"Frank Kotler" <spamtrap@[EMAIL PROTECTED]
> wrote in message
news:JN2Ej.7276$rR1.5825@[EMAIL PROTECTED]
> rhyde@[EMAIL PROTECTED]
wrote:
> > One curious thing I've noticed is that the presence/absence of a 0x66
> > size prefix byte, for 16-bit only instructions, is all over the map.
> > For example, consider the following two instructions:
> >
> > mov ds, ax
> > mov ax, ds
> >
> > Clearly, there are only 16-bit versions of these two instructions.
>
> True, "mov ds, ax" and "mov ds, eax" - with and without the prefix, IOW
> - do exactly the same thing. "mov ax, ds" and "mov eax, ds" are *not*
> the same, and the prefix is relevant.
>
> > Some assemblers *always* put a 0x66 size prefix byte in front of the
> > encodings, some never do, and at least one (MASM) puts size prefix
> > bytes before one but not the other.
>
> That's interesting... Nasm went round-and-round on the issue a while
> back. Referring to segreg as a *destination*, Intel said "most
> assemblers" emit the size prefix, and you could use "mov ds, eax"
> (absurd, on the face of it) to avoid it. It sounded like they were
> saying you should do it, but if you read closer, they almost said that
> those "most assemblers" were "doing it wrong" to emit the useless prefix
> - or making us write something that *looks* like a size-mismatch to
> avoid it. We took an informal survey, and Masm was about the only
> assembler that *was* doing it, at that time (Nasm used to, but stopped).
> Sounds like Masm has stopped, too. Who's doing "both"?
>
Wow... That's not what I got from their doc's. What I got was, "If you
use
the 16-bit form of mov to a segment register in 32-bit mode, instead of
using the 32-bit form of mov to a segment register in 32-bit mode, some
assemblers will generate an unecessary 0x66 operand size override prefix
due
to the 16-bit segment register in the instruction." Since Randall was
referring to 16-bit instructions, I thought he was referring to 16-bit
mode
too...
This is what I would expect an assembler to do:
BITS 16
mov ds, ax ; no 0x66
mov ds, eax ; yes 0x66, but unneeded
mov ax, ds ; no 0x66
mov eax, ds ; yes 0x66 - required because of cpu dependent 32-bit
operation
BITS 32
mov ds, ax ; yes 0x66, but unneeded
mov ds, eax ; no 0x66
mov ax, ds ; yes 0x66 - required to ensure 16-bit only operation
mov eax, ds ; no 0x66
The reason I expect that is because the address and operand size prefixes
can be used to execute 16-bit code in a 32-bit segment and vice-versa.
So,
I expect the assembler to place the overrides properly, even if
unecessary.
But, the override prefixes are required to ensure the proper operation for
ax/eax.
Rod Pemberton
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