       if (x == 0 && y == 1) /* EXT */ {
              console.log("Exited with code: ", d);
              this.stop();
              return;
            }
            switch(instr) {
              case 0x00E0: /* CLS */ this.clearScreen(); break;
              case 0x00EE: /* RET */ jump(pop()); break;
              default: unknown(instr); break;
            }
          break;
          case 0x1: /* JMP */ jump(a); break;
          case 0x2: /* CAL */ push(); jump(a); break;
          case 0x3: /* SEI */ 