Re: Why won't the Unidisk 3.5 work with . . .

[nparker@cie-2.uoregon.edu (Neil Parker)]

In article <hy7d5qL.rogtha@delphi.com> rogtha@delphi.com writes:
>Why won't the Unidisk 3.5 work with the 3.5 Bit Copy on Copy II Plus?

To put it simply, the Unidisk 3.5 is too smart for its own good.

The Unidisk 3.5 has its own internal CPU and a small amount of memory, and
this internal CPU has sole access to the actual disk drive hardware.  In
order to do a bit copy on the Unidisk 3.5, you would have to write a program
to read and write entire tracks from the drive's internal memory, and
transfer the complete track image back and forth between the drive and your
computer's main memory.  Since the program would need access to the drive's
hardware, it would have to run in the drive's memory.

The problem is that the drive only has about 2.5K of RAM.  But even the
shortest tracks near the middle of the disk contain over 5K of raw data
apiece--there's no way to make an entire track fit into the drive's
memory.


If you really wanted to work hard at it, you could probably come up with
some scheme for transferring bytes directly between the computer's main
memory and the drive hardware via a special-purpose routine in the drive's
memory, but it wouldn't be easy, especially on a standard 1 MHz Apple IIe,
since the computer would have to communicate with the drive at the drive
CPU's speed of 2 MHz.

Given the difficulty of this job, the authors of Copy II+ decided not to
bother.  There isn't much copy-protected software for the Unidisk 3.5
anyway, because designing a decent copy-protection scheme for the drive is
almost as difficult as designing a nibble-copier for it, for exactly the
same reasons.

               - Neil Parker
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