SSSD?

["Joel" <joelbuckley54.nospam.@hotmail.com>]

Joel wrote:

> Of course, the real spec for an Apple II disk is SSSD.

Roger Johnstone wrote
> Single-density? When I was still in school I had a floppy disk which
> used to give a lot of I/O errors. I was young and disks cost about the
> same as a box of them cost now (and this was circa 1987), so I continued
> to use it. Years later I came across the disk again and for the first
> time noticed the label. It was a single-density disk! Single-density
> 5.25" drives that I've heard of typically got about 80KB on a disk.

Joel replies:

I believe the SSSD spec is correct.
When the Disk ][ was introduced, there was no double density.
The Disk ][ was a single density drive using GCR encoding to increase
storage capacity.
The Apple's 5+3 13-sector 113k floppy  and 6+2 140k floppy has the same bit
density as the standard 4+4 70k
After DSDD drives came into wide use on the IBM, everybody started using
them for the Apple as well.

Here is a comparison of floppy data capacity, historically speaking.
IIRC:
This is by memory, so I may have errors like # sectors vs tracks in 3.5 HD,
but I'm pretty sure of the total capacities.
Also, Apple 3.5 had variable sectors/track, average was 10.

spt - sectors/track
bps - bytes per sector
tpd - tracks per disk
s - sides (or heads)
cap - capacity (bytes) and kilobytes

format coding spt bps tpd s cap cap (k)
standard 4+4 8 256 35 1 71680 70
Apple DOS 3.2 5+3 13 256 35 1 116480 113.75
Apple DOS 3.3 6+2 16 256 35 1 143360 140
Apple 40 trk (non-std) 6+2 16 256 40 1 163840 160
Apple SSDD3.5 DD 10 512 80 1 409600 400
Apple DSDD3.5 DD 10 512 80 2 819200 800
Apple DSHD3.5 HD 18 512 80 2 1474560 1440

SSSD5.25 4+4 8 256 35 1 71680 70
SSSD5.25 4+4 8 256 40 1 81920 80
DSHD5.25 HD 32 256 80 2 1310720 1280
SSSD5.25 4+4 9 256 40 1 92160 90
DSSD5.25 4+4 8 256 40 2 163840 160
DSSD5.25 4+4 9 256 40 2 184320 180
SSDD3.5 DD 9 512 80 1 368640 360
DSDD3.5 DD 9 512 80 2 737280 720
DSHD3.5 HD 18 512 80 2 1474560 1440
DSXD3.5 XD 36 512 80 2 2949120 2880

At its introduction, Apple's 13-sector format added almost 60% capacity over
the industry standard.
The 16-sector upgrade made it almost double. Impressive, for a while.
Alas, while the rest of the industry went 40 tracks, double-sided, double
and high density, Apple stood still.
Also, by doing so much disk control in software, with critical timing, (as
well as integrated video circuitry), you couldn't easily upgrade system
performance by simply putting in a faster processor.
IBM drives became fast and cheap with higher capacity, Apples remained slow
and expensive and low capacity.

With the 3.5 drives, Apple non-standard format added a more modest 25%
capacity.
But once again they fell behind when the industry went to high density.

Lack of intercompatibility made file transfers between the two groups a pain
in the butt.
This format was mainly used by some CP/M computers of that time.
But it was also supported in early versions of PC-DOS/MS-DOS.