On Wed, 17 Jul 2002, Daniel Sheltraw wrote:
> Mark
>
> Thanks for the response. Please see comments below.
>
> > > I have a general VGA question. I see from Ferraro\'s book that
> > > the legacy port 0x3C2 (bit 7) reports on whether a vertical
> > > retrace interrupt has occured. Is this bit cleared after reading
> > > it so that it can be used properly in the case of shared
> > interrupts?
> >
> > In legacy mode it\'s cleared explicitly by clearing bit 4 of
> > Cr11.
>
> OK great. I noticed that Ferraro\'s book says 3C2 is an EGA port.
> I am unclear about whether this means that most cards will support
> this port. I see you address this below.
>
> > > Do most cards support this port and bit?
> >
> > If they\'re in legacy mode they will. But many cards operate
> > in their extended modes and don\'t even enable legacy access.
>
> I have not found a card that simply does not allow legacy port
> address. Could you please tell those that you are familiar with
> so that I might stear clear of them if neccessary.
>
Perhaps I should say the driver rather than the card. No modern driver
will use legacy VGA to program the card. Port I/O is very expensive
and the vga method of programming registers (through indexed registers
a byte at a time) is annoying. The drivers will often disable access
to vga when operating because they themselves will not access it.
In a multicard environment where there is only one vga space but more
that one vga compatible card, disabling vga on all but one card is
a requirement, and the motherboard bios will usually do that at
bootup. In general you can't expect that this sort of thing is
accessible.
Mark.
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