[Coco] GFL football 2 on Cocoflash

[Darren A]

On Mon, Jan 15, 2018 at 1:02 AM, RETRO Innovations wrote:

> On 1/15/2018 1:17 AM, Arthur Flexser wrote:
>
>> The ROM contents of $FFF0-FFFF can be read by moving the ROM segment to a
>> different memory location using the MMU.
>>
> Can someone to try this and confirm it, because the COco3 schematic does
> not show this being possible.  The MMU address lines only appear at the Z
> address lines (DRAM multiplexed address lines), and the ROM is directly
> connected to A0-A14.  It *may* be possible to remap the ROM to 0-$7fff, but
> I don't think you can use the MMU to do it.
>
>

On the CoCo 3, ROM can be mapped into both the upper and lower 32K of the
CPU address space, but you cannot move the 8K pages around within either
32K section.  For example, the upper-most 8K of cartridge ROM space
(E000-FFFF) which includes the area normally hidden by the hardware page
(FFxx) can be made to appear at 6000-7FFF, but nowhere else.

The rules that determine whether RAM, Internal ROM or External ROM are
enabled for locations within 0000-7FFF are the same as for 8000-FFFF.
These include the RAM/ROM map mode (FFDE/DF), the MC0,MC1 bits of FF90 and
the MMU page selections. The exception is the MC3 bit of FF90 which does
not have any effect on 7E00-7EFF.

There is a brief article about this on the CoCopedia site:
MMU_RAMROM_Mode <http://www.cocopedia.com/wiki/index.php/MMU_RAMROM_Mode>


- Darren