David Johnson-Davies on the Technoblogy blog builds a machine-code monitor that you program from a hexadecimal keypad using a simplified instruction set. He writes:
It’s a good project for learning about the fundamentals of machine code, and will also appeal to people who like programming challenges. The simplified machine code, called MINIL, is designed to be easy to learn and understand. It’s similar to the Little Minion Computer used in some universities to teach students about machine code. The same method could be used to emulate other simple processors, such as the SC/MP, 6800, 8080, or 6502.
Once you type in the opcode the Tiny Machine-Code Monitor displays the assembler statement alongside it on the display, allowing you to check that it’s what you intended. Because you enter the opcodes in hexadecimal it avoids the need for a full alphanumeric keyboard, while giving you the advantage of seeing the assembler program on the display.
The project is based on the Microchip/Atmel ATtiny85, a small microcontroller with alot of capability (used in the first Adafruit Trinket and Gemma boards).
(Ed. nice to see work by a fellow 6800 era programmer)