Not content with your new, 1.4-billion-transistor Ivy Bridge processor? Maybe it runs too hot, or lacks the polygon-pushing powers that you require? Well, I’ve got just the thing for you: How about you augment it with a DIY 8-bit CPU?
One Kyle Hovey has decided to build his own 8-bit CPU from scratch, out of breadboard, NVRAM chips (non-volatile RAM), surface-mount transistors, and lots and lots of wire. This in itself isn’t particularly spectacular — electronic engineering students have been doing it since the ’70s — but Hovey has also decided to chronicle the entire build, so that you can follow along.
Hovey’s 8-bit ALU (arithmetic logic unit) is constructed completely out of transistor-transistor logic (TTL), as are the registers. The microinstructions (the op code, the ISA) that actually control the ALU are stored in the NVRAM. A ring counter, which continuously loops through six different outputs, drives the fetch/decode/execute cycle.
Output-wise, Hovey cheats a little — he uses an Arduino to convert binary to BCD, a task that would be tricky with TTL — but considering he decided to use three awesome Soviet-era IV-9 Numitron tubes for the computer’s display (pictured below), we forgive him. An Arduino is also used to upload the microinstructions to the NVRAM (in the picture above, the NVRAM chips are just about visible on the right hand side of the breadboard).
Ultimately, though, if you choose to follow in Hovey’s footsteps, you’re going to have to be pretty savvy with TTL and the pinouts of ancient chips — Hovey doesn’t provide detailed wiring schematics, after all. For this project, Hovey’s bibles were Digital Computer Electronics (first published in 1977), which introduces the SAP1 instruction set architecture used by Hovey, and the TTL Cookbook (first published in 1972).
The last step, according to Hovey’s blog, is to design a steampunk-style case for his 8-bit computer. If it even comes close to rivaling Datamancer’s seminal steampunk laptop, it will be a job well done.
