nes-emu/ppu/registers/PPUData.js

A NES emulator

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.default = void 0;
var _registers = require("../../registers");
var _constants = _interopRequireDefault(require("../../constants"));
var _helpers = require("../../helpers");
function _interopRequireDefault(e) { return e && e.__esModule ? e : { default: e }; }
/**
 * PPU Data Port (<> read/write)
 *
 * Read/Write VRAM data here. `PPUAddr` will be incremented after any read/write.
 * Reads are delayed by 1.
 */
class PPUData extends _registers.InMemoryRegister {
  /** When a context is loaded. */
  onLoad() {
    this.buffer = 0; // (used to simulate read delay)
  }

  /** Reads a value from PPU address space, delayed by one read (except for the Palette RAM). */
  readAt() {
    if (this.context.isDebugging) return this.buffer;
    let data = this.buffer;
    const ppuAddress = this.context.ppu.registers.ppuAddr.address;
    this.buffer = this.context.ppu.memory.readAt(ppuAddress);

    // (if the PPUAddr is inside Palette RAM area, skip the buffer)
    if (ppuAddress >= _constants.default.PPU_ADDRESSED_PALETTE_RAM_START_ADDRESS) data = this.buffer;

    // (reading increments `PPUAddr` as a side effect)
    this._incrementAddress();
    return data;
  }

  /** Writes a `byte` to PPU address space and increments `PPUAddr`. */
  writeAt(__, byte) {
    const ppuAddress = this.context.ppu.registers.ppuAddr.address;
    this.context.ppu.memory.writeAt(ppuAddress, byte);
    this._incrementAddress();
  }
  _incrementAddress() {
    const {
      registers
    } = this.context.ppu;
    registers.ppuAddr.address = _helpers.Byte.force16Bit(registers.ppuAddr.address + registers.ppuCtrl.vramAddressIncrement);
  }
}
exports.default = PPUData;