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Index > DOS > how to tune screen refresh rate under dos vbe 2.0?

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cylo



Joined: 06 Nov 2004
Posts: 20
cylo
hello!

i have programed a little graphic program with the svga.bgi unit for msdos (640x480x256 up to 1024x768x256)... but it is always fixed at 60Hz.

the program runs on a single board computer with a vesa vbe v2.0 compatible grafic chip.

i didnt figure out how to get this run at 75Hz or higher... Crying or Very sad

if i install windows i can go up to 1280x1024@85Hz... but i need dos! Embarassed

any idea?
Post 06 Nov 2004, 17:54
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neonz



Joined: 02 Aug 2003
Posts: 62
Location: Latvia
neonz
VBE 2.0 does not have standartized API for changing refresh rate. VBE 3.0 have API for changing refresh but it is supported only by very new adapters (3dfx, nVidia, etc...).
Post 06 Nov 2004, 18:07
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cylo



Joined: 06 Nov 2004
Posts: 20
cylo
so the only way is to program the chip directly? what registers i have to look for?
Post 06 Nov 2004, 18:27
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MCD



Joined: 21 Aug 2004
Posts: 604
Location: Germany
MCD
Lookily, many standard VGA-registers are accessible in VESA-modes too.
You tweak a bit around and test what changing of standard VGA-registers makes what changings in your VESA mode.

I used this way to emulate a vesa 512*384 mode starting from a 640*480/400 mode by simply writing something to miscellaneous output register, but this requires somewhat experimenting with graphics card, and when writing wrong things, the screen will often become blank. (Ctrl+C/Break or Ctrl+Alt+Delete Wink )

Another thing I did was changing the refresh rate of the standard vga 80*25/50 text mode by just setting a vesamode BEFORE setting to a textmode => this changes external pixel clock, which must be enabled in some VGA CRTC-register.

But all of these tweaking should be accomplished with lots of testing on many different machines, since I'm not sure how they work on other PCs, e.g. they are not (well) documented. (I think so)

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MCD - the inevitable return of the Mad Computer Doggy

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Last edited by MCD on 29 Nov 2004, 18:04; edited 1 time in total
Post 29 Nov 2004, 18:02
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MCD



Joined: 21 Aug 2004
Posts: 604
Location: Germany
MCD
I assume that you have a copy of Ralf Brown's Interrupt list. A "must-read"
Post 29 Nov 2004, 18:03
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