[Coco] POKE 65495,0

[L. Curtis Boyle]

On Mon, 29 Jan 2007 07:05:14 -0600, Robert Gault  
<robert.gault at worldnet.att.net> wrote:

> Diego Barizo wrote:
>> According to a text in
>> http://www.old-computers.com/museum/computer.asp?st=1&c=508
>> the double speed poke did not really switch the machine to 1.8 MHz.
>>
>> That doesn't seem right... or is it?
>>
>> Diego
>>
>>
> The statement is partially true and partially false. There are two
> double speed switches on a Coco1 or Coco2, $FFD6/$FFD7 $FFD8/$FFD9. The
> first lets ROM access occur at 2MHz but RAM (user programs) stays at
> 1MHz. This switch does not exist on the Coco3. The second runs all
> memory at 2MHz.
>
> The article claims, "Also if you were using the upper 32k of ram, the
> double speed poke would wipe the memory." which is wrong. I ran some of
> my ml programs using the $FFD9 switch which ran both RAM and ROM at
> 2MHz. Video was lost during this period but immediately returned to
> normal when the clock rate was slowed. So RAM was not wiped.
>


I think I know how that latter bit (wiping memory) started... You COULD  
lose RAM contents running the full (65497) double speed POKE _if_ you were  
running piggy-backed chips. My Coco 1 had 2 sets of piggy-backed 16K RAM  
chips originally, and it would scramble memory (and usually outright  
crash) if you ran it for more than a few seconds. It also had problems  
with paging graphics RAM into the 2nd bank... it would just display  
garbage if the start address of the screen was >$3fff. Rainbow had a  
little PMODE 0 demo of a an umbrella rotating in 3-D that used something  
like 12 pages, and on my machine (at the time) it would get through about  
half of them (the halfway point was actually in the middle of a graphics  
page, so it would display the top half of the image fine, and the bottom  
was garbage). Musica had an option for full double speed (losing the video  
refresh) for better quality music, and it would crash within 5-10 seconds  
of running it that way.
     All of these troubles disappeared when I got my machine upgraded to  
64K with 1 set of 4164's. This was a D board Coco 1.


-- 
L. Curtis Boyle