In article <3AD5B280.74FCE3A6@NOmlSPAM.com>,
Brian J. Bernstein <bernstbj@NOmlSPAM.com> wrote:
 
> If my rusty Apple II memory still works correctly, I think the use of
> DOS 3.2/3.3 was an upwards-compatbility issue. i.e., if you had one of
> the original Disk ][ controllers, it wouldn't support DOS 3.3 without a
> ROM replacement. I believe that every Apple II disk controller, even the
> IIgs, is capable of working with 13 sector DOS 3.2 disks.
 
There were two small ROM's on the Apple II disk controller:
 
1. The "State Machine ROM" which wasn't connected to the 6502 address
bus but which implemented the disk controller logic: that ROM was
indeed upwards compatible so the 16-sector State Machine ROM could
deal with 16-sector as well as 13-sector disks.  The difference
between the 13-sector and the 16-sector State Machine ROM's was: the
13-sector State Machine ROM was able to deal with only one zero ("0")
bit between one ("1") bits in the raw data written to disk, while the
16-sector State Machine ROM was able to deal with up to two
consecutire zero ("0") bits between one ("1") bits in the raw data
written to disk.  If these rules were violated, the byte read back
from disk could be different from the byte written to disk, leading
to data corruption.  This meant that the 13-sector State Machine
ROM's allwed fewer leal byte values on disk than the 16-sector State
Machine ROM.  Which meant that for each byte on disk, only 5 user
data bits could be encoded with the 13-srctor State Machine ROM,
while the 16-sector State Machine ROM allowed 6 user data bits to be
encoded in each byte on disk.  Thus, each sector of 256 bytes user
data required fewer bytes on disk with the 16-sector State Machine
ROM than with the 13-sector State Machine ROM -- and that's why
the 16-sector disks could store more user data than 13-sector
disks: the bit density on the disk was not higher on 16-sector
disks -- the overhead was just lower.
 
1. The Boot ROM was not upwards compatible -- a 13-sector Boot ROM
could boot only 13-sector disks, and a 16-sector Boot ROM could boot
only 16-sector disks.
 
On my own Apple II+ I piggy-backed a 13-sector and a 16-sector Boot
ROM, with all the pins on one soldered to the corresponding pin on
the other, with one exception: the +5 Volt pin, where I had a switch
which delivered +5 Volts to one or the other of the two Boot ROM's.
This allowed an easy way to switch between a "13-sector disk
controller" and a "16-sector disk controller".  Yes, my Apple II+ was
old enough to originally be supplied with 13-sector disk controller
ROM's.  When I later bought the Apple Pascal system, I got the
16-sector controller ROM's.
 
-- 
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Paul Schlyter,  Swedish Amateur Astronomer's Society (SAAF)
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