23. Encoding Sixteen Bit Operands

32-bit programs don't use 16-bit operands that often, but they do need them now and then.
To allow for 16-bit operands, Intel added prefix a 32-bit mode instruction with the operand size prefix byte with value 66h.
This prefix byte tells the CPU to operand on 16-bit data rather than 32-bit data.

There is nothing programmer has to do explicitly to put an operand size prefix byte in front of a 16-bit instruction:
- the assembler does this automatically as soon as 16-bit operand is found in the instruction.
However, keep in mind that whenever you use a 16-bit operand in a 32-bit program, the instruction is longer by one byte:
```
    Opcode     Instruction
    --------   ------------
    41h        INC ECX
    66h 41h    INC CX
```
Be careful about using 16-bit instructions if size (and to a lesser extent, speed) are important, because
1. instructions are longer, and
2. slower because of their effect on the instruction cache.

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