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Index > Main > Pentium ALU doesn't like ESP :)

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AlexP



Joined: 14 Nov 2007
Posts: 561
Location: Out the window. Yes, that one.
AlexP
Maybe most people know this, but I tried using ESP as a memory operand *4 for a table lookup, and I get a FASM error... Apparently you can't use ESP in this manner, thought I'd point out that funny little thing in my day.
Post 28 Mar 2008, 21:22
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17331
Location: In your JS exploiting you and your system
revolution
Yes, ESP cannot be used as an index register. It doesn't make sense to use your stack pointer as an index offest anyway so why do you want it?
Post 29 Mar 2008, 02:59
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Xorpd!



Joined: 21 Dec 2006
Posts: 161
Xorpd!
Yes, the SIB byte encodings that would otherwise index ESP, xx100xxx, instead mean 'no index'. You can encode R12 as an index, however because the processor combines the REX byte with the SIB byte to determine the index register. Encoding of R13 as a base is somewhat awkward because of what happens to its little brother RBP. As a consequence of their exceptional encodings, I simply avoid using RSP, RBP, R12, and R13 for addressing.
Post 30 Mar 2008, 23:47
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