Mistakes

Don't design opcodes in your head

You WILL screw up and your binary encodings WILL overlap. You definitely need some tool to design instruction opcodes for anything more complicated than just 1-byte instructions.

Turned out this mistake propogates further when implementing assembler.

Load/store instructions MUST be able to use register as address

I have no idea how we didn't notice this mistake. I guess it is not too bad for a CPU with 16 bytes of memory, but for something bigger it is unacceptable.

Jump instructions need more love

Jumping 3 instructions back and 7 forward is too limiting. Seriously, that's stupid.

Also, in my current design, jumping offset of 0 will move CPU to an infinite loop.

Implementing assembler with regex is not fancy

It is probably easier to implement than writing proper lexer/parser, but error reporting can be weird.

Two registers are really not enough

Just imagine, you are trying to write a loop, you use R1 as your index variable and R0 for your loop condition. Now you have already ran out of registers and you didn't even start writing loop body!