IO and Special Group

These are the instructions that don't fit into any of the other categories. I'll mention them for completeness, but I think we're going to have to revisit their implementation when we start to emulate the Space Invaders hardware.

Another use for NOP is to modify code. Let's say you need to modify the game's ROM code. You can't just delete opcodes you don't want because to modify all the CALL and JMP instructions (they would be wrong if any of the code moves). Using NOPs could allow you to get rid of code. Adding code is much more difficult! It can be done by finding space in the ROM somewhere and changing an instruction to JMP.

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