[Coco] What would a CoCo successor have to have as a minimum?

[Theodore (Alex) Evans]

On 11/20/2010 01:40 PM, jdaggett at gate.net wrote:
> On 21 Nov 2010 at 0:28, Mark McDougall wrote:
>
>> The real problem that everyone has missed here is how you're going to map a
>> huge chunk of video memory into the 6809 address space, and how you're going
>> to move any decent portion of the screen around in real time.
>
> I sure have not.
>
> A 640x480 8 color screen requires 307200 bytes. There is almost no way a 6809/6309 could
> do that other than running at about 250MHz or changing teh data buss width.. I realize the
> limitations of an 8 bit databus.

???

A 640x480 16 color screen requires only 153600 bytes, if it takes 10 
cycles to move a single byte, the CPU could replace the entire screen 
roughly 150 times in a second at 250MHz.  This seems more than 
sufficient.  In any case, if one is making a CoCo4, one does not have to 
just just unaccelerated bitmap graphics.  You could have an auxiliary 
processor that is capable of performing functions similar to the GFX2 
commands used under OS-9 Level II.  This would make higher resolutions 
and greater color depths practical.  As for the data bus width, the CoCo 
3 already uses a 16-bit data bus for memory access by the GIME.

How many times do you think the CPU needs to be able to completely 
redraw the screen in a second?