Cloning a 6502 Apple-1 in just 930 logic gates

The C74-6502 is somewhat similar. A 6502 built using 74xx chips, that runs 20x faster than the 6502 (20Mhz vs 1MHz).<p>And correct enough that he drops it into both a VIC20, and a C64, and it works.<p><a href="https:&#x2F;&#x2F;c74project.com&#x2F;" rel="nofollow">https:&#x2F;&#x2F;c74project.com&#x2F;</a><p>About ~130 74xx chips vs the Gigatron&#x27;s 33, and SMT instead of through-hole.<p>Edit: A link to an old school web-ring of other homebrew (mostly) ttl logic CPUs: <a href="https:&#x2F;&#x2F;www.homebrewcpuring.org&#x2F;ringhome.html" rel="nofollow">https:&#x2F;&#x2F;www.homebrewcpuring.org&#x2F;ringhome.html</a>

&quot;It needs just 930 logic gates (packed into 33 standard 7400-series ICs) to create a computer that beat &#x27;complex&#x27; 1980s home computers like the VIC-20 in terms of both CPU power and graphics.&quot;<p>Given that 7400 chips existed at the time, why did no contemporary microcomputer go this route? Would it just have been cost prohibitive?

&gt;&quot;Turning 930 logic gates into a working 6502 Apple-1 clone? Like the venerable IBM 360&#x2F;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&#x2F;30 CPU, the Gigatron normally pretends to be a 16-bitter using its microcoded instruction set. Unlike the IBM, though, the Gigatron&#x27;s instruction set is not compatible with anything else.<p>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.<p>Short answer: yes. In fact, you can make it into an entire Apple-1 clone without the use of a 6502.<p>[...]<p>Marcel wrote the Gigatron&#x27;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&#x2F;Apple-1 mode to demonstrate how a compatible Apple-1 including all its display hardware can be done in only 930 logic gates. Hmm!<p>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&#x2F;Apple-1 microcode next to all the other features of the latest Gigatron v5a ROM.&quot;

&gt;&quot;Like the venerable IBM 360&#x2F;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&#x2F;30 CPU, the Gigatron normally pretends to be a 16-bitter using its microcoded instruction set.&quot;<p>Could someone elaborate on how exactly the IBM 360&#x2F;3 and Gigatron &quot;elevate&quot; their eight hardware instructions into a larger ISA via microcode?

What&#x27;s fascinating about this is that the home computer &quot;revolution&quot; could have started a few years sooner had people just realized they could make a CPU out of 33 74xxx series IC chips. These were relatively low cost by the early to mid 70s. It took a generation of skill to go back and see what could be done with just these logic building blocks.

I wonder how big the difference is between current logic gate performance vs. what we have available when the 6502 was &#x27;new&#x27;.

TL;DR: this is a very simple computer running an emulator. Cool project! (The approach is similar to the way many of FPGA applications are done these days.)

I still recall the shock discovering that Micro-soft BASIC in the Apple ][ did a linear search for the line number from, the beginning of the program, on each GOTO or GOSUB. It would have been super-easy to memoize the search result at the branch site, but Bill couldn&#x27;t be bothered.