gistfile1.c

#include <stdio.h>

/* registers */
#define NUM_REGS 4
int regs[NUM_REGS];

/* program instructions */
int prog[] = {
  0x1005, /* loadi r0 #5 */
  0x1101, /* loadi r1 #1 */
  0x2221, /* add r2 r2 r1 */
  0x3001, /* sub r0 r0 r1 */
  0x4002, /* jnz r0 #2 */
  0x0000  /* halt */
};

/* program counter */
int pc = 0;

int fetch() 
{
  return prog[pc++];
}

int opcode = 0;

/* operands */
int reg1 = 0;
int reg2 = 0;
int reg3 = 0;
int imm = 0;

void decode(int instr)
{
  opcode = (instr & 0xF000) >> 12;
  reg1 = (instr & 0x0F00) >> 8;
  reg2 = (instr & 0x00F0) >> 4;
  reg3 = (instr & 0x000F);
  imm = (instr & 0x00FF);
}

int running = 1;

void eval()
{
  switch (opcode) {
  case 0:
    printf("halt\n");
    running = 0;
    break;
    
  case 1:
    printf("loadi r%d #%d\n", reg1, imm);
    regs[reg1] = imm;
    break;

  case 2:
    printf("add r%d r%d r%d\n", reg1, reg2, reg3);
    regs[reg1] = regs[reg2] + regs[reg3];
    break;
    
  case 3:
    printf("sub r%d r%d r%d\n", reg1, reg2, reg3);
    regs[reg1] = regs[reg2] - regs[reg3];
    break;
    
  case 4:
    printf("jnz r%d #%d\n", reg1, imm);
    if (regs[reg1] != 0) {
      pc = imm;
    }
    break;
  }
}

void showregs() 
{
  int i;
  printf("regs = ");
  for (i = 0; i < NUM_REGS; i++)
    printf("%04X ", regs[i]);
  printf("\n");
}

void run()
{
  while (running) {
    showregs();
    int instr = fetch();
    decode(instr);
    eval();
  }
}

int main (int argc, const char* argv[])
{
  run();
  return 0;
}