CODEWITHSUNDEEP
assemblyx86-64machine-codeinstructionsinstruction-encoding
user16566250
user16566250

Reputation:

Encoding 8-bit operand size? Is there a prefix for that like for 16-bit?

In instruction encoding Default sizes are:

operand size is 32 bit
address size is 64 bit

We can use the legacy prefix:

0x66 – Operand-size override prefix

to make operand size 16. What if I want to make it 8 bits not 16?

Upvotes: 0

Views: 371

Answers (1)

Nate Eldredge
Nate Eldredge

Reputation: 57922

What if I want to make it 8 bits not 16?

You can't do this with prefixes. Instructions that support an 8-bit operand size do so with an entirely separate opcode, not with an override prefix.

For instance:

   0:   01 d9                   addl    %ebx,%ecx
   2:   66 01 d9                addw    %bx,%cx
   5:   00 d9                   addb    %bl,%cl

The 32-bit add is opcode 01, with a mod/rm byte of d9. The 16-bit add is identical but with the 66 operand size prefix. However the 8-bit add is opcode 00 instead.

The explanation for this is historical. The 16-bit 8086 CPU supported 8-bit and 16-bit operands, and used separate opcodes for the two: 00 for addb and 01 for addw. (This is still what you get when you run a modern chip in real mode, like in a boot sector.) The 32-bit 80386 wanted to add 32-bit operands while still supporting both 8 and 16, but there was no room for so many more opcodes, so for 32-bit protected mode they made all the formerly 16-bit instructions act as 32-bit instead, with an override available to go back to 16 bit, and they left the 8-bit instructions alone. (In real mode the operand size override has the opposite effect: 01 is addw and 66 01 is addl.)

Upvotes: 3

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