One Bit, One Instruction Discrete CPU

There is a certain benefit to being an early adopter. If you were around when Unix or MSDOS had a handful of commands, it wasn’t hard to learn. Then you learn new things as they come along. If you started learning Linux or Windows today, there’s a huge number of details you have to tackle. You have the same problem trying to learn CPU design. Grappling with the design of a 16-bit CPU with a straightforward data path is hard enough. Throw in modern superscalar execution, pipelining, multiple levels of microcode, speculative execution, and all the other features modern processors have and you’ll quickly find yourself lost in the details.

[Michai Ramakers] wanted to build an educational CPU and he took a novel approach. The transistor CPU uses only one instruction and operates on one bit at a time. Naturally, this leads to a small data path, which is a good thing if you’re only using discrete transistors. His website is a ground-up tutorial in building and using the tiny computer.

The programming of the device is a bit odd. Each instruction is 32 bits wide (the CPU has a Harvard-like architecture where data and instruction memory is different). The instructions have only two parts. One is a data memory address and the second is a program address. The CPU inverts the value at the data address and then either loads the next instruction or the one specified by the second part of the instruction, depending on the value of the bit after inversion.

Each instruction is effectively an “invert and jump if result is zero.” Most one-instruction CPUs use a transfer architecture or a logical/mathematical function and conditional jump (usually subtract or nor). This design fits in the latter category.

We’ve covered many one instruction computers in the past. There’s a wide variety in how they work. We’ve even seen another 1-bit machine. I wouldn’t suggest building it out of transistors, but I once did my own take on a single instruction 32-bit CPU that programmed in Forth, supported interrupts, and could run pretty much the same kind of tasks any modern CPU could do.

Filed under: computer hacks