Enterprise 128

Embedding

The Enterprise 128 is emulated by JSep, a pure-JavaScript emulator by Gábor Lénárt (LGB). Its Z80 core is JSSpeccy's; everything Enterprise-specific - the Nick display chip, the Dave sound/timer chip, the four-register 16 KB memory paging and the I/O map - is original JavaScript. The engine was written to drive itself from an HTML page, so the only integration work is to take over its loop and feed it ROMs locally.

Boot. The engine keeps the whole machine in plain globals, so booting is: install the system ROMs into the guest memory map, initialise the Z80 tables, then call the engine's own enterpriseInitEmulation() - after which we run our own frame loop instead of its setTimeout one:

// self-host the ROMs (no CDN): a synchronous, side-effect-free binary read
window.installRom = function (name, segment) {
  var xhr = new XMLHttpRequest();
  xhr.open("GET", romBase + name, false);
  xhr.overrideMimeType("text/plain; charset=x-user-defined");
  xhr.send(null);
  var s = xhr.responseText, off = segment << 14;
  for (var i = 0; i < s.length; i++) memory[i + off] = s.charCodeAt(i) & 0xff;
};
z80_init();
enterpriseInitEmulation();          // ROMs in, Nick/Dave reset, palette built

The machine is plain objects. Everything the debugger needs is a live global. There is no wasm heap to reach into:

Debugger integration

The plug-in (ep128-debug.js) describes the machine to the shared debugger and nothing more:

• Registers are assembled live each refresh from the Z80's byte halves - PC, SP, AF, BC, DE, HL, IX, IY, the shadow set, I, R, IM and the S Z H P/V N C flags - and written back into the same fields.
• Disassembly uses the shared z80 decoder.
• Memory is the 64 KB the CPU currently sees, read side-effect-free through Nick's paging, so you watch exactly what the Z80 does - EXOS in segment 0, IS-BASIC paged in, RAM above.
• Single step runs one Z80 instruction (event_next_event = tstates + 1); frame step runs one video frame. Breakpoints are a PC set checked once per instruction inside a breakpoint-aware frame (Nick and Dave stay live so the machine keeps making progress); watchpoints are a host-side check wrapped around the single guest memory-write path. When no breakpoints or watchpoints are set the loop runs a whole frame at full speed with no per-instruction overhead.

Architecture

The Enterprise 64/128 (badged "Enterprise", by Intelligent Software / Elan, 1985) is a Z80 home computer built around two custom Hungarian-designed gate arrays and a paged memory map:

• Z80A @ 4 MHz - the CPU. It sees 64 KB at a time as four 16 KB pages; each page register (ports 0xB0–0xB3) selects one of up to 256 16 KB segments of the 4 MB address space, mixing ROM and RAM freely.
• Nick - the display chip. It reads a Line Parameter Table from RAM and can change resolution, colour mode (2/4/16/256) and palette per scanline, giving the Enterprise its flexible, colourful screen.
• Dave - sound and system timing: three tone channels plus noise, the 1 Hz/50 Hz/1 kHz interrupt sources that drive EXOS, and part of the memory decoding.
• EXOS - the Enterprise Operating System in ROM: a device/channel-based OS. With the IS-BASIC ROM paged in, a cold machine boots straight into IS-BASIC, an unusually complete structured BASIC.

This build boots EXOS 2.1 with IS-BASIC 2.1 in a 128 KB machine, landing at the IS-BASIC prompt with no disk.