[solved] Invalid compilation: OR ecx, 0x10000

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GrayFace

Joined: 07 Mar 2015
Posts: 4

GrayFace 07 Mar 2015, 04:17

The input file consists on 1 line:
or ecx, 0x10000
The output I get is (when decompiled):
or cx, 0
(2 bytes of garbage after that)

What is going on? Maybe FASM dafults to some wacko DOS model of 2-byte registers?
In any case it's going crazy, writing 2 high-order bytes of the operand number as new instruction. I thought a popular assembler wouldn't break in one of the first tests I run.

P.S. Windows, 32 bit, fasm versions tested: 1.71.22 and 1.71.37

07 Mar 2015, 04:17

GrayFace

Joined: 07 Mar 2015
Posts: 4

GrayFace 07 Mar 2015, 05:09

And
or ax, 0x8000
turns into
or eax, 0x????8000
Fantastic!

07 Mar 2015, 05:09

GrayFace

Joined: 07 Mar 2015
Posts: 4

GrayFace 07 Mar 2015, 05:31

Appending use32 at the beginning have solved the problem.
So, 32/16 bit operation codes are actually swapped in 16-bit mode?

07 Mar 2015, 05:31

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20759
Location: In your JS exploiting you and your system

revolution 07 Mar 2015, 07:32

GrayFace wrote:

So, 32/16 bit operation codes are actually swapped in 16-bit mode?

Yes. There is the operand size and address size prefixes. It is all explained in the Intel/AMD manuals.

07 Mar 2015, 07:32

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