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Mode Y

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Dizzy

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Aug 16, 1993, 9:07:55 PM8/16/93
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Hi,

I've got a couple of questions about Mode Y.
I know it is 320x200x256x4 pages.
But, can you write to screen memory just like mode 13h or is it like mode X.
Also, can you use the latches like mode x, or not?
Umm, and finnally, what does unchained and chained really mean?
Thanks


Well Nuff said
Ciao
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Christian Wagner

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Aug 17, 1993, 10:07:44 AM8/17/93
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In article <930817010...@buxton.teaching.cs.adelaide.edu.au>, t0s...@teaching.cs.adelaide.edu.au (Dizzy) writes:
|> Hi,
|>
|> I've got a couple of questions about Mode Y.
|> I know it is 320x200x256x4 pages.
|> But, can you write to screen memory just like mode 13h or is it like mode X.
You write to the screen like in Mode X.

|> Also, can you use the latches like mode x, or not?

Yes, you can.

|> Umm, and finnally, what does unchained and chained really mean?

chained: A VGA card's memory is divided into 4 planes in *ANY* VGA graphics mode. If
you use mode 13h (320x200x256) you don't have to care about the 4 planes.
You simply calculate the adress of a pixel and write it to the VRAM.
The VGA card calculates out of the adress of the pixel in which plane it
belongs. This is what is called 'plane chaining' mode.
This chaining is done in a strange way - so only 64 KB of the 256 KB VRAM can
be used.

unchained: If you switch off chained mode, you have to do all plane select
calculations by yourself. It isn't simply sufficient to pass an adress to
write a pixel but you have to select a plane where the pixel should be
written to as well. unchained mode takes longer for writing single pixels
because you have to specify an adress and a plane mask, but it's faster
when writing multiple pixels into a single horizontal scanline. With a
stosw for example you can write 8 pixels with a single memory access to the
VRAM. Which is 4 times faster than a stosw in mode 13h where you can only
write 2 pixels with a single stosw. 4 times faster! That isn't bad, is it?
Another advantage is that you have full 256KB to write video data to.
Since 320x200x256 takes 64KB you can store 256KB/64KB=4 pages in VRAM.
Switching between these pages is very fast, since all the pages are already
stored in VRAM and don't have to be transported to it.

ChriZ

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