Re: [Tuxnes-devel] more help w/ asm

[Jim U. <ji...@3e...>]

At 03:40pm on 2001 April 11, Rigel did write:
> I was wrong, then, because I meant load into ebx the value at the mem location held in edi.
 
> So if I say, 
> leal	4(%ebx),%edi
> then I mean (in C),
> edi = ebx + 4

Exactly.  With lea you can do a mul, add, AND a shl (by
2,4,or 8) in only one instruction, but it can also be
used to just do a single mov, add, or shl.  lea is a
special instruction that performs all address
computations, then moves this address result into a
register.  

For example, 
  leal 4(%ebp,%ebx,2),%edi
does (in C)
  edi = ebp + ebx*2 + 4;

What you want is movl (%edi), %ebx.   This is like saying ebx = *edi.
This is also equivalent to movl 0(%edi), %ebx -- the 0 is superfluous.

In intel syntax, 
  lea eax, [ebx]  ==  mov eax, ebx.

You may just be confused by the AT&T syntax, but here are some resources:

LEA documentation in intel syntax at:
http://webster.cs.ucr.edu/Page_asm/ArtofAssembly/CH06/CH06-1.html#HEADING1-136

Scaled indexed addressing modes at:
http://webster.cs.ucr.edu/Page_asm/ArtofAssembly/CH04/CH04-3.html#HEADING3-49

Register indirect and indexed addressing modes at:
http://webster.cs.ucr.edu/Page_asm/ArtofAssembly/CH04/CH04-2.html#HEADING2-35

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