Message-ID: <8D53104ECD0CD211AF4000A0C9D60AE301434031@probe-2.acclaim-euro.net>
From: Shawn Hargreaves <ShawnH AT Probe DOT co DOT uk>
To: djgpp AT delorie DOT com
Subject: Re: VGA Performance
Date: Mon, 14 Jun 1999 10:19:32 +0100
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Reply-To: djgpp AT delorie DOT com

Yellow writes:
> Does anybody  have an idea of how long does it take to transfer a 2
> MByte bitmap from system RAM to a VGA video controller video RAM when
> that controller is plugged into a PCI / PMC slot on the mainboard?

My experience is that this depends far more on the graphics card than
on your processor or bus type. The only way to find out for sure is
to time it and see: write a little loop that copies memory from system
ram to the screen, and see how long it takes on your hardware.

The results can be very different depending on what video mode you use.
For example most SVGA boards are much faster in SVGA than in VGA modes
(it is not unusual to see an order of magnitude speed improvement when
you switch from a 320x200 VGA mode 13h to a 320x200 VESA mode). You
may also find that the transfer times are better when using a linear
framebuffer, compared to banked SVGA access.

> Is there any valid reason to think that transferring 2 MB in the 
> opposite direction ( from VGA RAM to system RAM ) could take longer?

Very much so. Graphics cards are not usually optimised for reading,
and it is often very slow (on the Matrox that I measured, reads were
about 15 times slower than writes).


	Shawn Hargreaves.