Re: Apple II Disk Drive Question

[dempson@actrix.gen.nz (David Empson)]

Jack Tseng <tseng4@hotmail.com> wrote:

> How many sectors and track are max in Apple II Disk Drive?

I assume you are talking about the original 5.25" drive (Disk ][) and
later revised models (UniDisk 5.25, Disk //c and Apple 5.25 Drive), not
the 3.5" drives.

The original format, as used by DOS 3.2.1 and earlier, was 256 bytes per
sector, 13 sectors per track, 35 tracks per disk (single sided), for a
total capacity of 116480 bytes.

DOS 3.3 and Apple II Pascal introduced the 16-sector format (also used
by ProDOS), which involved some improvements in the logic state
sequencer on the disk controller card.  The increase in sectors per
track brought the total capacity to 143360 bytes.

Most Apple 5.25" drives are capable of a few more tracks, so a
nonstandard disk could be created on a specific drive with 36 to 40
tracks, but it might not be readable on all drives.  One of my drives
managed 39 tracks but I haven't done a wide survey.

Some third party drives could manage 40 tracks consistently.

> Is it MFM or GCR 

GCR.  Address fields in both formats are written using 4-and-4 encoding
(effectively FM, with interleaved clock and data bits).

On 13-sector disks, data fields are written using 5-and-3 encoding with
a 32-byte lookup table of valid disk bytes.  Each byte must have bit 7
set and no adjacent zero bits.

On 16-sector disks, data fields are written using 6-and-2 encoding with
a 64-byte lookup table of valid disk bytes.  The rules are relaxed
somewhat to allow a single pair of adjacent zero bits in the byte.

> ? How fast is it? Max bps?

Rotation speed is about 300 rpm.  Data rate is nominally 250 kbps (4
microseconds per bit), derived from the Apple II CPU clock which runs at
about 1.023 MHz, with one bit every 4 clock cycles, so about 255.75
kbps.

-- 
David Empson
dempson@actrix.gen.nz