every instruction set architecture has an encoding format and rules for how to encode every instruction, register, and memory address. it's usually the assemble

every instruction set architecture has an encoding format and rules for how to encode every instruction, register, and memory address. it's usually the assembler's job to ensure this is done correctly and to the spec but there's tons of room for undefined behavior by doing it yourself and using a prefix on an instruction that doesn't support it, or just encoding an opcode that doesn't exist, etc.