A New Forty Year Old Video Game

Defender on a Commodore PET (hardware 1981, software 2020); Williams Defender (1981 Arcade Version)

In the past I participated in an annual weekend game programming jam here in Toronto called TOJam. An amazing event where incredible gamers, coders, artists, etc create video games from scratch over a long-weekend. This inspired me this weekend to go 8-bit “retro” and complete a version of my favourite arcade game, Williams Defender (released in 1981) on my original Commodore PET home computer (also from 1981).

I am always intrigued by the unsung behind-the-scenes technology and people. There are many famous people from that 8-bit computing era (notably Bill Gates and Steve Jobs), but many others were involved as well. Commodore was founded as a typewriter repair shop by Jack Tramiel, and over the years branched into office furniture and calculators. Recognizing that home computers were the next calculator, Jack/Commodore bought Chuck Peddle’s MOS Technology processor company and Chuck designed the Commodore PET. The PET is one of the original home computers, and was released in 1977 at the same time as the Jobs/Wozniak Apple II and Radio Shack TRS-80; people refer to the PET, Apple II and TRS-80 machines as the 1977 Trinity. The PET featured an 8-bit 6502 processor, monochrome screen, had a fixed “PETSCII” character set, and a fully-loaded machine came with 32 kilobytes of memory.

Chuck Peddle’s 6502 processor was the heart of the 8-bit computer era. Super-functional (although no multiplication/division) and dirt-cheap ($25), it became the workhorse chip inside most home computers, arcade machines and home video game machines of the late 1970s and well into the 1980s. The 6502 was used in the Apple II, Commodore 64 (and the earlier Commodore PET described above), the original Atari 2600 video game system and the even original Nintendo NES.

Defender is my favourite arcade video game, and its programmer Eugene Jarvis is another one of my behind-the-scenes heroes. He was a pinball machine designer working for Williams, saw the success of Space Invaders in arcades and convinced Williams to expand into the video arcade game business. His first video game, Defender, became a fixture alongside PacMan and Donkey Kong in arcades everywhere. Released in 1981, the game was ahead of its time on many fronts: a 16-colour giant screen, and two processors controlling the graphics and sound. It was one of the first “side scrollers” (later popularized by Super Mario Brothers), had more control buttons than was typical for the day, and the graphics and sound were best-in-class.

My challenge was to try to jam as much as possible from the original arcade game into the constrained PET hardware, and to do it as quickly as possible. While from the same 8-bit era, the PET is far more limited than the custom Defender hardware. The PET was built on 1977 hardware, and Defender on 1981 hardware — two full Moore’s Law 18-month double-the-transistor-count cycles. The PET has monochrome fixed character graphics vs Defender’s pixel-level 16-colour control, single vs dual processors, 40x25 character display vs 292×240 pixels and a keyboard only / no joystick.

The result? Well you have to squint a bit but it’s definitely playable— my PET game uses arrows (↑) as the humans on the planet, and pi symbols (π) as the evil aliens flying around seeking to grab the humans and turn them into mutants. Q, A and Z keys on the built-in PET keyboard replace the joystick.

You can try out Defender 2020 for Commodore PET in an online simulator. (Thanks to Norbert Landsteiner for hosting it on his retro PET website!)

If you want a more complete experience, you can download the Commodore VICE emulator (PC/Mac), download the Defender game here and drag the game into the emulator to get it going.

And of course if you want the full experience, you can try it on your own PET if you happen to have one kicking around. Thanks to members of the Toronto PET Users Group (TPUG) for testing it on a few different machines for me.