nes-emu/memory/MemoryMirror.js

A NES emulator

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.default = void 0;
var _WithLittleEndian = _interopRequireDefault(require("./WithLittleEndian"));
function _interopRequireDefault(e) { return e && e.__esModule ? e : { default: e }; }
/** A memory chunk that mirrors another `chunk`. */
class MemoryMirror {
  constructor(chunk, size) {
    let startAt = arguments.length > 2 && arguments[2] !== undefined ? arguments[2] : 0;
    let mirroredSize = arguments.length > 3 && arguments[3] !== undefined ? arguments[3] : chunk.memorySize - startAt;
    _WithLittleEndian.default.apply(this);
    this.chunk = chunk;
    this.startAt = startAt;
    this.mirroredSize = mirroredSize;
    this.memorySize = size;
  }

  /** Reads a byte from `address`, mirroring bytes. */
  readAt(address) {
    this._checkAddress(address);
    return this.chunk.readAt(this.startAt + address % this.mirroredSize);
  }

  /** Writes a `byte` to `address`, mirroring bytes. */
  writeAt(address, byte) {
    this._checkAddress(address);
    this.chunk.writeAt(this.startAt + address % this.mirroredSize, byte);
  }
  _checkAddress(address) {
    if (address >= this.memorySize) throw new Error("Unreachable address: 0x".concat(address.toString(16), "."));
  }
}
exports.default = MemoryMirror;