Re: Apple 2 memory

[mjmahon@aol.com (Michael J. Mahon)]

Vince Briel wrote:

>Is there a way to add more
>> memory and access it by "page flipping"?
>
>Yes, actually this is how the 16K RAM card you have works. Your 16K RAM card
>usually gets filled with Integer Basic and is bank-switched out with the ROM
>on your mainboard. I have a 32K Saturn card that does the same thing except
>their are 2 banks of 16K. It is possible to have a board with much much more
>banks but it would require software to control the switching.

In fact, the Saturn (and clone) 128K card is even more common than
the 32K card, and they both use the same bank switching scheme.

The idea is to store a value in a slot I/O location that selects one
of eight logical 16K cards to be active for reading or writing.  Since
each "16K card" presents 12K of linear address space plus an overlaid
4K sub-bank, the actual use of the total memory is a bit convoluted.

The general problem with bank-switched memory is that it must
be accessed by flipping bank select switches with software rather
than simply addressing it directly.  As a result, software must, in
general, be written to be aware of bank-switched memory.

Some uses map the extra memory to a more generic interface.
For example, RAM disk software, once it is installed in an OS,
allows any program to treat the extra RAM as a fast, volatile disk.

-michael

Check out parallel computing for 8-bit Apples on my
Home page:  http://members.aol.com/MJMahon/