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PET 2001 Emulator (masswerk)

2017 Freeware Online

The masswerk PET 2001 emulator is a browser-based Commodore PET 2001 emulator by Norbert Landsteiner, based on an original PET emulation by Thomas Skibo. It emulates the 6502 CPU and CRT display and adds keyboard, tape and disk image handling plus a built-in BASIC and 6502 assembler/disassembler.

Visit the official site ↗

Runs on: Web browser

PET 2001 Emulator (masswerk) Online Emulator

Play PET 2001 Emulator (masswerk) using JavaScript directly in your browser.

Configurations

Configuration	Emulator	Machine	OS	Legal
PET BASIC	PET 2001 Emulator (masswerk)	Commodore PET 2001		grey	Open ⛶

Machines emulated

Commodore PET 2001

Chips

MOS 6502

Notes

Embedding

masswerk's PET 2001 is a set of plain JavaScript modules. Vendor them and load them in dependency order, but skip its own controller (pet2001ctrl.js) - that owns the page UI and a requestAnimationFrame loop that can't be paused or stepped, which the debugger needs. Instead replicate the controller's run(): build the machine, hand it a canvas context and a keyboard, and drive it from a loop you control.

var ctx  = document.getElementById("screen").getContext("2d");
var keys = new PetKeys(null);          // physical keyboard → the PET key matrix
var pet  = new Pet2001();
pet.init(controllerStub, ctx, keys, { RAM_SIZE:0x4000, ROM_VERSION:"2" });
(function loop(){ pet.cycle(16667); requestAnimationFrame(loop); })();   // ~1 MHz / 60 Hz

The machine is plain objects. Everything the debugger needs is a method on the machine or its CPU:

Member	Kind	What it does
`pet.cycle(ticks)`	method	Run the machine for `ticks` clocks (steps I/O + CPU); returns <0 if the CPU jams. `cycle(1)` runs one instruction, the single-step primitive.
`pet.reset()`	method	Reset the machine.
`pet.read(a)` / `pet.write(a,v)`	method	CPU bus read/write, poke memory live.
`pet.getCPUStatus()`	method	Live 6502 state: `{pc,a,x,y,sp,sr, c,z,i,d,b,v,n}`.
`pet.setRegister(r,v)`	method	Write a register (`pc a x y sp sr`).
`pet.setFlag(f,v)`	method	Write a status flag (`c z i d b v n`).
`PetKeys(elementId)`	class	The keyboard: attaches window key events and drives `io.setKeyrows()`, the PET key matrix.

Because the CPU and memory are ordinary JavaScript, the debugger single-steps with cycle(1), reads and writes the registers through getCPUStatus / setRegister, and implements breakpoints and watchpoints as host-side checks around the loop, no changes to the emulator.

Debugger integration

The masswerk PET even ships its own debugger (petdebug.js) built on the same getCPUStatus / setRegister surface, which confirms the machine is fully introspectable from JavaScript. Our plug-in uses that surface directly.

Registers come from pet.getCPUStatus() each refresh and are written back with setRegister / setFlag.
Memory is pet.read/write; the hex view skips the $E8xx I/O window so auto-polling never trips a VIA side effect.
Single step is pet.cycle(1) (one instruction).
Breakpoints and watchpoints are host-side: the boot loop checks the PC against a set before each instruction, and wraps pet.write to catch a watched address.

Architecture

The PET 2001 is Commodore's first computer: a 6502, a discrete-logic video circuit driving a 40×25 character display, two 6520 PIAs and a 6522 VIA for I/O, and Commodore BASIC in ROM. masswerk's emulation keeps each as its own module:

cpu6502 - the 6502 core, cycle-stepped.
pet2001 - the machine: RAM, ROM banks and the bus routing that read/write follow.
pet2001video - the character-ROM video, rastered to the canvas.
pet2001io - the PIAs/VIA and the keyboard matrix (setKeyrows).
pet2001ieee - the IEEE-488 bus, used to load .prg programs.
pet2001roms - the character, BASIC and KERNAL ROMs, embedded.

It boots straight to ### COMMODORE BASIC ### - a complete, inspectable machine with nothing to load.