Re: The Apple ][ was cool!

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

william strutts quoted:

>Nope. The controllers existed when Woz designed his controller
>but the cost would have been prohibitive.  He thought he knew
>of a better way to get the cost down and get more data on the
>drive than was standard at the time.

Right.

>From Apple History site:
>"Other issues that Wozniak had to deal with involved a way to
>properly time the reading and writing of information to the disk. IBM
>used a complex hardware-based circuit to achieve this
>synchronization. Wozniak, after studying how IBM's drive worked,
>realized that if the data was written to the disk in a different fashion,
>all that circuitry was unneeded. Many floppy disks sold at that time
>were "hard sectored", meaning that they had a hole punched in the
>disk near the center ring. This hole was used by the disk drive
>hardware to identify what section of the disk was passing under the
>read/write head at any particular time. Wozniak's technique would
>allow the drive to do self-synchronization ("soft sectoring"), not have
>to deal with that little timing hole, and save on hardware."
>http://apple2history.org/history/ah05.html

This passage, while not strictly wrong, contains an error and is
certainly misleading about the nature of Woz's "invention".

First, hard sectored disks have many holes around the center ring.
There is one hole for each "hard sector" plus one extra hole close
to one of the sector holes to mark the "index" or start of the track--
that is, sector zero.

Second, eliminating hard sectoring saves very little in drive or
controller cost, since the drive only requires a photosensor
(about $1), and the controller only requires a one-bit input to sense
where the track starts and when a sector starts.  (Of course, the
user continues to bear some cost burden for purchasing hard-
sectored disks--pretty steep now, since the world abandoned them.)

The main innovations in Woz's controller are:

1) The use of a 2MHz state machine formed by a shift register and
a 256 byte ROM as a digital "data separator" which dynamically
synchronizes with the varying-speed read bitstream and extracts
the 8-bit "nibbles" written to the disk, and

2) The use of GCR (Group Code Recording) to store more data
bits per magnetic transition than was customary with the single-
density recording of the time.  Standard (at the time) FM recording
achieved one data bit per two transition cells, for four data bits
per eight cells, while Woz's initial scheme achieved five data bits
in the same eight cells (5+3 encoding).  Later, with DOS 3.3, the
encoding was changed to 6+2, for six data bits in eight cells.

3) The elimination of seek control from the drive, substituting
complete software control over head positioning.  (This also made
the Apple drives proprietary, since they used a much simpler
analog board.)

These innovations resulted in the following advantages:

1) No dedicated (expensive at the time) floppy disk controller chip,

2) 50% greater data capacity than standard single-density disks, and

3) No seek electronics in the drive, and higher speed software-controlled
seeks, through use of an acceleration-deceleration table.

Soft-sectoring eliminated the photosensor from the drive, although
this was not so much an innovation as a design choice.

Putting the format of the disk almost totally under the control of
software enabled a vast array of non-standard modifications, in the
form of copy protection schemes, which were not only the bane of many
Apple users but also the subject of countless delightful hours of reverse
engineering enjoyment.  ;-)

-michael

 Email:  mjmahon@aol.com
 Home page:  http://members.aol.com/MJMahon/