c128

Embedding

There is no drop-in pure-JavaScript Commodore 128, so this machine is assembled from parts: the readable c64js 6510 core (the 8502 is a 6502 with extra I/O-port pins, so the same interpreter runs C128 code unchanged), the c64js VIC-II text renderer for the 40-column screen, the two CIA 6526s for the keyboard matrix and the 60 Hz system interrupt, and a new c128-mmu.js that models the MOS 8722 MMU. Vendor src/** and load the scripts in dependency order, then drive the machine yourself instead of any built-in loop so the debugger can pause and single-step.

Boot. Install the four system ROMs, point the 8502 at the MMU as its bus, point the VIC-IIe at a <canvas>, reset, then run your own loop built on mos6510.process() (one 8502 instruction, returns its cycle count):

c128.mmu.installRoms(c128.roms.basiclo, c128.roms.basichi, c128.roms.kernal);
memoryManager.character = new Rom(0, c128.roms.character4k); // VIC char fetches
mos6510.init(c128.mmu);        // bus = the MMU; reads reset vector $FFFC -> $FF3D
vic2.setScreenCanvas(canvas);
vic2.init(vicMemoryManager);
(function frame(){                          // 19656 VIC sub-cycles per frame
  for (var i = 0; i < 19656; i++) {
    if (cpuCycles === 0) cpuCycles = mos6510.process();
    cpuCycles--; vic2.process(i, 0);
  }
  memoryManager.cia1.process(19656);          // CIA1 Timer A -> the system IRQ
  requestAnimationFrame(frame);
})();

The 60 Hz system interrupt comes from CIA 1 Timer A, exactly as on the C64: the C128 hardware IRQ vector ($FF17) tail-calls the editor IRQ, which scans the keyboard and acknowledges the timer with LDA $DC0D. Feeding the timer each frame is what makes the cursor blink and the keyboard type.

Debugger integration

Because every part is plain JavaScript, the debugger reaches the whole machine with no wasm heap and no core patches:

Breakpoints are a host-side Set the loop checks against mos6510.register.pc before each instruction; watchpoints wrap the bank-0 RAM write path and pause when a watched address is written. Single-step just calls mos6510.process() once. The disassembler and register/hex views are the shared mos6502 framework code, the 8502 decodes identically.

Architecture

The Commodore 128 is really three machines in one box; this build emulates the native C128 mode (8502 CPU, BASIC 7.0, 40-column VIC-IIe). The Z80 (CP/M mode) and the 8563 VDC 80-column screen are not rendered; the VDC is stubbed "ready" so the KERNAL's 80-column init falls through and the 40/80 key reads as 40 columns.

• mos6510 - the 6502/8502 CPU interpreter (c64js). process() runs one instruction.
• c128.mmu - the MOS 8722 MMU: the configuration register ($FF00/$D500), the load-config registers ($FF01-$FF04), 128K RAM in two banks with a common-RAM region, and ROM/I-O banking of $4000-$FFFF.
• vic2 - the VIC-IIe, rastered into the canvas for the 40-column text screen (standard character mode).
• cia1 / cia2 - the two 6526s: keyboard matrix + Timer A system IRQ, and the VIC bank select.
• c128.roms - BASIC LO ($4000), BASIC HI ($8000), editor+KERNAL ($C000) and the character ROM, the stock Commodore images.

The three system ROMs plus the character ROM are Commodore firmware, self-hosted here and cited in README.txt; they are treated as grey and takedowns are honoured.