I need some clearence with regs EDX vs DL

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Jay

Joined: 12 Aug 2014
Posts: 2

Jay 12 Aug 2014, 20:25

I was given of this piece of code, that should verify the checksum of a file, but i do not understand 1 thing, the mov of a byte in dl and the add operation of edx this code has sense if dl and edx were the same, but they are not! So, here the question. Is there some kind of relation between dl and edx ?

Code:

proc verifyChecksum stdcall, file_address:DWORD, file_size:DWORD

        pushad
        mov ebx,dword [file_address]
        mov eax,[ebx]
        add ebx,4
        mov ecx,[file_size]
        sub ecx,4
        ;checksum is in eax
        ;pointer to file in ebx
        ;size of file in ecx
        sub edi,edi
vs_calc_cs:
        sub edx,edx             ; zero edx reg
        mov dl,byte [ebx]       ; take 1 byte from the area pointed by ebx
        add edi,edx             ; add edi and edx
        inc ebx                 ; increment the address( ebx )
        dec ecx                 ; decrement the counter
        jnz vs_calc_cs          ; check if the last op return 0, if 0 EXIT_SUCCESS, if !0 cycle again
        ;calculated checksum is in edi
        cmp edi,eax
        jne vs_exit_error

vs_exit_success:
        popad
        mov eax,1
        ret

vs_exit_error:
        popad
        sub eax,eax
        ret

endp

12 Aug 2014, 20:25

cod3b453

Joined: 25 Aug 2004
Posts: 618

cod3b453 12 Aug 2014, 21:29

Yes, dl is the 8 least significant bits of the (r/e)dx register (see http://sandpile.org/x86/gpr.htm for full view). The mixture of registers can be due to the limited permutations of encodable operations and/or the algorithm. Here the algorithm needs to carry up to 32bits in the add but the mov could have been movzx edx, byte [ebx].

12 Aug 2014, 21:29

Jay

Joined: 12 Aug 2014
Posts: 2

Jay 13 Aug 2014, 11:46

ok, since dl represent the first byte of edx the sum with edi has sense. thx

13 Aug 2014, 11:46

DOS386

Joined: 08 Dec 2006
Posts: 1898

DOS386 17 Aug 2014, 16:27

Welcome to the FASM forum Smile

> if dl and edx were the same, but they are not!

ADD EDI,DL unfortunately doesn't work Sad

When you master primitive checksums, you might want to look at CRC32, MD5 and SHA256 Smile

17 Aug 2014, 16:27

Foxxy

Joined: 14 Jul 2014
Posts: 42
Location: Somewhere over the rainbow...

Foxxy 27 Aug 2014, 20:54

DOS386 wrote:

Welcome to the FASM forum

> if dl and edx were the same, but they are not!

ADD EDI,DL unfortunately doesn't work

When you master primitive checksums, you might want to look at CRC32, MD5 and SHA256

I recently completed a CRC32 checksum utility written in assembly! Smile

I was looking around the internet on how a SHA256 checksum works and the code behind it, any resources?

27 Aug 2014, 20:54

shutdownall

Joined: 02 Apr 2010
Posts: 517
Location: Munich

shutdownall 28 Aug 2014, 10:03

http://nayuki.eigenstate.org/page/fast-sha2-hashes-in-x86-assembly

28 Aug 2014, 10:03

jhonny6721

Joined: 07 Aug 2014
Posts: 26

jhonny6721 01 Sep 2014, 19:35

You must avoid mix registers parts. In other words, you must avoid add a register that contains a 8 bit value with a 32 bit value. Because, only the value is stored in the first 8 bits. The rest of bits can have a wrong value.

By this reason, if you need to perform any operation with 8 bits or 16 bits part, you must set to zero the whole register.

Please, I´m sorry for my english. I am not a natice speaker

01 Sep 2014, 19:35

shutdownall

Joined: 02 Apr 2010
Posts: 517
Location: Munich

shutdownall 01 Sep 2014, 21:41

Well mixing register is not that unusual.
XLAT for example but also MUL and DIV operate with different register sizes.
CBW, CWDE, MOVZX and MOVSX helps with loading 32 bit registers from 16 or 8 bit registers.

01 Sep 2014, 21:41

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