From: csaba@vuse.vanderbilt.edu (Csaba A. Biegl) Subject: Re: How to switch back to VGA-text from SVGA-text mode??? Date: Sat, 27 Mar 1993 01:23:47 GMT
In article <zmbenhalC4IL7M.BFp@netcom.com> zmbenhal@netcom.com (Zeyd M. Ben-Halim) writes:
>From: zmbenhal@netcom.com (Zeyd M. Ben-Halim)
>Subject: Re: How to switch back to VGA-text from SVGA-text mode???
>Date: Fri, 26 Mar 1993 21:04:34 GMT
>In article <1993Mar26.171516.3195@prime.mdata.fi> kennu@mits.mdata.fi (Kenneth Falck) writes:
>>In article <1993Mar24.224133.12392@primerd.prime.com> cummings@hammer.prime.com (Kevin Cummings) writes:
>>>Since video hardware varies from machine to machine (and card to card)
>>>the only "portable" method of doing this is for the system to provide a
>>>common interface to the video hardware. This method is currently the INT 14
>>>code in your video BIOS. Sadly, OSes like LINUX do not use any of the BIOS
>>>code in your machine. Therefore, you need a program to do the work for you.
>>
>>You know, this is something I've been wondering for some time now.
>>Why exactly is it that BIOS calls can't be made under Linux?
>
>Because nobody has bothered to write to interrupt handlers in protected-mode
>to run real-mode code. It certainly CAN be done, only those who want it don't
>have enough knowledge about 386 programming, whilst those who know have a
>religious thing about not supporting the BIOS :-)
There is on more thing to consider: most SVGA cards come with some clock
tweaking, image centering TSR-s that you have to install in your
AUTOEXEC.BAT to get decent graphics. Without these you will end up
with a 56Hz vertical refresh 800x600 mode or a 1024x768 interlaced mode
since the ROM BIOS-es by default support the lowest frequency modes in
order to be compatible with any monitor. It is the TSR which augments and/or
takes over the mode set INT 10H BIOS call to provide higher refresh rates
and non-interlaced modes.
For this reason interfacing to the video ROM BIOS from Linux would be of
very limited value. (NOTE: There are a few cards which do it differently:
by storing the mode parameters in EEPROM. For these the ROM BIOS interface
would be more useful.)
For the majority of the cards the BIOS interface would be useful only if
Linux booted from DOS AFTER you had a chance to install your video TSR.
Furthermore, the Linux kernel would have to keep a copy of the low memory
where the video TSR was installed. It's getting more complicated, doesn't it?
On the other hand: such a scheme WOULD solve most of the video miseries
people have with Linux and X... It would even make the Diamond Stealth-s
usable.
Csaba Biegl
csaba@vuse.vanderbilt.edu