nes-emu/cpu/addressings/indirect.js

A NES emulator

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.getIndirectAddress = exports.default = void 0;
var _helpers = require("../../helpers");
var _getValue = _interopRequireDefault(require("./_getValue"));
function _interopRequireDefault(e) { return e && e.__esModule ? e : { default: e }; }
/**
 * "Indirect" addressing mode.
 *
 * The parameter is an absolute memory address to look up another address.
 * The byte read from memory gives the least significant byte of the final
 * address, and the following byte gives the most significant byte.
 *
 * This addressing mode has a bug:
 * If `address` falls on a page boundary ($xxFF), it fetches the least significant byte from
 * $xxFF as expected, but takes the most significant byte from $xx00.
 */
var _default = exports.default = {
  id: "INDIRECT",
  parameterSize: 2,
  getAddress: (_ref, address) => {
    let {
      cpu
    } = _ref;
    const msb = _helpers.Byte.highPartOf(address);
    const lsb = _helpers.Byte.lowPartOf(address);
    const low = cpu.memory.readAt(address);
    const high = cpu.memory.readAt(lsb === 0xff ? _helpers.Byte.to16Bit(msb, 0x00) : address + 1);
    return _helpers.Byte.to16Bit(high, low);
  },
  getValue: _getValue.default
};
const getIndirectAddress = (_ref2, address) => {
  let {
    cpu
  } = _ref2;
  const start = _helpers.Byte.force8Bit(address);
  const end = _helpers.Byte.force8Bit(start + 1);
  const low = cpu.memory.readAt(start);
  const high = cpu.memory.readAt(end);
  return _helpers.Byte.to16Bit(high, low);
};
exports.getIndirectAddress = getIndirectAddress;