讀指

read_inst 讀指令

word memory::read_inst(int adr){
  if((adr >= 0x10000000 && adr < 0x11000000) && adr % 4 == 0){
    int index = (adr - 0x10000000) / 4;
    return (*inst)[index];
  }
  std::cerr << "error: trying to read instruction from address: " << adr << '\n';
  std::exit(-11);
}

先判斷是不是在合法的地址內，取偏移地址，得到指令
001111 00000 11101 0010001111111111
op 001111 lui
把0010001111111111存放到11101號寄存器

對象化

執行（lui 把16位寄存器存放到rt號寄存器高16位）

然後pc + 4（一條指令4B）

R型ADD（寄存器）

op	rs	rt	rd	sh	fun
000000	10000	00010	00010	00000	100001

void cpu::ADD(const instruction& inst){
  s_word r1 = r[inst.src_s];
  s_word r2 = r[inst.src_t];
  s_word res = r1 + r2;

  if((res < 0 && r1 >= 0 && r2 >= 0)||(res >= 0 && r1 < 0 && r2 < 0)){
    std::cerr << "exception: arithmetic error" << std::endl;
    std::exit(-10);
  }

  r[inst.destn] = res;
  pc_increase(4);
 }
void cpu::ADDU(const instruction& inst) {
	word r1 = r[inst.src_s];
	word r2 = r[inst.src_t];
	word res = r1 + r2;
	r[inst.destn] = res;
	pc_increase(4);
}

結果

I型ADD（立即數）

void cpu::ADDI(const instruction& inst){
  //TODO: check immiatde sing extension
  s_word r1 = r[inst.src_s];
  s_word imi = sign_extend_imi(inst);
  s_word res = r1 + imi;

  if( (res <= 0 && r1 > 0 && imi > 0) || (res >= 0 && r1 < 0 && imi < 0) ){
    std::cerr << "exception: arithmetic error" << '\n';
    std::exit(-10);
  }

  r[inst.src_t] = res;
  pc_increase(4);
 }
void cpu::ADDIU(const instruction& inst){
  word r1 = r[inst.src_s];
  word imi = sign_extend_imi(inst);
  s_word res = r1 + imi;
  r[inst.src_t] = res;
  pc_increase(4);
 }

word cpu::sign_extend_imi(const instruction& inst){
  word imi = inst.i_imi;
  return (imi >= 0x8000) ? (0xFFFF0000 | imi) : imi;
 }