[Coco] Coco 1 and 2 RAM Expanders (was: Bubble Memory)

[Tormod Volden]

On Wed, Aug 16, 2017 at 8:35 AM, RETRO Innovations
<go4retro at go4retro.com> wrote:
> On 8/15/2017 1:31 PM, Zippster wrote:
>>
>> Well that’s true.  If you want the RAM usable by your video pages (VDG),
>> you’ll have to
>> interface it directly to the SAM (and data latches) so that interleaved
>> memory access can be used.
>>
>> You can still bank it with some additional logic outside the SAM of
>> course, but the
>> lower address lines have to handled by the SAM.  You’d also need a
>> connection to deal
>> with the latches handling RAM data output to the CPU data bus and VDG.
>
> I think my question is more:  If you have access to ~64kB of Video memory in
> "bank" 0, if it were, do you really need access to more RAM for the VDG?

For most purposes, no.

>>
>>
>> I think the easiest way is probably to add additional ram that can’t be
>> accessed for
>> video by asserting SLENB to access expansion RAM (this can even be added
>> at the cart port).
>
> I don't think SLENB pages out RAM, just IO.

SLENB disables internal RAM read, but not writes (as long as the SAM
believes there is RAM at the address). It means 8000-FFEF is "safe" in
SAM map 0.

Tormod