5 Jun
2009
5 Jun
'09
3 p.m.
Ethan Dicks wrote:
400K/800K have variable numbers of sectors per track.
That was done
in the original single-sided drives (400K only) by varying the drive
speed. Since I don't remember hearing double-sided drives change
pitch, they must have done it the way Commodore did with their 5.25"
floppies (starting with the CBM 2040 in the late 1970s), by changing
the bit clock in software (the CBM drives have a "divide-by-N" 16-pin
TTL counter chip, but the lower two bits of "N" are attached to a
6522, so the firmware gets to decide (track by track) to divide the
clock by 12, 13, 14, or 15. In Macs, that functionality is built into
the IWM (Integrated Woz Machine) AFAIK).
No, the Apple 800K drives varied the spindle motor speed also. They
were quieter, so it is less obvious.
Later Macs can read/write PC-compatible floppies, so
there must be a
way to select a bit-clock-divider that is compatible with PC-formatted
media. The rest is all software.
The Apple "SuperDrive"/FDHD floppy drives that supported 720K/1440K
MFM
format in addition to 400K/800K GCR format used variable motor speed for
GCR, and fixed for MFM.
Eric