[Coco] 256K SuperROMPak (32 Banks)

[Tormod Volden]

On Wed, Feb 11, 2015 at 6:50 AM, RETRO Innovations wrote:
> So, what are the options to map RAM into the address space from the cart
> port?  That is the key, right?
>
> I admit my signal understanding is weak, but I would assume SLENB would do
> it, so you could do:
>
> map a latch into IO space on the cart.
> Have one of the bits of the latch drag SLENB active (via some logic that
> only makes SLENB go active on writes on some address range (0000-7fff?)
> Then, any writes to some address range would trigger SLENB, which would map
> the FLASH into the address space.  Hitting the latch again would move the
> FLASH out of the write space.
>
> Someone is going to shoot me design down, but surely something like that
> would work.

I assume you are talking about the need to write to special EEPROM
addresses for programming ($5555 etc).

(On the other hand, for writing to the cartridge address space, the
only issue is that CTS is not asserted on writes, so you have to do
your own address decoding in the cartridge instead.)

The only catch here is that SLENB does not deactivate writes to
internal RAM. The SAM (talking CoCo1/2/Dragon here) always latches the
data bus into the internal RAM according to its memory map. SLENB only
deactivates the ROMs ands PIAs. This might not be a blocker though
because only a few addresses are needed for programming the EEPROM so
you can live with the shadow RAM writes by taking a few software
precautions.

Tormod