Stuff done at Cat's Eye Technologies on older computers, or more often, an emulation of such a computer on a modern computer instead.

Some of the information here is not specific to Cat's Eye Technologies and may be moved to The Dossier at a future date.

Apple II

Rundown: The Apple II series of microcomputers included the Apple ][+ and Apple //e. The CPU was a 6502. Applesoft BASIC (with a manual online here) was a dialect of BASIC that was available for (in fact, generally shipped with) these computers. There was also an "Integer Basic".

Emulators: AppleWin is an emulator for the Apple II, built for Windows, written in C++ and distributed under the GPL. Linapple is a port of AppleWin to Linux. has a list of other emulators.

Apple Befunge

Apple Befunge

Apple Befunge is a variant of Befunge for the Apple II which resembles Befunge-93, with some extra Befunge-96-esque instructions and some Apple II-specific instructions. It was written for the Apple ][+, but should also run on an Apple //e.

Commodore VIC-20

Rundown: The CPU was a 6502. Commodore BASIC 2.0 was in the ROM; this is the same BASIC that was in the Commodore 64.

Emulators: The xvic executable from VICE, written in C99 and distributed under the GPL, is a generally recommended emulator for the VIC-20.

Dungeons of Ekileugor

Dungeons of Ekileugor

Dungeons of Ekileugor is a roguelike written for the unexpanded Commodore VIC-20, which, despite the limitations of that platform, supports a respectable set of the usual "dungeon furniture": reasonably generated dungeon levels with tunnels and rooms whose contents are hidden until you enter, monsters, treasure, potions, traps, chests, combat with experience points, etc.

Commodore 64

Rundown: The CPU was a 6510, which was a slightly modified 6502. Commodore BASIC 2.0 was in the ROM; this is the same BASIC that was in the VIC-20. The Commodore 64 Programmer's Reference Guide is an invaluable reference, or is it merely a guide? Lots of C64 stuff can also be found at

Emulators: The x64 executable from VICE, written in C99 and distributed under the GPL, is a generally recommended emulator for the Commodore 64. Also, JaC64 is a GPL Java application which emulates a Commodore 64. Cat's Eye Technologies uses it to showcase our Commodore 64 games as online installations, using Java Web Start. We forked it to fix some bugs and because mainline development seemed stalled.

Bubble Escape

Bubble Escape 2K

Bubble Escape is a video game written for the Commodore 64 where the player must guide a bubble through a multi-screen maze. It was originally designed and implemented in Commodore BASIC 2.0 in the mid-80's, and rewritten in 6502 assembly language in the late 00's. The assembly version was pared down so that (crunched) it fit into 2K, christened "Bubble Escape 2K", and submitted to the Mini Game Compo 2009 where it won first place in its class.


DiskSumo DiskSumo (transfer).png)

DiskSumo is a program to transfer disk images off a Commodore 64 over the RS-232 port via the XMODEM protocol at 300 baud.


SITU-PAN is a front-panel switches simulator for the Commodore 64. It displays 8 virtual LEDs and 8 virtual dip switches on the screen. The LEDs display the bit pattern at the current address in memory. Via the keyboard, the switches may be toggled, a new bit pattern written into memory, and the address advanced. In this way, machine code programs may be entered into memory, and then run. SITU-PAN was written during RetroChallenge 2015/07 for the purposes of entering SITU-MON into an emulated C64.


The common thread of the above 3 architectures is that they all have a 6502 processor. Here is MOS Technologies' 1976 manual for the 650x series (PDF). There are also a couple of C compilers that target 6502, famously cc65, but also scc6502, a Small-C for 6502 and LUnix.

Some things Cat's Eye Technologies has done have been 6502-specific but not as specific to any one architecture.



SITU-MON is a machine language monitor for 6502-based systems, written (by hand!) during RetroChallenge 2015/07 and entered into an emulated Commodore 64 using SITU-PAN.



SITU-SOL is a vaguely Forth-like language which was designed and implemented (by hand!) during RetroChallenge 2015/07, and entered into an emulated Commodore 64 using SITU-MON.


