broken-neees/lib/apu/DPCM.js

A really broken NEEES emulator that introduces glitches and random bugs on purpose!

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.default = void 0;
var _byte = _interopRequireDefault(require("../byte"));
function _interopRequireDefault(e) { return e && e.__esModule ? e : { default: e }; }
/**
 * DPCM (Δ-Modulation) channel handler.
 * Manages sample playback: fetching bits, looping, and outputting variations.
 */
class DPCM {
  constructor(dmcChannel) {
    this.dmcChannel = dmcChannel;
    this.startFlag = false;
    this.isActive = false;
    this.buffer = null;
    this.cursorByte = 0;
    this.cursorBit = 0;
    this.dividerPeriod = 0;
    this.dividerCount = 0;
    this.sampleAddress = 0;
    this.sampleLength = 0;
  }

  /**
   * Called each APU cycle to handle DPCM timing, sample fetching,
   * bit processing, output variation, and looping.
   */
  update() {
    if (this.startFlag) {
      const dividerPeriod = dpcmPeriods[this.registers.control.dpcmPeriodId];
      this.startFlag = false;
      this.isActive = true;
      this.cursorByte = -1;
      this.cursorBit = 0;
      this.dividerPeriod = dividerPeriod;
      this.dividerCount = dividerPeriod - 1;

      // sample address = %11AAAAAA.AA000000 = $C000 + (A * 64)
      this.sampleAddress = 0xc000 + this.registers.sampleAddress.value * 64;
      // sample length = %LLLL.LLLL0001 = (L * 16) + 1 bytes
      this.sampleLength = this.registers.sampleLength.value * 16 + 1;
    }
    if (!this.isActive) return;
    this.dividerCount++;
    if (this.dividerCount >= this.dividerPeriod) this.dividerCount = 0;else return;
    const needFetch = this.buffer === null || this.cursorBit === 8;
    if (needFetch) {
      const nextByte = this.cursorByte + 1;
      if (nextByte >= this.sampleLength) {
        this.isActive = false;
        this.buffer = null;
        if (this.registers.control.loop) this.start();
        return;
      }
      this.cursorByte = nextByte;
      this.cursorBit = 0;
      let address = this.sampleAddress + this.cursorByte;
      if (address > 0xffff) {
        // (if it exceeds $FFFF, it is wrapped around to $8000)
        address = 0x8000 + address % 0x10000;
      }
      this.buffer = this.cpu.memory.read(address);
    }
    const variation = _byte.default.getBit(this.buffer, this.cursorBit) ? 2 : -2;
    const newSample = this.dmcChannel.outputSample + variation;
    if (newSample >= 0 && newSample <= 127) this.dmcChannel.outputSample = newSample;
    this.cursorBit++;
  }

  /**
   * Sets the startFlag so playback begins on next update.
   */
  start() {
    this.startFlag = true;
  }

  /**
   * Stops playback immediately by moving cursor to sample end.
   */
  stop() {
    this.cursorByte = this.sampleLength;
  }

  /**
   * Returns remaining bytes in sample, or 0 if inactive.
   */
  remainingBytes() {
    if (!this.isActive) return 0;
    return Math.max(0, this.sampleLength - (this.cursorByte + 1));
  }
  get cpu() {
    return this.dmcChannel.cpu;
  }
  get registers() {
    return this.dmcChannel.registers;
  }
}

/**
 * A list of all DPCM periods. The `DMCControl` register points to this table.
 */
exports.default = DPCM;
const dpcmPeriods = [214, 190, 170, 160, 143, 127, 113, 107, 95, 80, 71, 64, 53, 42, 36, 27];