Simple 8085 Computer

This project builds a compact 8-bit computer around an Intel 80C85, marking the processor’s 50th anniversary alongside that of Tiny BASIC. Its design aims to keep parts count, cost, and board complexity low: an 8 KB ROM and 8 KB RAM, a 4.9152 MHz clock, 2400-baud serial communication, and one 8-bit input and output port form the core system. The author says the computer can be assembled with six ICs, including basic logic, rather than relying on dedicated serial hardware or programmable logic for address decoding. A monitor program and Tiny BASIC reside in ROM, letting users interact with the machine and run programs.

The project demonstrates the computer by generating Mandelbrot sets in both Tiny BASIC and 8085 assembly. It also explores undocumented 8085 instructions: replacing 12 instructions in an assembly Mandelbrot program with the extended instructions ARHL and RDEL reduces execution time from 43.34 to 40 seconds. A second example uses self-modifying code to switch an instruction between left and right accumulator rotation, changing an LED’s direction according to the carry flag. The build is intended as a practical way to study the 8085’s extended instruction set and self-modifying programs, with hardware files, a bill of materials, monitor software, and example programs available for further experimentation.