Apple // Questions

Eric Smith eric at
Sat Apr 23 19:16:31 CDT 2005

Sellam wrote:
> ProDOS was made with large capacity disk drives in mind.

Well, for what passed as "large capacity" at the time.  Back then,
the common 5.25-inch drives were 5MB and 10MB.

When the Apple III operating system SOS was being developed (1979
or so), it was designed to support up to 32MB disks.  The decision
was not so much because 32MB was a target, but because 2^16 blocks
of 512 bytes each works out to 32MB.  The block number can be
conveniently stored in two bytes.

That was before any 5.25-inch disks had been introduced, so at the
time hard drives were generally 8-inch or 14-inch, and were very
expensive.  In 1981 the ST506 5MB 5.25-inch drive was introduced at
a price of around $1000 (not including a controller, which was also
expensive).  Other drive vendors followed suit shortly thereafter, and
Apple introduced the Profile 5MB drive for use on the Apple III.
If I recall correctly, the initiall list price of the Profile was
somewhere between $2500 and $3500.  It had an internal controller,
and only needed a simple parallel interface card in the computer.

About two years later, ProDOS was introduced.  It inherited the SOS
file system design, thus sharing the same 32MB limit.  At the time,
5MB and 10MB drives were common, and 20MB drives were starting to
appear.  Clearly there wasn't very much headroom.


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