Changing strings

Index > Main > Changing strings

Author

Thread

denial

Joined: 12 Sep 2004
Posts: 98

denial 27 Oct 2005, 10:19

Hi...

to change the string in a byte array reserved through rb, I always used sprintf from the WinAPI. But as I want to be independent from APIs, I want to use such kind of code from now on:

mov [myvar+0],'A'
mov [myvar+1],'B'
mov [myvar+2],'C'
...

Well its pure and simple, and thus I thought its also fast in execution. But I wanted to know: Is there a way that gets executed faster?

And for copying a string from one var into another, I use this:

mov al,[srcvar+0]
mov [destvar+0],al
mov al,[srcvar+1]
mov [destvar+1],al

How can this be made faster in execution?

Thank you...

27 Oct 2005, 10:19

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8359
Location: Kraków, Poland

Tomasz Grysztar 27 Oct 2005, 10:41

There are String operation instructions for such purposes.

27 Oct 2005, 10:41

denial

Joined: 12 Sep 2004
Posts: 98

denial 29 Oct 2005, 12:04

thank you. how could i forget about them. Embarassed

i'm sorry...

29 Oct 2005, 12:04

Reverend

Joined: 24 Aug 2004
Posts: 408
Location: Poland

Reverend 01 Nov 2005, 09:29

Code:

        push    ecx edi esi
        mov     esi, src
        mov     edi, dest
        mov     ecx, size
        rep     movsb
        pop     esi edi ecx

You can even optimize it, and use 'movsd' in 32-bit or 'movsq' in 64-bit

01 Nov 2005, 09:29

okasvi

Joined: 18 Aug 2005
Posts: 382
Location: Finland

okasvi 11 Nov 2005, 08:12

Reverend wrote:

Code:

        push    ecx edi esi
        mov     esi, src
        mov     edi, dest
        mov     ecx, size
        rep     movsb
        pop     esi edi ecx

You can even optimize it, and use 'movsd' in 32-bit or 'movsq' in 64-bit

so which one is it? in code you have movsb... im not at home so i could test it but i want to know before i memorize either one of those... Neutral

movsd/movsb...

_________________
When We Ride On Our Enemies
support reverse smileys |:

11 Nov 2005, 08:12

Reverend

Joined: 24 Aug 2004
Posts: 408
Location: Poland

Reverend 12 Nov 2005, 14:43

The version I posted is correct for all strings (of course for correct arguments). But if you know the string you want to copy is of length dividable by 4 (ie. 4, 8, 12, 16, 20, ...) you can use movsd instruction which copies 4 bytes at once. Then you must also divide size by 4:

Code:

;       if we know that size is dividable by 4
        push    ecx edi esi 
        mov     esi, src 
        mov     edi, dest 
        mov     ecx, size / 4
        rep     movsd
        pop     esi edi ecx

12 Nov 2005, 14:43

El Tangas

Joined: 11 Oct 2003
Posts: 120
Location: Sunset Empire

El Tangas 13 Nov 2005, 20:58

And remember to align memory variables, for example dword variables should be in addresses divisible by 4 (use the "align" keyword in fasm). This is very important for good performance.

13 Nov 2005, 20:58

< Last Thread | Next Thread >

Forum Rules:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum