Notes from a scratchbuilt 8-bit computer

8 Bit Spaghetti documents Kyle's project to scratchbuild an 8-bit transistor-transistor logic computer. He's embarked on the project to teach himself more about how computers work. Judging from the images, this thing is pretty much the definition of "glorious hairball." Reading Kyle's notes is a potted textbook on how computers work at the fundamental level

I set out to learn as much as I could about how to build an 8-bit computer and soon found that looking back to the invention of the computer was the way to go. I emailed the creator of the Magic-1 computer to get some advice on where to start. He pointed me to a textbook from the late 70′s: Digital Computer Electronics by Albert Malvino. Without that book this entire project would not have been possible. If you are looking to build your own 8-bit computer I highly recommend purchasing this book. It starts at the bare logic level and moves on up to registers, then counters, then the actual structure and operation of a simple-as-possible computer. My computer that I’m building right now is based on the SAP-1 architecture with some modifications that add needed functionality.
This project would also not be possible without my mentor whose expertise with electrical engineering has proven extremely invaluable in working on my project. I am extremely grateful that he has the patience and generosity to help me with my computer.

8 Bit Spaghetti (via Make)

I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.

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tylerkaraszewski

This is essentially what you do in the “Computer Organization” class that Computer Science and Computer Engineering majors have to take. As a former CS student, it is both one of the more challenging and more interesting courses in the curriculum.
nixiebunny

I’m always amazed when people try to build such a complex circuit on those solderless breadboards. Maintenance looks hopeless, especially when it’s such a hairball.

This is what Wire-Wrap® was made for.

However, you can’t buy the boards and sockets cheaply these days except as surplus junk, which gives you the added task of unwrapping the previous circuit.
- James B
  
  When I was in engineering school, one of the developers of the original IBM PC gave a talk to our class. He put a slide on the overhead projector showing a photograph of the prototype he had worked on. It was about an inch deep of dense Wire-Wrap®. I think you would have liked it.
http://ae4rv.com/ royaltrux

Hats off, amazing. Frankly, the inside of a Cray 1 doesn’t look much different except for wire color.
http://gspirits.com/ Zod

Reminds me of my first computer…wire-wrapped instead of bread-board…an RCA Cosmac Elf Complete with 4 display Nixie tubes and a 16 key hex keyboard. I upgraded it with an S-100 buss that I daisy chained to half a dozen add-ons!
- http://ae4rv.com/ royaltrux
  
  Ha! You’re old! My first computer was a Commodore 64. Oh yeah, I’m pretty old, too.
- Culturedropout
  
  Nixie tubes? But… those don’t display hex. Are you sure you’re not thinking of the dot-matrix LED displays most people used?
  - http://gspirits.com/ Zod
    
    http://www.tubeclockdb.com/data%20sheets/B8971.pdf
    I had a whole box of these thing back in the early ’70s! Long gone now…
  - kjs3
    
    He’s probably thinking of a Numitron or glass envelope VFD type display. Looks like a Nixie, but somewhat different operating principle.