## NANDputer: Functionally Complete and Absolutely Necessary

One of the statements you read a lot when you’re learning about digital logic is that NAND gates possess functional completeness, which means that they can be combined to implement any Boolean function.

What is a NANDputer? it’s a computer made out of nothing but NAND gates of course! I dunno why, but I thought it’d be fun to make this. I first had to work out how various parts of a CPU would be made out of NANDs, did a bunch of tests and went to town.

The design took about 2 months to come up with and make. At the bottom of the post is a few statistics on gate usage and count of each type (2 input, 3 input, 4 input, etc). As I suspected, the quantity vs. gate input count follows a pretty steep curve, with most gates being 2 inputs, and the fewest being 13 input gates.

Everything on the design is made out of NAND gates, even the 7 segment decoding. The last PCB though has a few non-NAND gate chips like an NES PPU and a serial chip and stuff, but it’s just a peripheral board and is not part of the NANDputer proper. (Eventually I want to make a NAND UART and replace that peripheral board).

The basic architecture of the computer is actually fairly conventional. There’s an accumulator, instruction skipping (like on PIC) for decision making, a full ALU (and, add, or, xor, subtract, add with carry, subtract with borrow, set all bits, clear all bits, shifting), 8 bit registers, separate RAM/ROM areas (harvard arch), and bit set/clearing. There’s a 3 level stack, and even an interrupt!

While the CPU architecture is fairly conventional, the way it is implemented isn’t. I went with a bit-serial setup on here to save gates. The ALU for example is only 1 bit, with a “latching” carry so operations are performed a bit at a time on the 8 bit registers/memory. The program counter is also bit-serial, and on the first youtube video you can see the carry propagating during the incrementing of it.

Neat design and awesome build — way more exciting than the Galaxy Nexus IV Episode 2, or whatever it’s called.

Happy Friday!

*pun most definitely intended.

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