9 May
2011
9 May
'11
3:10 p.m.
> What is the distance from the center of the
spindle to the "directory
> track" (giving myself LOTS of wiggle room) on an 8" disk? Surely radius
> is more important than diameter :-)
On Mon, 9 May 2011, Tony Duell wrote:
IIRC, most 8" drives were 48tpi, and had 77
cylinders. What I don't know
is the radius of the outside cylinder (which would be track 0 on side 0
IIRC). It's got to be A little under 4", of course. I don;t have a spare
8" disk to rip apart and measure the diameter of the magnetic disk
itself, nor do I know how far in track 0 is fro mthe edge of the disk.
Once we know that, obvious places pf the directory are on track 0, track
1 (allowing track 0 to be used for a bootstrap) and track 38 (in the
middle). Which would be 0, 1/48" and 38/48" in from the position of track 0.
Or:
Cylinder 0 side 0, with only a sector or two for boot (NS-DOS)
Cylinder 1 or 2, side 0 are more common in 8" CP/M like formats than
cylinder 0 side 1! In some cases, the DIRectory track is deliberately
placed in the same place as it was in for the single-sided format, AND
there are more than a few formats where side 0 is used first, and side 1
provides tracks 77 - 153, or tracks 153 - 77
Or: the same people who came up with 1,024,000 bytes per Megabyte (or
"35mm equivalent" for focal lengths of lenses on digital cameras) could
easily insist on measuring to the first track AFTER the DIRectory ("system
overhead doesn't count")
Therefore, it is certainly not past the current behaviors of certain
clueless lusers to call it (8") a "3.5 inch radius" disk :-)
Would you get 6" width of a 5.25" drive if you included the AT mounting
rails?
Why don't they use the circumferential measurement of the disk? :-)
or "square inches"? :-)
Do the extra access slots on a "twiggy" diskette give it twice as many
tracks? :-)
How much truth is there in the "bar napkin" story of how the size was
chosen for 5.25" disks?
--
Grumpy Ol' Fred cisin at xenosoft.com