[Coco] Video memory past the two bits at $FF9B...

[Robert Gault]

Paul T. Barton wrote:
> Guru's:
> 
> Since my NoCan-8MB contains video
> memory bits from 0..F, Is there
> a way to modify NitrOS9 to use it?
> ---
> 
> I've used debug like this:
> 
> DeBug
> DB: .ff9b
>     FF9B 17
> DB: =0F      gives a new video page
> ROM Memory   <---<debug complaining?
>     FF9B 17
> DB: =03      comes back gracefully.
> ---
> 
> Any number put there ($FF9B) gives a
> unique video page. Kinda fun to watch it.
> Does not crash as long as 03 is put back.
> ---
> 
> Not expecting a NitrOS9 re-write,
> Paul T Barton
> 

There is no support in NitrOS-9 for memory over 2Megs and is not likely 
to be so in the future. It would require a major rewrite of system code 
if possible. However, there is no reason that a user could not access 
and use this extra memory through special drivers/programs.

It is imperative that a user program turn off interrupts when accessing 
memory over 2Megs. If this is not done, NirOS-9 could overwrite this 
memory or crash. The problem would exactly match early Disk Basic ml 
programs which did not anticipate more than 128K Coco3 memory. These 
early programs made no provision for masking or using the extra bits in 
the MMU registers.

The user program should not rely on any NitrOS-9 functions while the 
extra memory is engaged. This includes all I/O, audio, video, etc. These 
functions probably can be accessed with custom software but then might 
as well run from Disk Basic as pure machine language.