Intel 4004 (Busicom 141-PF)

Embedding

There is no upstream runtime to vendor here. The Intel 4004 core is written from scratch as three plain-global JavaScript modules, loaded in order:

// 1. the ROM image, 2. the MCS-4 chip set, 3. the Busicom harness
<script src="busicom-rom.js"></script>   // window.BUSICOM_ROM: 1280 bytes (5x 4001)
<script src="mcs4.js"></script>          // window.MCS4: CPU/ROM4001/RAM4002/Shifter4003/Board
<script src="busicom.js"></script>       // window.Busicom: printer + keypad + loop

Boot. Construct the machine on a <canvas> and run your own loop. One refresh() is a 16 ms burst of ~1481 machine cycles followed by the printer drum step; you own it, so the debugger can pause and step it:

var m = new Busicom(canvas, BUSICOM_ROM, { demo: demoKeys });
(function loop(){
  for (var i = 0; i < m.speed; i++) m.refresh(bps); // bps = breakpoint Set
  m.render();
  requestAnimationFrame(loop);
})();

The machine is plain objects No wasm heap, so the debugger reaches everything directly:

Debugger integration

The debugger plug-in (i4004-debug.js) reads window.EMU_BOOT and calls EmuKit.defineMachine. Because the whole machine is ordinary JavaScript, single-stepping is just m.stepInsn(1), registers are read and written straight off m.board.cpu, and execution breakpoints are a host-side Set the loop checks against the program counter at each instruction boundary.

A new i4004 disassembler (/debugger/src/cpus/i4004.js) decodes the 4004's 8-bit opcodes, a high nibble (operation) and low nibble (register / condition / immediate), flagging the five two-byte instructions (JCN, FIM, JUN, JMS, ISZ) and rendering everything else as one byte.

Architecture

The Busicom 141-PF was the calculator the 4004 was designed for. This emulator models the whole MCS-4 board that shipped inside it:

• 4004 - the 4-bit CPU: a 12-bit program counter, a 4-bit accumulator and carry, sixteen 4-bit index registers (R0..R15) and a 3-level address stack. Instructions take 8–16 clocks (10.8–21.6 µs).
• 4001 ×5, 256-byte ROM chips holding the 1280-byte calculator firmware, each with a 4-bit I/O port. Those ports scan the keypad and clock the printer.
• 4002 ×2 - RAM chips (registers + status nibbles) with output ports that fire the printer hammers and light the status LEDs.
• 4003 ×3, 10-bit shift registers: one scans the keypad matrix, two cascade to select which of the 18 printer columns the hammers strike.
• Printer, a rotating drum of 13 character bands. The firmware spins the drum and fires the hammers as the wanted character passes, printing a number one column at a time onto the paper tape.

The 4004 core is a faithful re-implementation (ported behaviour-for-behaviour from the veniamin-ilmer Rust MCS-4 model), which is exactly what makes it a clear debugging target: every nibble of state is visible and steppable.