Nibbler 4 Bit CPU (opens in new tab)

(bigmessowires.com)

87 pointskephra10y ago19 comments

19 comments

Nibbler also lacks any address registers, which means it can’t support any form of indirect addressing, nor a hardware-controlled stack. All memory references must use absolute addresses.

Wow!

kens10y ago

The Nibbler is pretty cool. Lacking indirect addressing and a stack isn't that strange. The IBM 1401 mainframe (1959) for instance doesn't have indirect addressing or a stack. [] Self-modifying code was used to get around this. E.g. you store the return address into a jump instruction at the end of the subroutine. However, the big difference with Nibbler is it's not a von Neumann architecture, so you can't modify the code.

[] Indirect addressing was an optional feature on the IBM 1401 for an additional monthly fee.

On the topic of processors made from 74xx TTL, I'll point out the Datapoint 2200 desktop computer. This had a board of TTL chips equivalent to the 8008 microprocessor. The 8008 was actually designed to replace this TTL-based processor. Trivia: the x86 is little-endian because when building a serial computer from TTL it's easier to use little-endian. https://en.wikipedia.org/wiki/Datapoint_2200

The Nibbler is different from the PDP, Datapoint, etc in that the Nibbler has very, very few chips; almost all the logic is done in microcode. This is the same trick used to build a "single-chip" processor from the Four Phase AL1. https://en.wikipedia.org/wiki/Four-Phase_Systems

PythonicAlpha10y ago

Yes, it is a very limited design, but the thought, that it was implemented just using (very few!) standard 74xxx TTL logic is really amazing.

And still it runs with 1MHz, the same speed of my first computer ;) with a dedicated integrated CPU.

makomk10y ago

It's implemented using a few standard 74xxx TTL logic chips and a 256x16 microcode ROM. It's important not to ignore the microcode ROM because that's where all the smarts that turn it from an ALU unit and a bunch of registers and miscellaneous logic into a CPU that can actually execute instructions. All of the instruction decoding and the sequencing of all the operations that make up a CPU cycle is done directly by the microcode ROM.

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ChuckMcM10y ago

I have a 12 bit CPU at home that is implemented out of standard 74xxx TTL chips, its called a PDP-8/e :-). Not to take away from the Nibbler design, its great, but using TTL logic to build CPUs was a thing for a pretty long period of time (computer wise), probably 10 years from 1971 - 1981. 1981 was the year IBM introduced the IBM PC and a lot of people realized that bespoke CPU architectures were a luxury they could no longer afford.

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