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Index > Main > repe cmpsb "bug?"

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wht36



Joined: 18 Sep 2005
Posts: 106
wht36
While writing up a parser for commandline arguments I noted a curious
REPE CMSPB behaviour:

- if first letter don't match: ESI and EDI +1, NZ, ECX-1
- if first letter MATCHES but second letter don't match: same as above!!

For example:
Code:
        mov     esi,test1
        mov     edi,test2
        mov     ecx,4
.scan:  cmpsb
        loope .scan

test1   db 'abcd'
test2   db 'afgh'    


gives the same result as

Code:
        mov     esi,test1
        mov     edi,test2
        mov     ecx,4
.scan:  cmpsb
        loope .scan

test1   db 'abcd'
test2   db 'efgh'    


This bothers me because I want my commandline argument parser to parse partial arguments (so the user can use the option /extract and /e synonymously), and this unexpected behaviour of REPE CMPSB took me sometime to resolve.

Anyway, I wonder how many people are aware of this curious behaviour of REPE CMPSB.
Post 22 Dec 2006, 10:08
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17347
Location: In your JS exploiting you and your system
revolution
I think you had better check your DS and ES segment registers. Don't forget it is DS:ESI and ES:EDI that are used.
Post 22 Dec 2006, 10:34
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wht36



Joined: 18 Sep 2005
Posts: 106
wht36
Thanks, I think it's okay now. My mistake Wink
Post 22 Dec 2006, 10:42
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