15. Opcode Design Trade-offs

• Encoding operands is always a problem, because instructions have large number of operand combinations.

• For example, an x86 MOV instruction requires a two-byte opcode.

• However, Intel noticed that two instructions

      mov memory, eax   ; store register in a variable
      mov eax, memory   ; load register from a variable
  

  occur very frequently. These instructions store EAX register into memory and load EAX from memory.

• As a result, x86 provided a special one-byte versions of dedicated MOV instructions to reduce program clutter.

• Note that Intel did not remove the two-byte version of these instructions, but compiler or assembler would always emit the shorter of the two instructions.

• By doing so, Intel has made an important trade-off with the MOV instruction encoding:

  • giving up extra opcodes in order to provide a shorter version of the MOV sub-family.

• Intel used this trick all over the place to make decoding instructions shorter and easier.

• This decision dates back to 1978. Today's design could use extra bytes, but the cost of memory was high in 1978!