[solved] fasm bug

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Ali.Z

Joined: 08 Jan 2018
Posts: 719

Ali.Z 19 Apr 2020, 05:46

fasm1:

Code:

use32
mov     dword [edi],esi ; 2 bytes
mov     dword [ds:edi],esi ; 2 bytes

; the bug
mov     dword [es:edi],esi ; 3 bytes

the default segment for EDI is es - extra segment, so fasm should not add es prefix while assembling [es:edi].

but it should add ds prefix for [ds:edi]

fasm also assemble [cs:esp] with no issues, but in reality this is invalid because there is no replacement for stack operations.
im not sure what other segment bugs fasm have.

there is no segment override for CS:EIP, SS:ESP, and ES:EDI, that is what i know Confused

i love fasm

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Asm For Wise Humans

19 Apr 2020, 05:46

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 Apr 2020, 05:52

IIRC ES is only the default for EDI with the string instructions like MOVS and STOS. Question

19 Apr 2020, 05:52

Ali.Z

Joined: 08 Jan 2018
Posts: 719

Ali.Z 19 Apr 2020, 06:09

okay i checked the manuals, thats true the default destination segment for string operations is ES:EDI.
but normally is DS, last time i checked the manuals was very long time ago.

sorry for this, you can remove the thread if its so stupid.

_________________
Asm For Wise Humans

19 Apr 2020, 06:09

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