[Coco] [Color Computer] Re: POKE 65495,0

[Darren A.]

>From: Robert Gault <robert.gault at worldnet.att.net>
>
>   Don't ignore the facts to fit the data sheets. Several of us have
>posted that with Coco1 F-board machines, we could run programs in full
>double fast mode with disabled video. When the crunching of the programs
>finished, they returned to normal speed and video without using the
>intermediate half-fast setting none the worse for the experience.
>   There must be a reason why some Cocos can get away with this while
>others can't. My programs at least made no attempt to refresh memory.
>
>--

Here is a simple BASIC program you can use to see how 'immune' your coco 1/2 
is to the loss of RAM refresh supplied by the SAM:

10 PMODE 4:PCLS 0:SCREEN 1,1
20 POKE 65497,0
30 SOUND 255,255
40 POKE 65496,0
50 GOTO 50

Running this program will cause loss of RAM refresh (and video) while 
playing a long sound. Afterwards it restores video so you can see if any 
pixels in the PMODE 4 screen have decayed. You need to press BREAK to exit 
the program.

If you end up with a solid black screen with a white border then your CoCo 
is probably immune.

If your CoCo crashes or has some non-black pixels in the PMODE 4 screen, 
then your DRAM is decaying while refresh is disabled.

I have three CoCo 1 F-boards, one 64K CoCo 2 and one TANO Dragon. The 
Dragon, CoCo 2 and one of the F-boards seem to be immune. The other two 
F-boards crash every time unless I reduce the duration value of the SOUND 
command down to around 160 or less. Decayed pixels (bits) can be seen on 
those CoCos with SOUND durations as low as 40.

Interestingly, if I replace the SOUND command with a long FOR-NEXT loop 
(20000 iterations), no decay is observed. Very little RAM access occurs 
during the SOUND command so there is less opportunity for refresh to occur 
from reads.

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