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Many early computers encoded the [[Operation|operation]] and [[Addressing Mode|addressing mode]] as a distinct [[Byte|byte]] or [[Word|word]] called an [[OpCode|opcode]], followed by zero or more additional bytes for the arguments, where the number of additional bytes is implied by the opcode.
Other processors encode instructions as bit values packed with a instruction field. For example, specific bits within an instruction word specify the operation, other bits specify the [[Addressing mode|addressing mode]], and still other bits specify the register(s).
Thus, the length of each instruction may be variable (6502, x86_64) or fixed (ARM) -- a design decision which affects code density, execution speed, and [[Memory Prefetch|memory prefetch]] operations.
