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Re: contradiction in boot/i386/pc/boot.S

From: Pavel Roskin
Subject: Re: contradiction in boot/i386/pc/boot.S
Date: Tue, 08 Jan 2008 18:52:36 -0500

On Wed, 2008-01-09 at 00:37 +0100, Robert Millan wrote:
> On Tue, Jan 08, 2008 at 05:57:16PM -0500, Pavel Roskin wrote:
> > 
> > We can save 4 more bytes if we do "orb $0x80, %dl" unconditionally for
> > the hard drives.  However, I'm not sure if there are BIOSes that pass 1
> > in %dl and how critical it would be if it becomes 0x81.
> I think the assumption is that if the BIOS passes 0x01, etc, it's
> garbage and should be ignored completely.

I understand the assumption.

> > That's
> > something I'll rather not do without seeing the original bug reports.
> Same here.  Maybe Okuji will know..

When I tried to actually change the code, it turned out that
grub-mkimage doesn't change the boot sector, so grub-mkimage needs to be
adjusted, and maybe some other tools.

Alternatively, the logic should not be reverted, i.e. the adjustments
should be done for the hard drive installs.  This means that grub-setup
should write valid x86 code over NOPs.  It gets too complicated to
bother without having a very good incentive to save those 2 bytes.

Pavel Roskin

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