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Index > Main > [SOLVED|different segment regs] Why expression not resolved?

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ProMiNick



Joined: 24 Mar 2012
Posts: 656
Location: Russian Federation, Sochi
ProMiNick
Why expression [ebp+eax] not resolved into [eax+ebp]?
for example in mov instruction
Code:
mov     eax,dword[ebp+eax] ; 4 bytes: $8B,$44,$05,$00    

not resolved into
Code:
mov     eax,dword[eax+ebp] ; 3 bytes: $8B,$04,$28    

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Last edited by ProMiNick on 20 Apr 2022, 03:54; edited 1 time in total
Post 19 Apr 2022, 05:15
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 18447
Location: In your JS exploiting you and your system
revolution
The first register is taken as the base, and the second register as the index.

EBP can only be encoded as a base register with an offset.

fasm doesn't reorder the registers, even when the offset is zero.
Post 19 Apr 2022, 05:37
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 18447
Location: In your JS exploiting you and your system
revolution
I forgot to add that the reason for not swapping the register order is because of the different segment register used. EBP uses SS, EAX uses DS.

For Win32 this makes no difference normally, but fasm isn't only for Win32.
Post 19 Apr 2022, 07:33
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macomics



Joined: 26 Jan 2021
Posts: 462
Location: Russia
macomics
ProMiNick wrote:
not resolved into
Code:
mov     eax,dword[eax+ebp] ; 3 bytes: $8B,$04,$28    

Code:
mov     eax,dword[eax+ebp] ; 4 bytes: $17,$8B,$04,$28    
Post 19 Apr 2022, 16:08
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 18447
Location: In your JS exploiting you and your system
revolution
If you define the segment register then you get this:
Code:
use32
mov eax,[ds:ebp+eax]    ; 0x8b, 0x04, 0x28
mov eax,[ds:eax+ebp]    ; 0x8b, 0x04, 0x28

mov eax,[ss:ebp+eax]    ; 0x8b, 0x44, 0x05, 0x00
mov eax,[ss:eax+ebp]    ; 0x8b, 0x44, 0x05, 0x00    
Post 20 Apr 2022, 03:45
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