[Coco] Color basic 1.0, 1.2 differences.

[Darren A.]

>From: Robert Gault <robert.gault at worldnet.att.net>
>
>Phill Harvey-Smith wrote:
> ><snip>
> > Ahh, but I was talking about plain non-extended color basic, version
> > 1.0, which seems in software to only detect 4K or 16K rams. Do you know
> > what version of Color basic is in there ?
> >
> > Cheers.
> >
> > Phill.
> >
>
>The ROM is Color Basic 1.1 in my F-board Coco1. Looking at the source
>code for the 1.0 ROM, it reads a PIA at $A057 to determine 4 or 16K of
>RAM and sets the SAM at$A06A for 16K. However, at $A0AB the ROM has a
>routine which looks for the top of RAM by testing memory. It stops when
>memory can't be changed. I'm not sure what would happen if 32K or more
>of RAM was present, the SAM set for 16K, and data stored above $4000.
>
>All you need to do manually if more than 16K RAM is present would be to
>STA $FFDD, STA $FFDA after the initial boot is complete. That will
>indicate 64K dynamic RAM. This could easily be put in a modified disk
>ROM or entered from the keyboard.
>
-
According to the data sheet for the SAM, you need to properly setup the 
memory size control bits *before* RAM is accessed. These control bits don't 
just determine how much memory can be addressed, but how the SAM interacts 
with the the type of RAM chips (4K dynamic, 16K dynamic, 64K dynamic or 64K 
static are the possible options).

The Color Basic ROM sets these very early in the initialization sequence 
before any RAM is used. It does not appear that doing it later in a modified 
Disk Basic ROM or by a POKE command would be appropriate. You would have to 
modify the Color Basic ROM, which of course is what Tandy did in the 1.1 
version.

Darren

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