Re: [PATCH] Loading windows in macbook

[Yoshinori K. Okuji]

On Wednesday 25 June 2008 12:53:17 Bean wrote:
> On Wed, Jun 25, 2008 at 6:19 PM, Marco Gerards <address@hidden> wrote:
> > Bean <address@hidden> writes:
> >> On Wed, Jun 25, 2008 at 4:38 AM, Javier Martín <address@hidden> 
wrote:
> >>> El mié, 25-06-2008 a las 04:11 +0800, Bean escribió:
> >>>> 2. Macbook will halt if we disable a20, so I add a new option
> >>>> --keep-a20 to keep the a20 gate open.
> >>>
> >>> Does any OS fail to boot if it has A20 enabled when it is jumped to?
> >>> AFAIK some BIOS even had an option to enable it at POST time, so I
> >>> think we should make this new option the default and have an option
> >>> called "--reset-a20" or something like that.
> >>
> >> I don't know of any os that would fail if a20 is enabled. Normally,
> >> they try to enable it as soon as possible as they need to access upper
> >> memory. So making it default is not a bad idea.
> >
> > It's disabled for legacy reasons.  Some old DOS applications expect
> > addresses to wrap around.  And thus a20 gate should be disabled.
>
> Right, but new version of dos will enable a20 as well, so application
> that depend on such feature would have serious compatibility issue.
> Anyway, the question is whether we should leave it on by default. A20
> is a common issue for buggy bios, I think we should leave it alone as
> much as possible.

No, no, no. Please don't forget the principle: a boot loader should pass 
control in the same environment as when directly booted, as much as possible. 
All sane PC BIOSes start with a20 disabled, so it must be disabled.

Okuji