Galaxian

Embedding

Galaxian is the 1979 Namco arcade machine — the game that answered Space Invaders with a colourful, swooping alien attack and became the template for the fixed shooter, including its own sequel Galaga. The board is a single Zilog Z80 at 3.072 MHz driving a 256×224 raster rotated 90° in the upright cabinet. This build pairs the vendored DrGoldfire Z80.js CPU core with a plain-JavaScript model of the Galaxian board (a hand port of the 8bitworkshop GalaxianMachine) and drives the whole thing from a host-owned loop.

Boot. Constructing the machine loads the combined program/graphics/palette ROM image and resets the Z80, which starts at 0x0000 (XOR A; LD (0x7001),A; JP 0x1a55) and drops into attract mode:

var gx = new GalaxianMachine(window.GALAXIAN_ROM); // program + gfx + colour PROM
function loop(){                                // ~one 60Hz field per rAF
  var budget = 264 * 192;                     // scanlines x cycles/line
  while(spent < budget) spent += gx.runInstruction();  // draws scanlines, fires vblank NMI
  draw();                                     // rotate the 264x264 buffer to portrait
}

The machine object. GalaxianMachine exposes the board as ordinary fields and methods, so the host page can render, step and inject input:

Video. Each field the renderer walks 264 scanlines, drawing the 32-column tilemap, the eight hardware sprites, the shell/missile bullets and the twinkling starfield into an off-screen 264×264 buffer; the loop then rotates the visible 224×256 window into the upright cabinet's portrait orientation.

Debugger integration

Because the Z80 interpreter runs one instruction at a time in JavaScript and the host owns the loop, the debugger's single-step, breakpoints and watchpoints fall out naturally — the loop can stop, step and inspect between any two instructions.

window.EMU_BOOT.transport exposes the controls the shared debugger calls:

• pause / resume / isPaused — stop or restart the requestAnimationFrame loop. Resume steps off a breakpoint first, so it always makes progress.
• stepInsn(n) — call GalaxianMachine.runInstruction() exactly n times and redraw, so a single step advances the PC by one instruction (and still lets a field boundary fire its NMI).
• step(n) — advance n whole 60 Hz fields.
• breakpoints — a Set of PC values. When it is empty the loop runs a whole field at speed; when it is non-empty the loop checks cpu.get_pc() before each instruction and pauses before executing a watched address. An idle debugger costs nothing.
• watchpoints — the board's writeHook is called with the address of every CPU memory write; a write to a watched address pauses the loop.

The Z80 core needed one small addition for the debugger: get_pc(), a side-effect-free read of the program counter, so the run-loop can test PC against breakpoints every instruction without allocating a full getState() snapshot. Registers are read and written live through getState()/setState() — A F B C D E H L, the BC/DE/HL pairs, IX IY SP PC I R and the S Z H P/V N C flags derived from the flag object. Memory views read through readSafe, which returns ROM, RAM, video RAM and object RAM but never kicks the watchdog or trips an output latch, so inspecting memory never disturbs the machine. The plug-in (galaxian-debug.js) feeds all of this to EmuKit.defineMachine and reuses the shared z80 disassembler at /debugger/src/cpus/z80.js.

Architecture

The Galaxian board (Namco, 1979) is a single-CPU design, much simpler than the three-Z80 Galaga that followed it:

• CPU — one Zilog Z80 at 3.072 MHz. 16 KB of program+graphics ROM at 0x0000, 1 KB of work RAM at 0x4000.
• Video RAM — a 32×32 tile map at 0x5000 (only the visible 32×28 is shown), with per-column scroll and colour attributes in the 256-byte object RAM at 0x5800.
• Sprites & bullets — object RAM also holds eight 16×16 hardware sprites and eight one-pixel bullet generators (seven white player/enemy shells and one yellow missile).
• Starfield — a hardware pseudo-random star generator scrolls a twinkling background, enabled by a bit of the addressed output latch at 0x7004.
• Interrupt — a non-maskable interrupt is raised once per field at vertical blank, gated by the output latch at 0x7001; the game enables it after its power-on setup and runs its logic from the NMI handler at 0x0066.
• Inputs — three latches read at 0x6000 / 0x6800 / 0x7000: coin and player-one joystick/fire, the start buttons and player-two controls, and the dip switches (lives, bonus, coinage). All active-high.
• Watchdog — a counter that reboots the board unless the game keeps reading 0x7800; the model reproduces the kick and the reboot.

Sound (two AY-3-8910 PSGs on the Galaxian daughter board) is not emulated in this build; the PSG register writes are accepted and ignored.