[Xen-devel] Re: beginnings of allowing more than the basic 80x25 VGA screen resolution

[Matthias Huber <hubermat@xxxxxx>]

Gerd Knorr <kraxel <at> suse.de> writes:

> 
> Keir Fraser <Keir.Fraser <at> cl.cam.ac.uk> writes:
> 
> > > As far as I know, not only the VGA driver does not do any mode
> > > switching, even the VESA one doesn't (because the protected mode
> 
> Exactly.  A simple way to get more screen lines is to load a 8x8
> screen font though.
> 
> > real-mode assembly file. Any video card we care about these days
> > should support VBE 2.0/3.0 I think, and those interfaces don;t require
> > you to be runnign in real mode.
> 
> vbe 2.0 yes, not sure about 3.0.
> 
> For mode switching from protected mode you need 3.0 though, 2.0
> protected mode support is _very_ limited (you can do panning and
> palette register programming, but no mode switching).  And I think
> even for vbe 3.0 you need some real mode code to figure the protected
> mode entry point, so paravirtual linux wouldn't be able to do that
> without help from xen.

The VBE3.0 spec shows clearly how to figure out the protected mode entry point:
copy the video bios to another location and search for the PMInfoBlock
structure, identified by a four-byte signature "PMID". In the structure you find
the protected mode entry point.
At the moment I am working on initializing graphics mode via the protected mode
VBE interface. So far I found the PMInfoBlock, now I try to figure out how to
set up the needed 16bit-registers to do the lcall into the video bios (at the
protected mode entry point). Any code snippets e.g. for switching to a 16bit
stack selector prior to the lcall are wellcome...

Matthias Huber



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