Re: 8088 card and bank 15

Hello!

Mia Magnusson wrote:

> B.t.w. while looking at the circuit diagram my impression is that it
> would be rather easy to expand the memory on a CBM-II machine. It could
> be done with more banks and/or with larger banks. If opting for more
> banks the PLA 82S100 in U75 would have to be complemented with another
> with similar content but which treats BP0-3 differently to map the
> extra memory in other banks than 1-4. 

Yes.

> If opting for the same number of
> banks but larger memories in one or more banks it's just a matter of
> mainpulating BP0-3 on it's way to this PLA. Also if opting for larger
> memories there would be a need to substract 1 from the BP0-BP3 signals
> and then mux the resulting BP0 and BP1 (or maybe even more signals)
> onto "new" inputs on larger DRAMs. Seems rather straight forward.
> Either way it would probably not need much special attention to make it
> compatible with the 8088 board.

If you want to install larger chips (for example 21256 instead of
original 2164) you will need to provide 9-bit row/column addresses
instead of 8-bit, this requires an additional 1-bit latch and some glue
logic to make accesses and refresh work.

> Is the U75 PLA socketed on CBM-II boards? As I understood from this
> thread, the first two banks of DRAM is usually soldered directly to the
> board while the last two banks is usually socketed. Is this correct?

I have seen the PLA socketed on all boards I have.

The last two banks are socketed in high-profile machines or just
unpopulated in low-profile (in this case soldering sockets is necessary,
I did this on my machine but it took some work).

Regards,
Michau.


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