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

[Mike M. <mel...@pc...>]

On Mon, 9 Apr 2001, Rigel wrote:

> All right.  I know all about AT&T style syntax, I know many x86 opcodes, but I've never seen this type of addressing mode.  This found on line 148 of table.x86 (in the BRK inst.), and looks like this: 
> "leal   0x0(%ebp,%ebx,2),%ebp".  What's inside the parentheses?

	Okay, now I understand the question. That AT&T syntax is such a
pain to digest sometimes. From plowing through my reference material, the
above instruction means:
  ebp = ebp + (ebx * 2) + 0
I think that's it, but I'm not positive.

> Got one more.  there's a nice little couplet directly before the previous (lines 146,147) that looks like this:
> testl  $0xff,%edi
> setz   %bl
> 
> This sets bl when? is it when edi == 0xff ?

	It looks to me like the net effect of the couplet is:
  if (!(edi & 0xff))
    bl = 1;
The test instruction performs a bitwise 'and' between the 2 operands. Like
a cmp instruction, it discards the result but sets flags. The setz
instruction stores a 1 into the destination if the zero flag is set.

-- 
	-Mike Melanson