---------- Forwarded message ----------
From: pradeep sawlani <pradep@xxxxxxxxx>
Date: Jan 18, 2006 3:15 PM
Subject: Re: pls explain strcpy() assembly
To: Vijay Subramanian <subramanian.vijay@xxxxxxxxx>
On 1/18/06, Vijay Subramanian <subramanian.vijay@xxxxxxxxx> wrote:
> On 18/01/06, Rajaram Suryanarayanan <rajaram_linux@xxxxxxxxx> wrote:
> > Hi,
> >
> > Can anybody pls explain me step by step what is done by the assembly code
> > below ? Particularly, I want to know how "src" and "dest" get substituted in
> > the assembly code.
> >
> > static inline char * strcpy(char * dest,const char *src)
> > {
> > int d0, d1, d2;
> > __asm__ __volatile__(
> > "1:\tlodsb\n\t"
> > "stosb\n\t"
> > "testb %%al,%%al\n\t"
> > "jne 1b"
> > : "=&S" (d0), "=&D" (d1), "=&a" (d2)
> > :"0" (src),"1" (dest) : "memory");
> > return dest;
> > }
>
> The esi (source index register) is referenced as S. It is set to point
> to what src is pointing to. The "0" in front of src means look at the
> previous list (set of output operands) and use the 0th element.
> Similarly, dest references D which is the
> (destination index register).
>
> "1:\tlodsb\n\t"
> 1 is a label. lodsb loads a byte from what esi points to and
> increments esi. The byte should be in al register.
>
> stosb\n\t
> stosb moves the byte in al into the location pointed to by edi (D).
> edi will also be incremented .
>
> testb %%al,%%al\n\t
> To test for end of string test the byte in al register. If it is not
> null jump back to the
> top of the loop and repeat.
> This will continue till you hit the end of the string (NUL) in which
> case the jump will not take place. At this point the string has been
> copied.
>
> I am not sure why we need d0, d1 and d2. Can anyone give a hint? Thanks.
>
> The syntax will make sense if you read this
> http://www-128.ibm.com/developerworks/linux/library/l-ia.html
>
> Hope this helps.
> vijay
>
> --
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>
>
I think d0,d1,d2 are output variables they will contain values of
esi,edi and al registers. I dun why we require them.
Regards
Pradeep
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