copy extended memory.

Eric W. Biederman ebiederman at
Mon Oct 28 15:32:00 CET 2002

Adam Sulmicki <adam at> writes:

> FWIW, here's summary what I was able to gather so far
> > And the format of the GDT is:
> > dummy
> > GDT location
> > source GDT entry
> > target GDT entry
> ok, I think I got it nailed down, how about the way it is below?
> I'm not quite sure of your interpretation of #2 (GDT location), can you
> elaborate? Ie what would it be used for by BIOS. It seems to me like I
> already get this info from ES:SI.
>        CX     = counter of 2-byte words
>        ES:SI  = pointer to to a structure of size of 0x30 which
> 		contains 6 GDT entries.
> 1	0x00	Reserved  GDT	(all 0's)
> 2	0x08	FS & GS   GDT 
(I think this is actually for the indirect link)
> 3	0x10	DS (src)  GDT
> 4	0x18	ES (dst)  GDT
> 5	0x20	CS (bios) GDT
> 6	0x28	SS (bios) GDT
> of all those caller of the interrupt is required to only fill out
> #3 and #4 and leave all other blank.
> Because we are not passed indirect link with (size of GDT,ptr to GDT) we
> can't use it directly in our LGDT call (after filling out missing info).
> We have to either copy it over or add another level of indirection.

I would put the indirect link in table entry 0x00.  That way you can trivially
call lgdt with the address of the gdt...  Table entry 0x01 would do as well,
and I think that is what my documentation was suggesting.


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