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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, and still other bits specify the register(s)and other arguments.
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.
