"Turning 930 logic gates into a working 6502 Apple-1 clone?
Like the venerable IBM 360/30, the Gigatron uses a form of microcode to elevate its spartan eight hardware instructions into a comfortable instruction set you can live with. Like the 8-bit IBM 360/30 CPU, the Gigatron normally pretends to be a 16-bitter using its microcoded instruction set. Unlike the IBM, though, the Gigatron's instruction set is not compatible with anything else.
Which sparked a discussion: 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.
Short answer: yes. In fact, you can make it into an entire Apple-1 clone without the use of a 6502.
[...]
Marcel wrote the Gigatron's 6502 microcode quickly (no bugs detected so far) but wrapping the Apple-1 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-1 including all its display hardware can be done in only 930 logic gates. Hmm!
The 6502 microcode takes up about 1K of ROM cells, and could fit inside a fast late-70s ROM. But the Gigatron cheats a bit by using a biggish 128K EPROM from the 1980s. That leaves enough space to tuck in the 6502/Apple-1 microcode next to all the other features of the latest Gigatron v5a ROM."
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Which sparked a discussion: 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.
Short answer: yes. In fact, you can make it into an entire Apple-1 clone without the use of a 6502.
[...]
Marcel wrote the Gigatron's 6502 microcode quickly (no bugs detected so far) but wrapping the Apple-1 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-1 including all its display hardware can be done in only 930 logic gates. Hmm!
The 6502 microcode takes up about 1K of ROM cells, and could fit inside a fast late-70s ROM. But the Gigatron cheats a bit by using a biggish 128K EPROM from the 1980s. That leaves enough space to tuck in the 6502/Apple-1 microcode next to all the other features of the latest Gigatron v5a ROM."