Plus/4

Embedding

The Plus/4 here is a set of plain-global JavaScript modules (no bundler). Load the reused 6502 core (cpu6502.js), the base64-embedded ROMs (roms.js) and the machine (plus4-machine.js), then drive the machine from your own loop so the debugger can pause and single-step it.

Boot. Install the two 16 KB ROMs, reset (which loads PC from the $FFFC KERNAL vector), point the renderer at a <canvas>, then run your own loop built on mos6510.process() (one instruction, returns its cycle count) feeding Plus4.tick() so the TED timers and raster interrupt advance:

Plus4.setRoms(Plus4Roms.basic, Plus4Roms.kernal);
Plus4.reset();
(function frame(){
  Plus4.mem._frame = false;
  while (!Plus4.mem._frame) {                 // run one PAL frame
    var c = mos6510.process();          // one 8501 instruction
    Plus4.tick(c);                          // timers + raster IRQ
  }
  Plus4.renderInto(ctx, canvas.width, canvas.height);
  requestAnimationFrame(frame);
})();

The machine is plain objects. Everything the debugger needs is a live global:

Debugger integration

Because the CPU, memory and TED are ordinary JavaScript, the debugger integrates with no changes to the core. The boot loop publishes window.EMU_BOOT with a transport the shared framework drives:

• Single-step calls mos6510.process() once and ticks the TED by the returned cycles.
• Execution breakpoints are a Set of PC values; when non-empty the loop runs instruction-by-instruction and halts before any matching mos6510.register.pc.
• Write watchpoints hook Plus4.mem.onWrite, so any store to a watched address pauses the machine promptly.
• Registers are read and written straight off mos6510.register; memory views read through Plus4.busReadSafe so auto-polling never disturbs the TED.

The disassembler is the shared mos6502 decoder. The 8501 is a 6502 with an on-chip I/O port, and on the Plus/4 the banking is done through TED registers rather than that port, so the stock decoder is exact.

Architecture

The Plus/4 (Commodore 264 series, 1984) is a 6502-class machine whose single custom chip, the TED, does almost everything:

• mos6510 The reused 6502 interpreter standing in for the MOS 7501/8501. process() runs one instruction.
• Plus4 memory - 64 KB RAM with BASIC ($8000-$BFFF) and KERNAL ($C000-$FFFF) ROM banked in for reads; writes always reach RAM. Reads or writes of $FF3E/$FF3F switch the ROM in and out.
• Plus4.ted - the MOS 7360/8360: three interval timers, the raster counter and raster interrupt, the interrupt status/mask registers ($FF09/$FF0A) and a 40x25 text renderer reading the video matrix at $0C00, colour RAM at $0800 and the character generator inside the KERNAL ROM.
• keyboard - the 6529 column latch at $FD30 selects columns and the TED latch $FF08 reads the rows of the 8x8 matrix.

Sound (the TED's two-voice tone generator) is not emulated; everything the KERNAL needs to reach BASIC and scan the keyboard is.