nes-emu/cpu/instructions/checks.js

A NES emulator

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.default = void 0;
var _helpers = require("../../helpers");
const instructions = () => [
/**
 * Bit Test
 *
 * Tests if one or more bits are set in a `value`.
 * The mask pattern in A is ANDed with the value to set or clear the Z flag,
 * but the the result is not kept.
 * Bits 7 and 6 of the value are copied into the N and V flags.
 */
{
  id: "BIT",
  needsValue: true,
  execute: (_ref, value) => {
    let {
      cpu
    } = _ref;
    const mask = cpu.registers.a.value;
    const result = value & mask;
    cpu.flags.updateZero(result);
    cpu.flags.updateNegative(value);
    cpu.flags.v = !!_helpers.Byte.getBit(value, 6);
  }
},
/**
 * Compare
 *
 * Compares A with `value`, setting the flags:
 * Z (if A = `value`), N (if bit 7 of A - `value` is set) and C (if A >= `value`).
 */
{
  id: "CMP",
  needsValue: true,
  execute: CP_("a")
},
/**
 * Compare X Register
 *
 * Compares X with `value`, setting the flags:
 * Z (if X = `value`), N (if bit 7 of X - `value` is set) and C (if X >= `value`).
 */
{
  id: "CPX",
  needsValue: true,
  execute: CP_("x")
},
/**
 * Compare Y Register
 *
 * Compares Y with `value`, setting the flags:
 * Z (if Y = `value`), N (if bit 7 of Y - `value` is set) and C (if Y >= `value`).
 */
{
  id: "CPY",
  needsValue: true,
  execute: CP_("y")
}];
const CP_ = registerName => {
  return (_ref2, value) => {
    let {
      cpu
    } = _ref2;
    const source = cpu.registers[registerName].value;
    cpu.flags.z = source === value;
    cpu.flags.updateNegative(_helpers.Byte.force8Bit(source - value));
    cpu.flags.c = source >= value;
  };
};
var _default = exports.default = instructions();