This is neat! I had some thoughts recently about creating something similar as well.<p>For a simple but still practical instruction set, you can probably sanely get away with about two dozen or so different instructions, but at the expense of having larger programs (by number of instructions) to accomplish the same tasks compared to a more featureful instruction set.<p>Here's a fairly simple set that might be a reasonable place to start...<p>ALU: and, or, xor, complement, shift left/right, rotate left/right, add, subtract, multiply, divide, compare (or just use subtract and set flags for this one)<p>Memory: move (copy between registers), load and store (between memory and a register), maybe also push and pop for sanity if you want to provide explicit stack instructions.<p>Control flow: jump, branch (maybe depending on flags from ALU), call/return<p>It might also be helpful to look at the 8-bit AVR instruction set (quite popular for small microcontrollers, including what you might find on many Arduino boards). It contains about a hundred or so instructions, with all of the above list present in one form or another (although many are just aliases for some other instruction with the same opcode but otherwise fixed parameters). In the case of AVR, where code size is often the limiting constraint, having a larger vocabulary of instructions is useful for expressing a program in the smallest amount of space possible, but it certainly isn't necessary to have all of them just for completeness.