From: dstamp@watserv1.waterloo.edu (Dave Stampe-Psy+Eng)
Subject: VGA from hell!
Date: Sat, 26 Oct 1991 03:32:46 GMT
Message-ID: <1991Oct26.033246.13012@watserv1.waterloo.edu>
Organization: University of Waterloo


OK.  I ran some timing tests on my somewhat-super VGA Paradise card.
Talk about wait states!  This is the added times for various operations
as compared to memory accesses:

Write byte: add 0.93 uS
Read byte:  add 1.21 uS
Copy bytes (plane to plane, using internal latches to move 4 bytes at once)
            add 2.2 uS
Write to plane mask, bit mask, or color registers (out dx,ax)
            add 2.1 uS

Now, I'm running this on a 486/25, and these timings include a few
clock cycles because the VGA card buffer is always a cache miss.
These timings translate into (factoring in mask register accesses, etc):

83 mS to copy buffer into 320x200x256 color mode
42 mS to copy buffer to 320x200x16 color mode, 32 mS for 8 color, 21 mS
 for 4 color, etc. (also to clear buffer).
6 uS to write 1 pixel in 16-color mode
25 uS to draw 24-pixel horizontal line
350 uS to draw 24x24 polygon
 .
 .
You get the general idea.  Obviously, 10 frames/sec on a VGA card is the
best you can get.  The processor is running at 30% of its possible speed.
You CAN get 2800 polys/sec, but almost no time to do anything else.  
Interestingly, the 6uS single-pixel time predicts that line-drawing the
same scene (2800 polys, no hidden lines) takes the same amount of time!
And it's faster to draw both lines and polys in 256-color mode (1.3 uS/pixel)
(340 uS/24x24 poly)

Going to have to check out some new ideas on how to do it better: the
best idea is imcremental updates, but they won't work for eyephones,
as head movement means the whole screen must be redrawn.  Maybe drawing
to a buffer, and copying to the screen later... (>B-{(=


--------------------------------------------------------------------------
| My life is Hardware,                    |                              | 
| my destiny is Software,                 |         Dave Stampe          |
| my CPU is Wetware...                    |                              | 
| Anybody got a SDB I can borrow?         | dstamp@watserv1.uwaterloo.ca |
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