The Gigatron TTL-Logic Home Computer Gets an Apple I Clone Mode — with No Microprocessor in Sight

A year of hacking has given the Gigatron the ability to act like an Apple I — despite having no microprocessor at all, let alone a 6502.

The Gigatron TTL computer can now run Apple I code — including Microchess. (📷: Oscar Vermeulen)

Oscar Vermeulen has written up a successful project to turn the Gigatron, a microcomputer built from just 37 7400-series TTL chips and with no microprocessor in sight, into a functional clone of the original Apple I — despite not having a MOS Technologies 6502 on the board.

Launched two years ago, Marcel van Kervinck and Walter Belgers' Gigatron TTL Color Computer - to give the board its full name — is a "what if" project to investigate the creation of an '80s-style home computer using only discrete logic. There's no microprocessor on board; instead, everything is done in 37 7400-series TTL chips — giving its central processing unit total of 930 logic gates.

"As it turns out, for me as a kid of the 80s, the Gigatron compares roughly to a VIC-20 in terms of performance and graphics. It is, in fact, quite a bit better," Vermeulen explains. "Which is impossible. If you could make something like a VIC-20 using 1971 technology, without a microprocessor, without a graphics chip, on a circuit board half the size, then - then - then - this is a revolution, a work of genius that just happened not to have happened back in the day. And now you finally get the Gigatron's achievement! This is, indeed, insane."

While the Gigatron is designed to run specially-written software, an effort has been completed to turn it into a clone of Steve Wozniak's Apple I design — a project which should, in theory, have been impossible, given that in direct-comparative terms the MOS Technologies 6502 microprocessor at its heart is roughly twice as complex as the TTL chips on the Gigatron.

The secret sauce: Microcode, which sits between the machine's actual eight instructions and the user-facing and considerably larger instruction set. "Could the microcode also contain a 6502-compatible instruction set? That would prove that a 6502-compatible system could be done with much, much less hardware, even back in the 70s," Vermeulen writes. "Short answer: yes. In fact, you can make it into an entire Apple I clone without the use of a 6502.

"Marcel wrote the Gigatron's 6502 microcode quickly (no bugs detected so far) but wrapping the Apple I around it took about a year. The machine has become dual-core: You either use its colourful native vCPU microcode to embarrass 1980s home computers, or you boot it into 6502/Apple-1 mode to demonstrate how a compatible Apple I including all its display hardware can be done in only 930 logic gates."

Vermeulen's full write-up, including a tour of the 6502/Apple I replica functionality, is available on the Obsolescence Guaranteed blog; more information on the Gigatron itself can be found on the official website, where the machine can be purchased in kit form for €159 (around $172.)

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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