[Juan] sent us his writeup of a microcomputer he built using an Arduino UNO (AVR ATmega328p) and some off-board SRAM. This one’s truly minimalistic.
Have a look at the schematics (PDF). There’s an Arduino, the SPI SRAM, some transistors for TV video output, and a PS/2 connector for the keyboard. That’s it, really. It’s easily built on a breadboard in a few minutes if you have the parts on hand. Flash the Dan64 operating system and virtual machine into the AVR and you’re good to go.
Now we’ve seen a few 6502-based retro computers around here lately that use a 6502 paired with a microcontroller for the interfacing, but they’ve all been bulky three-chip affairs. [Juan] wins the minimalism prize by using a 6502 virtual machine implemented in the AVR to reduce the parts count down to two chips for the whole shooting match.
Using a 6502 virtual machine was a crucial choice in the design, because there are 6502 cross compilers that will let you compile and debug code for the microcomputer on your macrocomputer and then load it into the micro to run. This makes developing for the micro less painful.
How does it load programs you ask? The old-fashioned way of course, using audio files. Although rather than using the Kansas City Standard as in days of yore, he encodes the data in short and long pulses of square waves. This might be less reliable, but it sure saves on external hardware.
What else does the Dan64’s OS do? It’s got a limited shell that includes a 6502 assembler and disassembler, so you can program the microcontroller on the fly in its native assembly language if you want. And of course, no microcontroller is complete without raw memory access through peek and poke.