[Coco] [Color Computer] Hardware specifics regarding GIME , RAM and ROM
lamune at doki-doki.net
Wed Apr 29 23:15:02 EDT 2009
> I have a few questions regarding the GIME, RAM and ROM.
> 1) If I understant what I have read so far, if $FFDE is written to,
> then the
> memory will be in ROM mode, meaning $8000-$FEFF will be accessed from
> How does the MMU registers $FFA0-$FFAF affect this? Can those ROM
> be remapped to different memory slots? (Instead of $8000 being at
> can it be read from the equivalent of $18000, etc?)
No, the MMU affects RAM only.
> 2) When $FFDF is written to set RAM mode, are the ROMs no longer
> until $FFDE is written back to?
> 3) Also, if I understand correctly, there is 32K worth of ROM space,
> either 32K internal, 32K internal (Cartridge?), or 16K internal/16K
> external. Does bit 2 (MC2) of $FF90 determine WHICH external ROM (if
> is a multipack in use) is mapped for the 32k external or 16k external?
There is a 32k internal ROM on the CoCo3. And no, that register only
affects which range of addresses toggle the CTS signal at the cart slot.
32k internal would be no addresses mapped to CTS, 16k/16k would be
$C000-$FDFF mapped to CTS, and 32k external would mean $8000-$FDFF is
> 4) Is $FF00 - FFEF ALWAYS mapped for hardware no matter what MMU
> are set or RAM/ROM mode, etc? And are they always mapped to $7FF00 -
> And what about $FFF0-$FFFF, I understand they are mapped to
> the vectors in the ROM. Is this ALWAYS the case no matter what the MMU
> registers and RAM/ROM mode are set for?
$FF00-$FFFF always maps to the hardware and vectors, not $7FF00. You can
use that MMU block ($3F) of RAM completely- it makes no difference what
you set the MMU block that physical 6809 $DFFF-$FFFF is mapped to,
physical address $FF00-$FFFF is always the hardware.
Keep in mind the vectors by default point (mostly) to a non-mmu block of
memory at $FE00-$FEFF (which I believe is actually in MMU block $3F)
> 5) Lastly, I guess bit 3 of $FF90 controls whether the 256 bytes from
> $FE00-FEFF are always present at that address no matter if banks are
> switched around? If so, is this space mapped to $7FE00 to $7FEFF?
I believe that is the case
> I am trying to get a better understanding of how the GIME, RAM and ROM
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