nes-emu/ppu/registers/PPUStatus.js

A NES emulator

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.default = void 0;
var _registers = require("../../registers");
/**
 * PPU Status Register (< read)
 *
 * Reflects the state of various functions inside the PPU.
 * It is often used for determining timing.
 */
class PPUStatus extends _registers.InMemoryRegister {
  constructor() {
    super();
    this.addField("spriteOverflow", 5).addField("sprite0Hit", 6).addField("isInVBlankInterval", 7);
  }

  /** When a context is loaded. */
  onLoad() {
    this.setValue(0b10000000);
  }

  /** Reads the status flags, with some side effects. */
  readAt() {
    const value = this.value;

    // this has two side effects:
    if (!this.context.isDebugging) {
      // - it resets the vertical blank flag
      this.isInVBlankInterval = 0;

      // - it resets the write latch
      this.context.ppu.loopy.onPPUStatusRead();
    }
    return value;
  }

  /** Writes nothing (read-only address). */
  writeAt() {}
}
exports.default = PPUStatus;