Re: [PATCH] Use correct x86 reboot vector

[ebiederm@xmission.com (Eric W. Biederman)]

Davide Libenzi <davidel@xmailserver.org> writes:

> On Sun, 11 May 2003, Jamie Lokier wrote:
> 
> > Jos Hulzink wrote:
> > > On Sunday 11 May 2003 05:50, Linus Torvalds wrote:
> > > > Hmm.. Doesnt' a _real_ hardware reset actually use a magic segment that
> > > > isn't even really true real mode? I have this memory that the reset value
> > > > for a i386 has CS=0xf000, but the shadow base register actually contains
> > > > 0xffff0000. In other words, the CPU actually starts up in "unreal" mode,
> > > > and will fetch the first instruction from physical address 0xfffffff0.
> > > >
> > > > At least that was true on an original 386. It's something that could
> > > > easily have changed since.
> >
> > I got my info from an article on the net which says that a 386 does
> > behave as you say, but it is possible for the system designer to
> > arrange that it boots into the 286-compatible vector at physical
> > address 0x000ffff0.  It states that the feature is specifically so
> > that system designers don't have to create a "memory hole" (that's as
> > much detail as it gives).
> 
> Guys, mem[0xfffffff0,...] == mem[0x000ffff0,...] since the hw remaps the
> bios. Being picky about Intel specs, it should be f000:fff0 though.

The remapping is quite common but it usually happens that after bootup:
0xf0000-0xfffff is shadowed RAM.  While 0xffff0000-0xffffffff still points
to the rom chip.

Now if someone could tell me how to do a jump to 0xffff0000:0xfff0 in real
mode I would find that very interesting.

Eric