>

#define Perror(cond, format, ...) \
  Assert(cond, format ": %s", ## __VA_ARGS__, strerror(errno))

int main(int argc, char *argv[]) {
  PC = 0; R[0] = 0;
  Assert(argc >= 2, "Program is not given");  // 要求至少包含一个参数
  FILE *fp = fopen(argv[1], "r");
  Perror(fp != NULL, "Fail to open %s", argv[1]); // 要求argv[1]是一个可以成功打开的文件
  int ret = fseek(fp, 0, SEEK_END);
  Perror(ret != -1, "Fail to seek the end of the file"); // 要求fseek()成功
  long fsize = ftell(fp);
  Perror(fsize != -1, "Fail to return the file position"); // 要求ftell()成功
  rewind(fp);
  Assert(fsize < 1024, "Program size exceeds 1024 Bytes"); // 要求程序大小不超过1024字节
  ret = fread(M, 1, 1024, fp);
  Assert(ret == fsize, "Fail to load the whole program"); // 要求完全读出程序的内容
  fclose(fp);
  while (!halt) { inst_cycle(); }
  return 0;
}

uint8_t M[512];

Assert(fsize < 1024, "Program size exceeds 1024 Bytes");
ret = fread(M, 1, 1024, fp);  // BUG: 忘了改, 可能发生缓冲区溢出!

// main.c
#define MSIZE 512
ret = fread(M, 1, MSIZE, fp);

// inst.c
#define MSIZE 1024 // BUG: 这里忘了改
assert(PC < MSIZE);
uint32_t inst = *(uint32_t *)&M[PC];

if (((inst & 0x7f) == 0x13) && ((inst >> 12) & 0x7) == 0) { // addi
  if (((inst >> 7) & 0x1f) != 0) {
    R[(inst >> 7) & 0x1f] = R[(inst >> 15) & 0x1f] +
      (((inst >> 20) & 0x7ff) - ((inst & 0x80000000) ? 4096 : 0));
  }
} else if (((inst & 0x7f) == 0x13) && ((inst >> 12) & 0x7) == 0x4) { // xori
  if (((inst >> 7) & 0x1f) != 0) {
    R[(inst >> 7) & 0x1f] = R[(inst >> 15) & 0x1f] ^
      (((inst >> 20) & 0x7ff) - ((inst & 0x80000000) ? 4096 : 0));
  }
} else if (((inst & 0x7f) == 0x13) && ((inst >> 12) & 0x7) == 0x6) { // ori
  if (((inst >> 7) & 0x1f) != 0) {
    R[(inst >> 7) & 0x1f] = R[(inst >> 15) & 0x1f] |
      (((inst >> 20) & 0x7ff) - ((inst & 0x80000000) ? 4096 : 0));
  }
} else if (((inst & 0x7f) == 0x13) && ((inst >> 12) & 0x7) == 0x4) { // andi
  if (((inst >> 7) & 0x1f) != 0) {
    R[(inst >> 7) & 0x1f] = R[(inst >> 15) & 0x1f] &
      (((inst >> 20) & 0x7ff) - ((inst & 0x80000000) ? 4096 : 0));
  }
} else if (...) {  ...  }

uint32_t inst = *(uint32_t *)&M[PC];
uint32_t opcode = inst & 0x7f;
uint32_t funct3 = (inst >> 12) & 0x7;
uint32_t rd  = (inst >> 7 ) & 0x1f;
uint32_t rs1 = (inst >> 15) & 0x1f;
uint32_t imm = ((inst >> 20) & 0x7ff) - ((inst & 0x80000000) ? 4096 : 0);
if (opcode == 0x13) {
  if      (funct3 == 0x0) { R[rd] = R[rs1] + imm; } // addi
  else if (funct3 == 0x4) { R[rd] = R[rs1] ^ imm; } // xori
  else if (funct3 == 0x6) { R[rd] = R[rs1] | imm; } // ori
  else if (funct3 == 0x7) { R[rd] = R[rs1] & imm; } // andi
  else { panic("Unsupported funct3 = %d", funct3); }
  R[0] = 0; // 若指令写入了R[0], 此处将其重置为0
} else if (...) {  ...  }
PC += 4;

typedef union {
  struct {
    uint32_t opcode  :  7;
    uint32_t rd      :  5;
    uint32_t funct3  :  3;
    uint32_t rs1     :  5;
     int32_t imm11_0 : 12;
  } I;
  struct { /* ... */ } R;
  uint32_t bytes;
} inst_t;

inst_t *inst = (inst_t *)&M[PC];
uint32_t rd  = inst->I.rd;
uint32_t rs1 = inst->I.rs1;
uint32_t imm = (int32_t)inst->I.imm11_0;
if (inst->I.opcode == 0b0010011) {
  switch (inst->I.funct3) {
    case 0b000: R[rd] = R[rs1] + imm; break; // addi
    case 0b100: R[rd] = R[rs1] ^ imm; break; // xori
    case 0b110: R[rd] = R[rs1] | imm; break; // ori
    case 0b111: R[rd] = R[rs1] & imm; break; // andi
    default: panic("Unsupported funct3 = %d", inst->I.funct3);
  }
  R[0] = 0; // 若指令写入了R[0], 此处将其重置为0
} else if (inst->bytes == 0x00100073) {  ...  }

int main() {
  int s = 0;
  int i = 1;
loop:
  s = s + i;
  i = i + 1;
  if (i <= 100) goto loop;
  printf("s = %d\n", s);
  return 0;
}