db 66h?

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randomdude

Joined: 01 Jun 2012
Posts: 83

randomdude 19 Feb 2014, 18:32

i see in some hex edited files: 90h (NOPs) and 66h

what is the instruction 66h? ida doesnt translate it

19 Feb 2014, 18:32

shutdownall

Joined: 02 Apr 2010
Posts: 517
Location: Munich

shutdownall 19 Feb 2014, 20:44

66h is an operand size override (16/32 bit) to change default operand size for single instructions.

http://pdos.csail.mit.edu/6.893/2009/readings/i386/s17_01.htm

19 Feb 2014, 20:44

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20344
Location: In your JS exploiting you and your system

revolution 19 Feb 2014, 21:10

AMD used to promote alignment code that was db 0x66,0x90 with varying amounts of the 0x66's from zero to three times.

It is basically a do-nothing space filler when used in that combination.

19 Feb 2014, 21:10

shutdownall

Joined: 02 Apr 2010
Posts: 517
Location: Munich

shutdownall 20 Feb 2014, 09:58

Yes - it is simply a 16-bit or 32-bit NOP. Laughing

20 Feb 2014, 09:58

randomdude

Joined: 01 Jun 2012
Posts: 83

randomdude 20 Feb 2014, 18:39

ok thx

20 Feb 2014, 18:39

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