code: function (cpu, mmu, cc) {
      if (cc) {
          cpu.pc = mmu.readWord(cpu.sp);
          cpu.sp += 2;
  
          return 20;
      }
      cpu.pc += 1;
  
      return 8;
  }
docs: RET cc
  
  Description:
  Return if following condition is true:
  
  Use with:
  cc = NZ, Return if Z flag is reset.
  cc = Z, Return if Z flag is set.
  cc = NC, Return if C flag is reset.
  cc = C, Return if C flag is set.
  
  Opcodes:
  Instruction    Parameters    Opcode    Cycles
  RET            NZ            C0        8
  RET            Z             C8        8
  RET            NC            D0        8
  RET            C             D8        8