Re: RAM cards for IIe

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

Bill Garber wrote:

>I know I might say 64K when I mean 64 in some places
>but this should explain it for the most part. Sorry
>if my mind can see it, but my fingers can't type it.
>This is the gist of it though.  :o)
>
>First, let us recognize that 16 bits tops out at 64K, so
>therefore 64K of 64K banks would allow us to go to 4MB,
>and each bit that we logic out doubles our possible number
>of banks we can add, so one bit above 4MB gives us 8MB,
>and another bit gives us 16MB, however, this latching of
>bits also increases the amount of time needed to access
>these additional banks. A 1MHz system would be considerably
>slower the more time needed to latch these extra bits. Of
>course, an accellerated system would access them faster,
>the faster we can get it to run. Theoretically, we could
>have 64K of 64K of 64K banks if we so choose to, which
>would allow 256MB of bank-switched ram. Good Luck. You
>would be waiting forever for it to logic out which bank
>you would be accessing, not to mention the cache needed
>to store which bank of banks of banks had the byte you
>wanted to read/write. Where would we put all the needed
>hardware?, outside the machine in a separate box? Ok!

Actually, the bank select bits are set up in a separate
Store operation, so timing is never an issue.

The "natural" data width on the 6502 is 8 bits, so the
natural limit on number of banks is 256.

16+8 = 24 bits of AUX RAM address = 16MB.

I understand that the architecture (if not the implementation)
of the RAMWorks allows for 16MB, and only the relatively
high cost and low density of RAM prevented the manufacture
of a 16MB RAMWorks.

To go beyond 16MB would require at least two stores
to set up a >8-bit bank select register, and would go
beyond the architecture of the RAMWorks card, and
therefore would not be automatically used by any 
software.

And besides, 16MB of RAM is quite enough for an Apple II.  ;-)

-michael

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