Re: PATCH - assembler glitches
Martin Dalecki (dalecki@cs.net.pl)
Wed, 13 Oct 1999 04:17:47 +0200
Alan Modra wrote:
>
> This is a copy of something I posted a while back that should clear up the
> confusion over gas and x86 segment registers. If Martin's patch is
> applied, then you are forcing people to move to new binutils - which may
> be required anyway with the new bootsect stuff. Martin, please review
> your patch after reading the following. Your patch is looks mostly
> correct to me (for new binutils), with the exception of
> arch/i386/kernel/signal.c, where you will unnecessarily add a prefix.
>
> ==================================
> On Mon, 9 Aug 1999, Alan Cox wrote:
>
> > > - AFAIU, there is no point in using just the lower half of a register to
> > > fill in a segment register, with a movl to boot.
> >
> > I seem to remember there was. This came up before if I remember rightly
> > and Linus had a specific reason for it. Check with Linus.
>
> Compatibility with old binutils.
>
> For example, when transferring %eax to %ds:
>
> `movl %eax,%ds' is the "correct" instruction, but this fails to assemble
> on old binutils.
>
> `movw %ax,%ds' is also correct, but will generate an operand size prefix
> on older binutils. You don't want extra prefixes if you can help it.
>
> `movl %ax,%ds' gets a warning on new binutils, but assembles correctly on
> both old and new binutils. ie. without a prefix. This is the form that
> should be used despite the warning.
>
> Note: When transferring the other way, `movw %ds,%ax' and `movl %ds,%eax'
> actually do different things. The first form leaves the upper 16 bits of
> %eax unchanged (and costs you an operand size prefix), while the second
> form sets the top 16 bits of %eax to "an implementation defined value"
> according to Intel. If you don't care about the top 16 bits of %eax, use
> `movl %ds,%ax'. Again, note the missing `e' for compatibility with old
> binutils.
>
> By old binutils, I mean anything before April 1998 in the Cygnus tree, or
> earlier than 2.9.0.3 from H.J. Lu
Been there have done it already. However I would like to preffer just to
change
the note about the needed binutils, since this is what allmost all other
platforms are really needing (ASM comes first to mind if I read the
ChangeLog's
from binutils correctly). And anyway newer binutils are a real win
apparently
for everybody not just hacking on the kernel but writing user space code
too... (In esp C++)
--
Marcin Dalecki
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