nes-emu/cartridge/mappers/_3_CNROM.js

A NES emulator

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.default = void 0;
var _Mapper = _interopRequireDefault(require("./Mapper"));
var _memory = require("../../memory");
var _lodash = _interopRequireDefault(require("lodash"));
function _interopRequireDefault(e) { return e && e.__esModule ? e : { default: e }; }
/**
 * It provides bank-switching for CHR ROM only.
 * CPU $8000-$BFFF: 16 KB PRG ROM, fixed to the first page
 * CPU $C000-$FFFF: 16 KB PRG ROM, fixed to the second page (or mirror)
 * PPU $0000-$1FFF: 8 KB switchable CHR bank
 */
class CNROM extends _Mapper.default {
  static get id() {
    return 3;
  }

  /** Creates a memory segment for CPU range $4020-$FFFF. */
  createCPUSegment(_ref) {
    let {
      cartridge
    } = _ref;
    const unused = new _memory.MemoryPadding(0x3fe0);
    const prgRomFirstPage = this._newPrgBank();
    const prgRomLastPage = cartridge.header.prgRomPages === 2 ? this._newPrgBank(1) : new _memory.MemoryMirror(prgRomFirstPage, 0x4000);
    this._state = {
      page: 0
    };
    return _memory.WithCompositeMemory.createSegment([
    //                  Address range   Size      Description
    unused,
    //          $4020-$7FFF     $3FE0     Unused space
    prgRomFirstPage,
    // $8000-$BFFF     $4000     PRG ROM (first 16KB of ROM)
    prgRomLastPage //   $C000-$FFFF     $4000     PRG ROM (second 16KB of ROM or mirror)
    ]);
  }

  /** Creates a memory segment for PPU range $0000-$1FFF. */
  createPPUSegment(_ref2) {
    let {
      cartridge
    } = _ref2;
    return this._newChrBank(cartridge);
  }

  /** Maps a CPU write operation. */
  cpuWriteAt(address, byte) {
    if (address >= 0x8000) {
      this._state.page = byte;
      this._loadBanks();
      return;
    }
    this.context.cpu.memory.writeAt(address, byte);
  }

  /** Returns a snapshot of the current state. */
  getSaveState() {
    return _lodash.default.assign(super.getSaveState(), {
      page: this._state.page
    });
  }

  /** Restores state from a snapshot. */
  setSaveState(saveState) {
    super.setSaveState(saveState);
    this._state.page = saveState.page;
    this._loadBanks();
  }
  _loadBanks() {
    this.segments.ppu.bytes = this._getChrPage(this._state.page);
  }
}
exports.default = CNROM;