Recently I have designed yet another homebrew computer based on the 8bit 65c02 cpu, as with every other project I make, this is not motivated by logic or common sense, but rather what I felt would be a fun project to build and play around with. I’m currently dubbing it the MicroMicro.

This time I challenged myself with coming up with an extremely simple design for a computer, that was nevertheless fully standalone and “complete”, I’ve used only a relatively small number of integrated circuits, 12 in total: The cpu, bios rom, ram, and 9 other 74-series logic chips that together handle address decoding, video generation, sound, program saving/loading to tape, and reading the built-in keyboard and joystick ports.

The specs are as follows:

• CPU: 65c02@2/4/8Mhz
• Video: 256x240 black and white, via VGA
• Sound: 1-bit beeper
• RAM: 32KiB (24KiB for programs, and ~8Kib reserved for the frame buffer)
• OS: 8KiB BIOS, with text editor, program loading/saving and a (still undefined) programming language
• Storage: Tape Drive (headphone jack)
• Input: built-in 30% Mechanical Keyboard and 2 Atari-Style Joystick Ports
• Expansion: Port on the back of the machine.

Keyboard

On the KITTY I’ve used a 40-key keyboard, so for this little guy I’ve decided to go even smaller and use only 30 keys, placed in a regular 10x3 ortholinear grid. We have the 26 latin letters, space, enter and finally shift/alt modifiers that allow writing all of the different ascii numbers and symbols. It is challenge, but a fun challenge, to try and wrangle a comfortable layout out of something this limited (overall I feel I am on a good path to achieving this). Additionally it also makes the physical build a bit cheaper, which is always a plus!

Here I’m using a cheap ymdk dsa keycap kit and blue low profile redragon switches (the board can also be built using standard sized mx switches):

Software

As stated above, the BIOS will only be 8KiB in size, largely a challenge imposed on myself ;)

This code is still in very early development, it has a simple command line interface, as well as a wip text editor, it will also have a built-in programming language, though I am still undecided on which. Either it will be KitLang, the language I’ve begun developing for the KITTY, or some other equally minimal language, we only have ~4KiB of code available for our language, so the options are somewhat limited.

Of course, we also have an expansion port in addition to the base 8KiB BIOS, this can be used for additional ROM code that interfaces with the BIOS, adding in new commands or languages!

A kit?

Overall, this project is a decent candidate to be offered as a diy kit on a place like tindie, and I’ve even begun mocking up pages for a potential manual. The low number of parts could easily make this a sub-100€ kit, even including a potential pcb case, and every component is a easy to solder through hole part that is still in active production. Of course, as of writing, I’m still a student and the economy may not be looking so good in the near future due to <events>, so we’ll have to wait and see how things shape up. For now I will continue writing the bios and potentially coding or porting over some games, and spreading the word about this project.