nes-emu/cpu/registers/FlagsRegister.js

A NES emulator

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.default = void 0;
var _helpers = require("../../helpers");
const N_BIT /*  */ = 0b10000000;
const V_BIT /*  */ = 0b01000000;
const B1_BIT /* */ = 0b00100000;
const D_BIT /*  */ = 0b00001000;
const I_BIT /*  */ = 0b00000100;
const Z_BIT /*  */ = 0b00000010;
const C_BIT /*  */ = 0b00000001;

/** The processor status flags. **/
class FlagsRegister {
  constructor() {
    this.n = false; // negative
    this.v = false; // overflow
    this.d = false; // decimal mode
    this.i = false; // interrupt disable
    this.z = false; // zero
    this.c = false; // carry

    // (bits 4 and 5 -also called "B1" and "B2"- are not actual flags)
    // (they only appear when the flags are pushed to the stack)
  }

  /** Deserializes a `byte` into the status flags. **/
  load(byte) {
    this.n = !!(byte & N_BIT);
    this.v = !!(byte & V_BIT);
    this.d = !!(byte & D_BIT);
    this.i = !!(byte & I_BIT);
    this.z = !!(byte & Z_BIT);
    this.c = !!(byte & C_BIT);
    return this;
  }

  /** Serializes the status flags into a byte. **/
  toByte() {
    return (this.n && N_BIT) | (this.v && V_BIT) | B1_BIT | (this.d && D_BIT) | (this.i && I_BIT) | (this.z && Z_BIT) | (this.c && C_BIT);
  }

  /** Updates the Z and N flags depending on a `byte`. */
  updateZeroAndNegative(byte) {
    this.updateZero(byte);
    this.updateNegative(byte);
  }

  /** Updates the Z flag depending on a `byte`. */
  updateZero(byte) {
    this.z = byte === 0;
  }

  /** Updates the N flag depending on a `byte`. */
  updateNegative(byte) {
    this.n = _helpers.Byte.isNegative(byte);
  }
}
exports.default = FlagsRegister;