Re: Improve Video Counter???

[mjmahon@aol.com (Michael J. Mahon)]

Bryan Parkoff replied:

>    Thank you for the information.  It makes sense that Apple II can't
>use 320x200 because you said that it is under 300.  Apple Computer Inc
>made wise decision to choose 280x192 resolution.
>    You are right that 7 pixels per half video cycle for even and
>another 7 pixels per half video cycle for odd to form 80 column text
>and DHR pixels that they go into 560x192 resolution.

Actually, both bytes are read out simultaneously (in the same video
cycle) and latched to be shifted out at 14MHz.  Being able to read
multiple bytes simultaneously is the big advantage of multi-bank
memories.

>    Do you think that it is possible to have 256 colors per pixel in
>280x192 resolution and 560x192 resolution?  If I am going to implement
>that one pixel will be displayed during 1/7 video cycle to create one
>of 256 colors?

If you used the main bank graphics memory and 7 additional banks of
"auxiliary" graphics memory, then you would have 8 bits per pixel at
280 pixels per line.  These bits could be used to drive an RGB DAC
directly, or they could be used as an index into a 256 entry pallette
memory (of as many bits width as desired) which could drive a DAC.

If a total of 16 banks of memory were addressed in parallel, then
the pixels could be shifted by the 14MHz clock to produce 560
pixels per line.

But 8 parallel banks of memory is a lot, and 16 is even more.  ;-)
Just using dense DRAMs won't do the job, since all banks need
to be accessed in parallel.  A _lot_ of 64KB DRAMs would be the
way to do it, but it would be big and hot.

"Drawing" on such a multi-bank graphics screen would be hard
for the Apple, too, since a change to a pixel would, in general,
require changing bits in 8 or 16 bytes, all in different banks.

Other schemes for mapping memory for video display can be
imagined, which might simplify pixel updates, but they would
bear little resemblance to native Apple II video.

The practical limit for this kind of extension on the Apple II is,
in my opinion, two banks (or DHR) which can be thought of
as supporting 16 colors at 140 pixels per line.

-michael

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