Non-standerd vga modes

 BBS: Inland Empire Archive
Date: 05-17-92 (14:16)             Number: 181
From: MATT PRITCHARD @ 930/21      Refer#: NONE
  To: CORIDON HENSHAW               Recvd: NO  
Subj: Non-standerd vga modes         Conf: (2) Quik_Bas
MP@9> Not all cards can do 640 x 480 x 256 colors.

MP@9> The minimum amount of Video RAM on a VGA is 256K; In fact 256K is
MP@9> *ALL* the memory that IBM designed it to handle.  IBM didn't plan

MP@9> 360 x 480 (x256) mode requires about 180k of video memory.

CH> How high is the max res that you can get before hitting 256K?

360 x 480 is the max I know of that works on all VGA's.

I have an idea I'd like to try someday for 640x400x256, but it might overload
the VGA card.  There is another factor to the equasion:
that's video memory speed and Clock Crystals.  You have to
have a clock crystal that can drive the system at the
correct speed to pull out 1 screen of data and drive the
display every 1/60th or 1/70th of a second.  Except for the
25 and 28 mhz crystals which every VGA card has, there is
not a standard way to select the crystal speed.  Any VGA
card that has a 132 column mode had another crystal,
usually at 36 mhz one.  Also, if the memory isn't fast
enough, it can't spit out enought data every frame for a
high resolution image.

-Matt Pritchard

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