Re: Apple II+ 15 Slots?

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

Bryan Parkoff wrote:

>    I have noticed that C800-CFFF are not used for Apple II+.

No, the $C800-$CFFF space is shared by all slots.  The protocol for
using this "expansion ROM" space is described in the Reference Manual.

>  I plan to
>build a motherboard with 14 slots.  Each slot will be mapped to Cx00 like
>below.
>
>C100 - slot 1
>C200 - slot 2
>C300 - slot 3
>C400 - slot 4
>C500 - slot 5
>C600 - slot 6
>C700 - slot 7
>C800 - slot 8
>C900 - slot 9
>CA00 - slot 10
>CB00 - slot 11
>CC00 - slot 12
>CD00 - slot 13
>CE00 - slot 14
>CF00 - slot 15
>
>    Will 6502 CPU be able to manipulate 15 slots through 8 bit bus?  What is
>the difference between 8080 and 6502 which 8080 support 256 I/O.  It looks
>like that 8080 and later models have more I/O than 6502 does.  What do you
>think?

The 6502 uses memory-mapped I/O, which means that it supports a single
16-bit address space which can be divided in any desired way by address
decoders between memory addresses and I/O addresses.

Since the I/O address space is a subset of the memory address space,
it is correct to say that the 6502 provides less _memory_ address space
than other processors that support a separate I/O space with special I/O
instructions.  But the I/O address space on such processors is usually
limited to a much smaller space than 16 bits, so the 6502 actually has
the potential for a _larger_ I/O addressing capability than processors with
dedicated I/O address space.  (Of course, any processor can support
memory-mapped I/O, so the limitation is not fundamental.)

Memory-mapped I/O is a simplification and generalization of I/O control.
One address space is enough, so more than one is redundant.  And with
memory-mapped I/O, any memory-referencing instruction is potentially
an I/O-controlling instruction.  On non-memory-mapped I/O processors,
only the dedicated I/O instruction set (e.g.: IN and OUT) can control I/O.

>    I know that Apple //e and Apple //c will use C800-CFFF as built-in ROM.

And many peripheral cards for all the slotted Apple II's, as well.  This
space is not "available" for the use you intend.

-michael

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