DEC PDP-1

Embedding

masswerk's Spacewar! is one big self-contained engine (spacewar_pdp1_engine_ng.js) plus a code module - the Spacewar! 3.1 paper tape, base64-embedded in modules/spacewar_3_1.js. Give it a canvas called pdp1_canvas and a small preset object, and it boots the game on its own:

var PDP1Presets = { canvasId: 'pdp1_canvas', showSplashScreen: false, CRTWidth: 512 };
// <script src="spacewar_pdp1_engine_ng.js"> then the module <script> ...
// the engine auto-starts on window.onload - no start() call needed.

The engine owns its own setTimeout game loop, which a debugger can neither pause nor single-step. So this build vendors the engine with a small, clearly-marked _dbg hook that exposes the live PDP-1 and a breakpoint-aware, one-video-frame stepper. The boot shim waits for the game to start, calls PDP1Engine.stop() to silence the engine's timer, and drives everything from its own requestAnimationFrame loop instead:

Because the CPU and core store are ordinary JavaScript, breakpoints and watchpoints are host-side checks passed into _dbg.frame(), with no change to the emulation logic.

Debugger integration

The boot shim publishes window.EMU_BOOT with a full transport: pause / resume / isPaused, stepInsn(n) (one instruction), step(n) (whole frames), reset, plus breakpoints (program-counter values the frame stepper checks before each instruction) and watchpoints (core addresses, checked for change each instruction). A dedicated CPU decoder, /debugger/src/cpus/pdp1.js, disassembles each 18-bit word into native octal mnemonics: the memory-reference set (LAC DAC ADD SUB JMP JSP and friends) with the indirect flag, the augmented groups SKP / SFT / LAW, the IOT devices (DPY RPA RPB CKS), and the microcoded OPR operate word (CLA CLI CMA HLT, flag and PDP-1D register moves).

The core store is 4096 eighteen-bit words. The pdp1 decoder reads each word as three bytes, little-endian, so the plug-in exposes the store as a 12288-byte chip: byte 3n is bits 0–7 of word n, 3n+1 bits 8–15, and 3n+2 bits 16–17. Every instruction is therefore length three, and the register plug-in returns a byte-scaled pc() so the disassembly gutter, the program-counter highlight and breakpoints all land on word boundaries.

Architecture

The DEC PDP-1, delivered in 1959, was Digital's first computer and the machine on which Spacewar!, widely called the first video game, was written at MIT in 1961–62. It is an 18-bit, one's-complement machine with 4096 words of core, a single accumulator (AC) and in-out register (IO), and a Type 30 point-plotting CRT.

• Word and registers: 18-bit words, one's-complement arithmetic with an overflow flag (OV). The visible state is just AC, IO, PC and OV; the whole game runs in that.
• Instructions: the top six bits are the opcode (its low bit selecting indirect addressing) and the low twelve bits the address. Memory-reference orders (LAC, DAC, ADD, JMP…) sit alongside augmented orders whose address field is a function mask: SKP (conditional skips), SFT (shifts and rotates), OPR (the operate microcode) and IOT (in-out transfer).
• Display: the DPY IOT plots one point from AC (x) and IO (y). Spacewar! draws its stars and ships point by point, and masswerk's engine models the Type 30's dual-phosphor persistence so the trails glow and fade like the real tube.

The bundled program is the genuine Spacewar! 3.1 tape from 24 September 1962, loaded from its RIM paper-tape image, so the two ships, the gravity star, the torpedoes and hyperspace all behave exactly as they did at MIT.