Re: back-up disk anomalies

Tom Stiller <tomstiller@xxxxxxxxxxx> wrote:

In article <1i5etxy.118l6gcleabb2N%nospam@xxxxxxxxxxxxx>,
nospam@xxxxxxxxxxxxx (Richard Maine) wrote:

heron stone <heronDO@xxxxxxxxx> wrote:

i say ONLY 200 MBs
...

hell... i remember the original mac
for me, everything... system plus apps and docs existed
on a single 400Kb floppy

And I recall the days when a 128k floppy was a luxury on my Apple 2. It
was a fairly expensive luxury as the floppy drive cost somewhere around
$500. I forget exactly; it might "only" have been $400, but somewhere
around there.

As I recall, the 5.25" floppies contained 40 tracks of 16 sectors
(increased from 13) of 512 bytes each for a total of 320k.

Not on the Apple II. They were single sided, 35 tracks, originally 13
then 16 sectors per track, 256 bytes per sector.

The GCR encoding technique (invented by Woz, I believe) used on
13-sector and 16-sector disks was quite a novel invention, as it allowed
higher data density than could be achieved using the standard FM
technique (which required alternating clock and data bits), without
changing the bit cell timing (4 microseconds per cell) and without
requiring an expensive disk controller. If Apple had used FM encoding on
5.25" disks, they would have only got about 10 sectors per track, which
is 87.5 KB per disk. 13-sector GCR got 113.75 KB per disk, and 16-sector
squeezed 140 KB out of it.

The MFM technique used on single/double density 5.25" disks (e.g.
original IBM PC) had the same bit cell spacing (4 microseconds) but
adjusted the timing to allow bit cells to start on 2 microsecond marks.
Some clock bits could also be eliminated, and the combination resulted
in a direct doubling of capacity over FM. For a 40 track 5.25" floppy
using 512 bytes per sector, this allowed 9 sectors per track (slightly
more than could fit if using 256 byte sectors) resulted in 180 KB per
single sided disk, or 360 KB for double sided.

That was in 1981. The Apple II achieved a near doubling of the disk
capacity over FM around 1979 without having to use an expensive drive or
controller.

Apple continued to use GCR (with 512 byte sectors) on their 3.5" disks
(400 KB and 800 KB), but switched to MFM for the 1.44 MB high density
disks. MFM was also used on the 720 KB double density format. The main
reason for the switch was to improve compatibiity with other platforms.

--
David Empson
dempson@xxxxxxxxxxxxx
.

Relevant Pages