30 Jan
2012
30 Jan
'12
11:26 a.m.
On 30 Jan 2012 at 12:39, Jules Richardson wrote:
I expect that most machines of the era came with
either single or
double-sided drives and the OS/firmware was coded to treat the entire
disk as a single entity.
Exactly HOW the second side of a floppy was used by various
manufacturers was wildly variable--even the order of sectors was
subject to variation.
For example:
MS-DOS (and many other formats) start at the outermost cylinder and
use the lower side sectors, then the upper side sectors, then move to
the next cylinder.
Many CP/M formats use all of the sectors starting with cylinder 0 on
the lower side, then go back to cylinder 0 and use all of the sectors
on the bottom side.
Other CP/M formats use all of the sectors starting with cylinder 0 on
the lower side, then turn around on the upper side and work backwards
from the highest cylinder to cylinder 0.
The National Semi BLC 86 systems start at the middle cylinder (39 on
an 80-cylinder drive) to hold the directory, then allocate top and
bottom sides, alternately working up to cylinder 79 and down to
cylinder 0.
Some, such as the Cifer, add a logical "skew" to heads as well as
tracks, so that the first sector on each track and side is different.
And at least one (I'd have to go to my notes), interleaves the
sectors on the top and bottom side before moving to the next
cylinder.
I suppose arguments can be made for any of these.
--Chuck