Pepe's PCPU-NANO1 Is a One-Bit Processor Made From a Mere Five Transistors
Inspired by Motorola's MC14500B Industrial Control Unit, this two-instruction one-bit CPU comes complete with its own operating system.
Mononymous developer Pepe has built just about the simplest processor you're likely to find, inspired by the Motorola MC14500B: the one-bit, five-transistor PCPU-NANO1 — and, for good measure, developed an operating system compatible with a protoboard implementation.
"[The PCPU-NANO1 is a] minimalistic one-bit CPU made from NPN transistors and resistors," Pepe explains of the device, which is built using five individual transistors. "It requires an external program counter, registers, and clock."
It might be hard to see how a five-transistor device could be termed a "central processing unit," but Pepe's project takes its inspiration from a commercial product: Motorola's MC14500B Industrial Control Unit, a similarly one-bit processor released by the company in 1977 and which wasn't discontinued until 1995.
Where the M14500B supported 16 instructions, though, the PCPU-NANO1 only supports two: NOP, a null no-operation instruction; and NADJZ, which NANDs input and data and jumps if the result is zero. It's limited, then, but that hasn't stopped Pepe writing an operating system for the processor — using an Arduino UNO R4 Wi-Fi to replace the other missing parts of the system. "The OS uses Arduino as memory," Pepe explains, "and PCPU-NANO1 as the core for calculations.
The design of the PCPU-NANO1 has been released on GitHub, with the Nano-OS software in a separate repository — both made available under the permissive MIT license.