[ipxe-devel] Big real mode use in ipxe

Michael Brown mbrown at fensystems.co.uk
Sun Aug 19 15:34:50 UTC 2012

On Sunday 19 Aug 2012 16:07:05 Avi Kivity wrote:
> Which is exactly what happens here.  My understanding of big real mode is
> that to achieve a segment limit != 0xffff, you must go into 32-bit
> protected mode, load a segment with a larger limit, and return into real
> mode without touching the segment.  The next load of the segment will
> reset the limit to 0xffff.

Not quite.  You can't "return into real mode without touching the segment", 
since part of the process of returning to real mode is to reload the segment 
registers with real-mode values, and this happens _after_ setting CR0.PE=0.

Whenever CR0.PE=0, loading a segment register with value N will load the 
literal value (N<<4) into the base address for that segment, without changing 
the limit.  This is the trick that allows flat real mode (aka big real mode) to 
work; the limit remains at 4G even after loading the segment register with a 
real-mode value.

> (and that seabios needs changes to either work in
> big real mode, or to put the processor back into big real mode after
> returning from a PMM service.

If seabios switches into protected mode when performing a PMM service, then it 
_must_ leave the segment limits at 4G when returning to real mode.  To do 
otherwise will violate the PMM spec, and will break conforming clients such as 


More information about the ipxe-devel mailing list