Re: Improve Video Counter???

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

Bryan Parkoff replied:

<snip>

>    I appreciate that you have provided your idea.  I already thought
>that 256 color per pixel would use 8 banks for 280x192 resolution and
>16 banks for 560x192 resolution.  I prefer that it stays in bank one
>from $400 through $D600 while 6502 program stays in bank zero to
>manipulate display in bank one.  The problem is that it requires 6502
>MPU to spend 7 or 8 cycles.  Parallel is very ideal while I have alot
>of things in my mind to imagine how I can implement it.

There is less chioce than you seem to think.

The Apple II is based upon a 1MHz processor and 2MHz memory.
Half of the memory cycles are used by the processor, leaving just
1MB/sec for video.  If you want more pixels or more colors in a
40 microsecond line, you'll have to increase the bandwidth.

Since the memory access is locked to 1MHz, that means increasing
the number of DRAM chips that are accessed in parallel, which, to
keep things simple, means at the same address.

Sure, there are other ways to generate video, and other ways
to connect the video RAM to a processor, but the Apple II already
has chosen one, and that implies the limits I describe (for an
"organic" extension).

If all the video data is in one bank, as you suggest, you will need
to address it at 8 or 16 times the speed of an Apple II to get the
required bandwidth for a 256-color 280- or 560-pixel per line
display.  To integrate it with the Apple II processor, you would
actually need 9MHz or 17MHz access, so that the processor
gets an access every microsecond, as well.

These speeds are easily attained today, but such a memory
subsystem is quite incongruous with the Apple II design, and
the 1MHz 65(C)02 is incongruously puny as a method to
update such a video memory.

If you're thinking of a display suitable for a "modern" GUI,
then you will need at least an 8MHz to 16MHz processor
to keep the update rates comparable to existing 1MHz Apples.

This is beginning to sound like a very non-Apple II machine...

-michael

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