Re: What is max IIe RAM? 24meg possible? ;-)

[shack@crash.cts.com (Randy Shackelford)]

In article <2nar63$dfk@cleese.apana.org.au>,
Jonathan Pratt <ntt@cleese.apana.org.au> wrote:

>Theoretically, there's no real limit on the amount of memory you have in a
>][ system, but you need to be able to address it.  For a non //e (aux
>slot) memory card, the info is usualy addressed by bank switching blocks
>of 16 K in the same way as the language card use to do. (Titan cards were
>like this - they had multiple banks of 16k to give 32k, 64k or 128k of
>total expansion RAM)

Slinky cards like RAMFactor and the Apple II Memory Exp. Card give you access
to one byte at a time. You set a three byte address in the card's workspace
and call a firmware routine and you get back the contents of the byte at the
address and the address gets incremented for the next call. You write to memory
in a similar way. So these cards have a theoretical limit of 16 mb.

>The //e (aux slot) cards were a little bit easier to use since they
>actually switched blocks of 64k which can be addressed using the built in
>soft switches.  (you can address various parts of the auxillary RAM with
>the soft switches). For cards larger than 64k, you merely have another
>location (and there were two that were most popular) which switches
>between banks of 64k.  If the location held 00, then the bottom 64 k
>(though it doesn't matter) was addressed, an 01 was the next bank, and so
>on until the whole card was accessed (1meg = 16  {$00-$0F} banks). 
>relatively simple...

These cards have a bank select switch generally located at $C073 which selects
a 64K bank. The selected bank is accessed and behaves in exactly the same way
as the only aux bank in a 128K machine. This scheme also gives a maximum of 16
mb.
--
Randy Shackelford                                "That's right, keep dancing
shack@crash.cts.com                               on the minefield"
                                                                  -Al Bundy