Transistor computers: the Megaprocessor, the MOnSter 6502, and one you build in your browser
A transistor computer is a processor built from individual transistors instead of a single chip, so its logic is big enough to see. Two famous modern builds: James Newman's Megaprocessor in Cambridge, a 16-bit processor made from discrete transistors and covered in LEDs, about 10 m long and 2 m tall, with 15,300 transistors in the processor and another 27,000 in its 256 bytes of RAM, finished in June 2016 and now at the Centre for Computing History; and the MOnSter 6502, a working transistor-scale replica of the MOS 6502 on a 12 by 15 inch board, with 3,218 MOSFETs and 1,019 resistors in its functional part plus LEDs on its control lines and registers, demonstrated running assembly, BASIC and Forth. Go one step further back in time and you get relays: Harry Porter's Relay Computer at Portland State University uses 415 identical relays for an 8-bit machine with an ALU, eight registers and a 16-bit address bus. These machines make a processor visible by making it huge. digiwleea does the same thing in a browser: you place the transistors yourself and watch every wire.
What the big builds are good at
Seeing a computer think. On the Megaprocessor and the MOnSter 6502 the LEDs show data moving and logic switching as programs run, and the Megaprocessor can be single-stepped cycle by cycle; its creator's stated aim was to build a processor big enough to walk inside. If you can visit one, do: the Megaprocessor is on show at the Centre for Computing History in Cambridge. (digiwleea is an independent project, not affiliated with any of these builds or their makers.) Relays and transistors as switches are compared in relay vs transistor.
Build your own transistor computer, in the browser
digiwleea lets you make the same journey without a soldering iron. You place NMOS and PMOS transistors as switches and wire them into a CMOS inverter and a CMOS NAND gate, save those as parts, then build adders, latches, registers, RAM, a program counter and a control unit, through a 40+ level course where every part is checked against its truth table, with an AI tutor (Wleea). The 8-bit CPU you finish with is far smaller than these machines, and its transistors are ideal switches rather than real devices, but every one of them is yours and every wire shows its value live. For the 6502's own transistors, see the Visual 6502 comparison; for a breadboard build from chips, Ben Eater's 8-bit computer.
The machines side by side
| Megaprocessor | MOnSter 6502 | Harry Porter's Relay Computer | digiwleea | |
|---|---|---|---|---|
| Switches | Discrete transistors | Discrete MOSFETs | 415 relays | Simulated NMOS and PMOS transistors |
| Word size | 16-bit | 8-bit (a 6502) | 8-bit data, 16-bit address | 8-bit |
| Size | About 10 m by 2 m | 12 by 15 inch board | 415 relays, 111 switches, 350 LEDs | A browser tab |
| Transistor count | 15,300 (processor) + 27,000 (RAM) | 3,218 MOSFETs + 1,019 resistors | None: relays | As many as you place |
| Built by | James Newman | The MOnSter 6502 team | Harry Porter | You, level by level |
No sign-up is needed to start: open the lab and place your first transistor, or read the free theory course. The account (and the AI tutor) are optional.
More comparisons: Books, courses and projects
See for yourself: build a logic gate from transistors in the simulator, then follow the path from sand to an 8-bit CPU.
Open the digiwleea lab →Frequently asked
What is a transistor computer?
A computer whose processor is built from individual transistors rather than an integrated circuit. Many computers of the late 1950s and the 1960s were built this way; modern builds such as the Megaprocessor and the MOnSter 6502 do it to make the logic visible.
How many transistors does the Megaprocessor have?
Its vital statistics page lists 15,300 transistors for the processor and 27,000 for the RAM, with many of them driving LEDs.
Can I build a computer from transistors myself?
Yes, in a simulator: in digiwleea you wire NMOS and PMOS transistors into gates, then build adders, registers, memory and an 8-bit CPU, all in the browser.