Re: Making the ][+ better

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

Robert C. wrote:

>I have been looking at the similarities and differences between the
>][+ and the //e and wondered what, if anything, could be changed.
>
>The ][+ has a 1MHz 6502; the //e has the 1MHz 65C02. The two chips are
>pin for pin identical (Rockwell R65C02). However, the //e can access
>128K to 1Mb of memory; the ][+ can only access 48K of onboard RAM, 64K
>with the bank switched 16K board in slot 0.
>
>Can I "piggyback" a 65C02 processor onto the original 6502 socket of
>my Apple ][+, and add more memory on the "piggybacked" board?
>
>If the two chips are pin identical, how does the R65C02 access the
>extra memory?

The two processors are identical with respect to their addressing
capabilities.

The additional memory in the //e AUX slot is addressed by bank
switching built partly into the //e main board and, primarily, into
the AUX slot memory card.

The ][ and ][+ are also capable of addressing large amounts of
memory by bank switching, but in these machines, it is usually
mapped into the 16KB "language card" window (with a 4KB
"sub-bank" for each bank).

The Apple II Memory Expansion card (the "Slinky" card) uses
another approach: providing an address register on the card which
is auto-incremented on each access to facilitate reading or
writing sequential ranges of expansion memory, which can be
up to 1MB.

Note that _no_ 8-bit Apple II provides direct addressing of 
more than a 64KB address space at a time--the processor
architecture simply does not support it.

-michael

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