3 Jun
2007
3 Jun
'07
1:41 p.m.
On Sun, 3 Jun 2007, Roy J. Tellason wrote:
Well, my recollection is that the single-sided disks
on my Osborne held 183K.
The MFM/Double Density.
The single density was 40 track * 10 sectors per track * 256 bytes per
sector. (100K)
Allowing for the possibility of using 35, instead of 40 tracks (SA400),
and 128 byte sectors (due to the "overhead"/gaps, could usually only fit
18 per track) would give 80640 bytes (78.75K), although that could also be
obtained by 9 * 256.
A single-sided "pc" disk held roughly 180K
as well, so half of that would be
90K. Those were 9 sectors/track, and I seem to remember an 8 sectors/track
format as well, giving 80K? Not too far off, I guess.
The single sided "360K" formats were, indeed 160K (DOS 1.xx) and 180K
(DOS 2.xx) (40 * 512 * 8, 40 * 512 * 9)
But, "half" is inaccurate.
The name "Single Density" for FM did not exist until AFTER the name
"Double Density" came about. Just as "The Great War" was not known
as
"World War I" until "World War II" began to be discussed.
"Double Density" was a MARKETING name, produced by those who feared the
dissemination of the actual information about it more than cannon.
(Good sig quote!)
FM provided 1 bit of info for every two pulses; hence, some recording
engineers would prefer to see it called "Half Density". MFM permitted the
omission of some, but not all of the clock pulses, thereby permitting
squeezing twice the raw data transfer rate, and getting ALMOST, but
definitely not quite, twice the info in the same space.
Although you could get 10 * 256 on FM, you could only get about 18 256
byte sectors with MFM, and putting 10 512 byte sectors in MFM required
seriously compromising the size of the intersector gaps to squeeze them
in.
Information is more dangerous than cannon to a society
ruled by lies. --James
M Dakin
--
Grumpy Ol' Fred cisin at xenosoft.com