whats the opcode difference?

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Teehee

Joined: 05 Aug 2009
Posts: 568
Location: Brazil

Teehee

[see image]
When/why compiler will choose FF/0 instead 40+rd if using a register?

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09 Jul 2010, 13:34

b1528932

Joined: 21 May 2010
Posts: 287

b1528932

in long mode 40+ is REX prefix, only way to encode inc/dec is by modrm opcode.

09 Jul 2010, 14:21

sinsi

Joined: 10 Aug 2007
Posts: 709
Location: Adelaide

sinsi

In long mode lots of opcodes referring to the high 8-bit registers are prefix bytes to use r8-r15.

>in long mode 40+ is REX prefix
yes, from what I see it is 40..4F

This is why you see in the 64-bit mode column the letters N.E. bad crash

09 Jul 2010, 15:01

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 17667
Location: In your JS exploiting you and your system

revolution

Teehee wrote:

[see image]
When/why compiler will choose FF/0 instead 40+rd if using a register?

In 32-bit code fasm will choose the single-byte variant.
In 64-bit code fasm has no choice and "chooses" the two-byte variant.

09 Jul 2010, 15:59

Teehee

Joined: 05 Aug 2009
Posts: 568
Location: Brazil

Teehee

hmm.. so is there many ways to do the same?

Manual says FE /0 is to r/m8, but when i look to inc al it shows me:

Code:

FEC0           inc al

what that C mean? from where it comes?

Last edited by Teehee on 10 Jul 2010, 00:11; edited 1 time in total

10 Jul 2010, 00:05

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 17667
Location: In your JS exploiting you and your system

revolution

Teehee wrote:

what that C mean? from where it comes?

Well it is explained in the nice AMD/Intels manuals. modREGr/m - the mod part must be 11b for a register operand.

10 Jul 2010, 00:11

Teehee

Joined: 05 Aug 2009
Posts: 568
Location: Brazil

Teehee

oh.. i didn't see yet.. that must happen when you don't use linear reading Smile

sorry to annoy you, uncle revolution. Embarassed

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Sorry if bad english.

10 Jul 2010, 00:15

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