rdos/utils/emul.c

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

uint8_t mem[64*1024];

union regset {
   uint8_t r8[8]; // AL, AH, CL, CH...
   uint16_t r16[8]; // AX, CX, DX, BX, SP, BP, SI, DI
} regset;

uint16_t ip = 0x100;
uint16_t flags;
uint16_t scratch;
uint16_t reg;
int off;

void* rmptr;
#define RM8 *(uint8_t*)rmptr
#define RM16 *(uint16_t*)rmptr

#define R8(x) regset.r8[fixr8ref(x)]
#define R16(x) regset.r16[x]

#define syscall_number regset.r8[4]
#define AL regset.r8[0]
#define CL regset.r8[2]
#define DL regset.r8[4]
#define BX regset.r16[3]
#define SP regset.r16[4]
#define BP regset.r16[5]
#define SI regset.r16[6]
#define DI regset.r16[7]

const char* const regnames[] = { "AX", "CX", "DX", "BX", "SP", "BP", "SI", "DI", 0 };

// Dump registers
void dump() {
  printf("\n");
  int i;
  for (i=0; i<8; i++) {
    printf("%s=%04X ", regnames[i], regset.r16[i]);
  }
  printf("\n");
  printf("IP=%04X : %X\n", ip, mem[ip]);
}

int fixr8ref(int r) {
  return ((r << 1) & 0b110) | ((r >> 2) & 0b1);
}

uint8_t imm8() {
  uint8_t r = *(uint8_t*)&mem[ip];
  ip++;
  return r;
}

uint16_t imm16() {
  uint16_t r = *(uint16_t*)&mem[ip];
  ip+=2;
  return r;
}

#define BIT8  8
#define BIT16 16
void modrm(int size) {
  uint8_t mod = (mem[ip] >> 6) & 0b11; // highest 2 bits
  reg = (mem[ip] >> 3) & 0b111;
  uint8_t rm = (mem[ip]) & 0b111;
  ip++;
  if (mod == 3) {
    if (size == BIT8) {
      rmptr = &R16(rm);
    } else {
      rmptr = &R8(rm);
    }
   return;
  }
  if (mod == 0 && rm == 6) {
    rmptr = mem + (int16_t)imm16();
    return;
  }
  switch(rm) {
  case 0: rmptr = mem + BX + SI; break;
  case 1: rmptr = mem + BX + DI; break;
  case 2: rmptr = mem + BP + SI; break;
  case 3: rmptr = mem + BP + DI; break;
  case 4: rmptr = mem + SI; break;
  case 5: rmptr = mem + DI; break;
  case 6: rmptr = mem + BP; break;
  case 7: rmptr = mem + BX; break;
  }
  if (mod == 1) rmptr += (int8_t)imm8();
  if (mod == 2) rmptr += (int16_t)imm16();
}

int cond(int num) {
  int r = 0;
  switch(num >> 1) {
  case 2:
    r = !scratch;
    break;
  default:
    fprintf(stderr, "Condition code %d not implemented\n", num);
    exit(1);
    break;
  }
  // Each odd condition is the negation of the previous
  if (num & 1) r=1-r;
  return r;
}

void push(uint16_t v) {
  SP -= 2;
  *(uint16_t*)&mem[SP] = v;
}

uint16_t pop() {
  uint16_t v = *(uint16_t*)&mem[SP];
  SP += 2;
  return v;
}

void handle_syscall(int number) {
  switch(number) {
  case 0:
    exit(0);
    break;
  case 2:
    printf("%c", DL);
    break;
  default:
    fprintf(stderr, "Fatal: Unhandled syscall CL=%Xh\n", number);
    exit(1);
  }
}

void handle_intr(int number) {
  switch(number) {
  case 0x20:
    handle_syscall(0);
    break;
  case 0xE0:
    handle_syscall(CL);
    break;
  default:
    fprintf(stderr, "Fatal: Unhandled interrupt %Xh\n", number);
    exit(1);
  }
}

// Execute a single instruction
void step() {
  uint8_t opcode = mem[ip];
  ip++;
  switch(opcode) {
  case 0x70 ... 0x7F: // Jcc
    reg = ip + (int8_t)imm8();
    if (cond(opcode-0x70)) {
      ip = reg;
    }
    break;
  case 0x84: // TEST r/m8,reg8
    modrm(BIT8);
    scratch = AL & RM8;
    break;
  case 0x88: // MOV r/m8,reg8
    modrm(BIT8);
    RM8 = R8(reg);
    break;
  case 0xAC: // LODSB
    AL = mem[SI];
    SI++;
    break;
  case 0xB0 ... 0xB7: // MOV reg8,imm8
    reg = opcode-0xB0;
    R8(reg) = imm8();
    break;
  case 0xB8 ... 0xBF: // MOV reg16,imm16
    reg = opcode-0xB8;
    R16(reg) = imm16();
    break;
  case 0xC3: // RET
    ip = pop();
    break;
  case 0xCC: // INT3
    dump();
    break;
  case 0xCD: // INT imm8
    handle_intr(imm8());
    break;
  case 0xE8: // CALL rw
    reg = ip + (int16_t)imm16();
    if (reg == 5) {
      handle_syscall(CL);
    } else {
      push(ip);
      ip += reg;
    }
    break;
  case 0xEB: // JMP rb
    ip = ip + (int8_t)imm8();
    break;
  case 0xFA: // CLI
  case 0xFB: // STI
    break; // no-op, since we dont have interrupts
  default:
    ip--;
    dump();
    fprintf(stderr, "Invalid opcode at IP=%04X\n", ip);
    exit(1);
    break;
  }
}

int main(int argc, char** argv) {
  memset(&mem, sizeof(mem), 0);
  memset(&regset, sizeof(regset), 0);
  mem[0]=0xCD;
  mem[1]=0x20;
  push(0);

  FILE* fd = fopen(argv[1], "r");
  fread(mem + ip, 1, sizeof(mem) - ip, fd);
  while(1) {
    step();
  }
}