A homemade computer runs 6502 and Z80 software without either processor chip

Isetta runs software for two familiar old processors while containing neither processor. The homemade computer builds its CPU from ordinary logic chips and switches between 6502 and Z80 instructions through microcode. It has found a remarkably literal way to keep its options open.

The maker’s project page, posted by roelh, lists 42 integrated TTL circuits for the CPU and another nine for I/O. The machine runs ZX Spectrum games, several BASICs and SymbOS. Its published design excludes a microprocessor, microcontroller or FPGA from doing the central work.

The reason for borrowing instruction sets is practical. Inventing a new CPU usually leaves its maker responsible for the software too. Reproducing instructions existing programs understand gives the handmade hardware a library to try immediately. Building the whole restaurant is easier to justify when somebody has already written a menu.

That compatibility gives the soldering a familiar result: programs written for two different processors. The chip count becomes interesting because the computer can do recognizable things. A machine that runs a remembered game turns a circuit diagram into an experience somebody can compare, including all the ways the new implementation might still differ.

The November 6 microcode release makes the project easier to inspect below the program level. Its notes describe startup and work associated with video, keyboard, mouse and sound. That gives another builder something more useful than an impressive boot photo: a place to examine how all the small jobs are coordinated.

The demonstrations do not establish that every program for either processor runs unchanged. An instruction set is part of a computer’s environment, while software can also expect particular memory, graphics or timing behavior. The documented applications establish things the maker has demonstrated; a universal compatibility promise would require much more evidence than the project page supplies.

There is a kit route in the project notes, alongside hardware and software files. Before building it, read through the assembly material and check current availability before counting the weekend as booked. A soldering project can be wonderfully satisfying, but its idea of plug-and-play contains considerably more solder than the phrase normally admits.

The lasting appeal is being able to trace familiar behavior through a machine small enough to understand piece by piece. It gives an old program a new physical explanation, with plenty left for another builder to explore.

The design stores its programmable microcode in three flash chips. Updating those instructions is part of the architecture, which helps explain how the same board can support different processor instruction sets.

Both processors are absent. Class is in session.