REPE and CMPS

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matias_beretta

Joined: 28 Dec 2007
Posts: 1

matias_beretta 28 Dec 2007, 22:34

Hello, thanks for reading my topic...

I was reading the FASM manual and found this:

repe cmpsb ;COMPARE [ES:DI] and [DS:SI] until CX = 0

but can i use this???????????:

repe cmps byte [ds:si], [ds:di]

bye Smile

_________________
Matías Beretta

28 Dec 2007, 22:34

edfed

Joined: 20 Feb 2006
Posts: 4324
Location: Now

edfed 28 Dec 2007, 22:37

if you want to know, you have to try...

28 Dec 2007, 22:37

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 28 Dec 2007, 22:42

AMD64 Architecture Programmer’s Manual Volume 3: General-Purpose and System Instructions wrote:

The forms of the CMPSx instruction with explicit operands address the first operand
at seg:[rSI]. The value of seg defaults to the DS segment, but may be overridden by a
segment prefix. These instructions always address the second operand at ES:[rDI]. ES
may not be overridden. The explicit operands serve only to specify the type (size) of
the values being compared and the segment used by the first operand.
The no-operands forms of the instruction use the DS:[rSI] and ES:[rDI] registers to
point to the values to be compared. The mnemonic determines the size of the
operands.

So you can modify the segment register of the destination operand but the source is always ES.

Basicaly, when you override the default (you don't use [DS:SI] as dest), an extra byte is added before the instruction (the segment override prefix).

Code:

rep cmps byte [fs:si], byte[es:di]
; is equivalent to

rep fs cmps byte [ds:si], byte[es:di]

; also equivalent to

rep fs cmpsb

You don't need to bother about the last code, is just to show the encoding.

28 Dec 2007, 22:42

DOS386

Joined: 08 Dec 2006
Posts: 1898

DOS386 29 Dec 2007, 04:19

matias_beretta wrote:

Hello, thanks for reading my topic...
I was reading the FASM manual and found this:
repe cmpsb ;COMPARE [ES:DI] and [DS:SI] until CX = 0

The time has come that you study the Intel 80386 manual:

http://board.flatassembler.net/topic.php?t=3164

Of course you also can look into an 8086 manual, which is even smaller and easier to understand, but don't search stuff like EAX inside Laughing

Quote:

AMD64 Architecture Programmer’s Manual Volume 3: General-Purpose and System Instructions wrote:

Yes, but it's really not for beginners Rolling Eyes

_________________
Bug Nr.: 12345

Title: Hello World program compiles to 100 KB !!!

Status: Closed: NOT a Bug

29 Dec 2007, 04:19

rugxulo

Joined: 09 Aug 2005
Posts: 2341
Location: Usono (aka, USA)

rugxulo 25 Jan 2008, 07:19

Apparently, for string instructions, you can never override the destination's segment (always uses ES).

See here for a bit of info about why FASM doesn't complain.

25 Jan 2008, 07:19

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