Richard B. Johnson wrote:
> Well! We went over this two years ago. The problem is that segment
> overrides are specifically excluded for the LODS, LODSB, LODSW, and
> LODSD instructions on the '486. (Page 16-203, Intel Rag called
> Intel 486 Microprocessor Family, Programmer's Reference Manual).
http://developer.intel.com/design/pentium/manuals/24319101.pdf
Intel Architecture Software Developer's Manual, Volume 2: Instruction Set
Reference Manual, page 3-275:
"LODS/LODSB/LODSW/LODSD-Load String [...] Loads a byte, word, or doubleword
from the source operand into the AL, AX, or EAX register, respectively.
The source operand is a memory location, the address of which is read from
the DS:EDI [sic!] or the DS:SI registers (depending on the address-size
attribute of the instruction, 32 or 16, respectively). The DS segment may
be overridden with a segment override prefix."
While they're visibly not above typos, I'd be surprised if they mis-stated
anything as crucial as the support of segment overrides.
> The sreg field in the LODxx instruction does not exist! (page E-9).
(Page B-10 in the manual above.) Correct, but a segment override is
a _prefix_ (B-17, B-18), not part of the instruction's opcode per se.
> There is absolutely no reason whatsoever to use segment-overides in
> any of the code.
In 16 bit code ? Well, theoretically not, but I'm not sure turning
mov ax,es:[si]
mov bx,ss:[si+2]
mov cx,[si+4]
into (assuming CS == DS, and that ES, SS are a non-trivial distance
away from each other and from DS)
mov ds,[extra_segment]
mov ax,[si]
push cs ; there's no mov es,cs, and we probably
pop ds ; don't want to sacrifice a register to
mov ds,[stack_segment] ; avoid touching the stack
mov bx,[si+2]
push cs
pop ds
mov cx,[si+4]
makes the code more readable or more correct.
- Werner
-- _________________________________________________________________________ / Werner Almesberger, ICA, EPFL, CH werner.almesberger@ica.epfl.ch / /_IN_N_032__Tel_+41_21_693_6621__Fax_+41_21_693_6610_____________________/- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.rutgers.edu Please read the FAQ at http://www.tux.org/lkml/
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