25. Alternate Encodings for Instructions

• To shorten program code, Intel created alternate (shorter) encodings of some very commonly used instructions.

• For example, x86 provides a single byte opcode for

      add al, constant    ; one-byte opcode and no MOD-REG-R/M byte
      add eax, constant   ; one-byte opcode and no MOD-REG-R/M byte
  

  the opcodes are 04h and 05h, respectively. Also,

• These instructions are one byte shorter than their standard ADD immediate counterparts.

• Note that

      add ax, constant   ; operand size prefix byte + one-byte opcode, no MOD-REG-R/M byte
  

  requires an operand size prefix just as a standard ADD AX, constant instruction, yet is still one byte shorter than the corresponding standard version of ADD immediate.

• Any decent assembler will automatically choose the shortest possible instruction when translating program into machine code.

• Intel only provides alternate encodings only for the accumulator registers AL, AX, EAX.

• This is a good reason to use accumulator registers if you have a choice

  • (also a good reason to take some time and study encodings of the x86 instructions.)