vga16fb display size problems (Re: Live CD X autoconfiguration)

[Paul Sladen]

On Tue, 18 Jan 2005, Matt Zimmerman wrote:
> On Tue, Jan 18, 2005 at 07:11:43PM +0100, Marcus Bauer wrote:
> > It get's initialized a 80x30 but only 80x24 is shown on screen.
> According to Colin, there is currently no reliable way around this.  On some
> systems, it seems to just get it wrong, and a boot parameter is needed to
> work around the problem.

I don't know the answer;  but here are some pointers as to why might be
occuring...

The ''standard text'' mode when a PC comes up is actually 640x400 (80x25,
mode 02h/03h), this mode features non-square pixels and has a slightly
'stretched' appearence.  It is also 640*400/2 = 128,000 bytes in framebuffer
mode and will *just* fit on a 128kB VGA card.

Whereas, 640x480 (80x30 lines @ 8x16 pixels/character), features
square pixels and wants 3 (64kB) pages of VideoRAM, however because of the
weird 'planes' on the VGA card the RAM is not actually used linearly.

So, it could be:

  (a)  Used in VGA (not SVGA, not VBE) mode, the chip is doing a
       particularly good impression of a 128kB VGA adaptor.

  (b)  Only two pages (for whatever reason) are getting mapped.

  (c)  General weirdness to do the card already have been in that mode
       because it's a laptop with a fixed-size TFT and square pixels

My memory is telling that I've seen a similar issue with isolinux/grub on
the Install/or LiveCD and didn't put 2+2 together until I read the above.

I suspect it might be possible to deal with this mismatch by grepping the
VGA hardware registers or writing/reading to testif there is real-RAM at
those locations >128kB.  Knowning which machines exhibit the issue and
having access to one would be a real bonus.

Marcus, you mentioned '80x24' above, would you be able to recount and
confirm whether it is 24, or wether it might be 25 being displayed?

	-Paul
-- 
Is there no safe way to travel?  London, GB