SixtyPical program SixtyPical output

SixtyPical is a very low-level programming language, similar to 6502 assembly, which defines a set of static analyses via type-checking and abstract interpretation (liveness analysis of variables, i.e. memory locations.)

Amiga 500

Rundown: The CPU was a Motorola 68000, but this could be upgraded to a 68020, etc (and needed to be if you wanted memory protection). It had many custom chips with colourful names such as "Angus" and "Paula".

AmigaDOS 1.3 (with a manual online here) was the version of Amiga's operating system which was current when the Amiga 500 was a really popular home computer — mostly for video games, but also for graphics and video processing, such as ray-tracing and animation.

AmigaBasic was the name of the "advanced" variant of BASIC that shipped with AmigaDOS. It was developed by Microsoft.

Emulators: There's an emulator for the Amiga 500 architecture (and several other models such as the Amiga 2000) called UAE. It's written in C++ and available under the GPL, and while the build of it for windows, WinUAE, seems basically stable, every decade or so the Unix version gets forked into a new incarnation. A few years ago, E-UAE was the thing to use, but now it's apparently FS-UAE.

IBM PC compatible

Rundown: The CPU was a 8086, or 80286, 80386, 80486... or just "X86". Or "IA-32". Until you get to the 64-bit architectures — but that's not retrocomputing anymore. (yet.)

For any Cat's Eye Technologies project that claims to run on an "IBM PC compatible," probably a 486 with a BIOS, keyboard, and basic VGA is the minimum to make it worthwhile.

Some modern PC's can handle running that sort of legacy setup, but most don't. Or at least, I wouldn't risk it anymore. Better to run it under an emulator.

For emulators, specifications, and other resources, see Blurry Memories of DOS Programming.



BefOS is an "operating system" for this architecture. But it's incomplete and terrible. Generally you would instead be running...


Rundown: By Microsoft. There's a good free clone of it called FreeDOS which is written in (I'm guessing) C99 and assembler and distributed under the GPL. Can run .BAT files. And x86 machine code .COM files. DJGPP is a port of GCC to MS-DOS. Here is a random MS-DOS game programming link.

I wouldn't assume any modern Windows installation can handle running MS-DOS programs directly anymore. Better to run it under an emulator.

Emulators: there are a few recommended choices here:

For other emulators, specifications, and other resources, see Blurry Memories of DOS Programming.

Any Cat's Eye Technologies project which claims to need MS-DOS only needs something which emulates MS-DOS sufficiently well. This includes both the standard DOS INT 21h handler, and the standard DOS utilies like, for example, TYPE and DEBUG.COM.

There are only two projects of Cat's Eye Technologies' that require MS-DOS (or a compatible operating system) anymore, and these dependencies are only partial:


An example session with Shelta

Shelta is an extremely minimal Forth-like language with barely any semantics; it relies on inline machine code to write anything resembling an actual program in it. In the spirit of compilers for languages such as FALSE and brainfuck, a Shelta-to-8086 compiler was implemented (with help from Ben Olmstead) in less than 512 bytes of 80286 machine code. What's more, it's also been bootstrapped — that is to say, a Shelta compiler was written in Shelta, which was compiled with the original compiler, and then compiled again with the resulting compiler, producing a wholly self-hosted executable!

Shelta can be built under any system with a NASM assembler, but requires DOS to run, and the executables it produces also run only under DOS (unless you bypass that part of the stdlib).


An example session with ILLGOL

ILLGOL is an illness disguised as a programming language empire.

ILLGOL can be built and run on any system with a decent ANSI C compiler, such as gcc, but the executables it produces will only run under DOS.


The Z80 is another 8-bit CPU that was popular.

Here are some links:



Mildred is a 4MHz Z80-based homebrew computer with 40K of memory (8K EEPROM, 32K SRAM.)


It's actually a virtual machine designed by Infocom so they could port their interactive fiction onto a multitude of home computers.

Here are its specifications documents.

Emulators: Zplet is a Java application which emulates a Z-Machine. There are other, much better emulators, such as Frotz.

The Never-Ending Maze

The Never-Ending Maze

The Never-Ending Maze is an Infocom-style text adventure game based on the defining cultural phenomenon of a generation. (Possibly.)

TODO: other things that can redirect here: emulator, BASIC, vintage