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

[Rigel <ri...@an...>]

 
--

On Wed, 11 Apr 2001 16:51:44  
 Jim Ursetto wrote:
>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) 

correction here.  I think you mean a mult by 2, 4, or 8 (which is really shl by 1, 2, 3).  I only mention this because I confuse the two all the time...

>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:
>

all right, I think I've got it (all) figured out now (haha).
I understood mov all along (promise), just not lea.  I do like Intel syntax much better though, as it's a little more like other *real* (RISC) assembly languages.

This whole gaffe of mine just illustrates another reason I don't like x86 asm, which is that the instructions are misnamed.  I would have called lea, cea, for calculate effective addr. or maybe just wpos for weird piece of sh**.

I'll definitely look up that Art of Assembly stuff.  It should be a good reference.

Okay, with the zero and sign flag (edi) you just give it the latest result from the ALU, right?  But what about the carry flag (ebp) ?  do I just set it to 1 if there was a carry involved in the most recent instruction?

Thanks much,
	Rigel


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