[Coco] CoCo 32k rom architecture

[Barry Nelson]

Ok, but…

How does the rom map both before and after those bits are set? Does it start out with the top 16k half of the 32k rom mapped at $C000, or the lower half? After the bits are toggled, the rom maps to $8000-$FDFF, correct? At that point does the lower half of the rom map to $8000-$BFFF and the top half maps to $COOO-$FDFF? How does this all line up with the format of a .ccc file? Does the file have the lower address half first or does it start with the part of the rom that maps at $C000? Does the cartridge need any special wiring or special hardware?

 

>        Robert Gault robert.gault at att.net  <mailto:coco%40maltedmedia.com?Subject=Re%3A%20%5BCoco%5D%20CoCo%2032k%20rom%20architecture&In-Reply-To=%3C56A79F2B.1030901%40att.net%3E> 
Tue Jan 26 11:30:35 EST 2016 

 

>         > Barry Nelson wrote:
>         > 
>         > Sorry, but that article talks in great detail about designs for a 64k and 128k program pak, but only briefly mentions the 32k paks, which use a different system. I am not interested in breaking the 32k barrier at this time I am just interested in reaching it and in the workings of the standard 32k paks for the CoCo 3 and the format or the .ccc files for these paks. The larger ones can wait for now. I need to understand how the .ccc file layout is ordered, how the rom chip is ordered, and how the 32k mode is activated. Also on my mind is if the 32k paks use the SLENB line to control the bus and allow access to the extra memory space?
>         > 
 
>         $FF90 should give the Coco3 access to 32K External ROM by settings bits 0-1 high.
>         So the code ought to be:
>         orcc #$50
>         lda #%01000111
>         sta $FF90
>         access the external ROM
>         lda #%01000100
>         sta $FF90
>         andcc #$AF
 
>         If you want external ROM always ON, don't restore $FF90.
 
>         Robert