Re: assistance with disk ][ and modern 1.2M floppies

[david@uow.edu.au (David Wilson)]

dempson@actrix.gen.nz (David Empson) writes:
>> SD disks were only guaranteed to hold a maximum of 40 tracks

Unless you got some early ones with short slots that could only do 35.

I have never had problems using 80 tracks on DSDD disks (with a drive
designed to do this ie HD drive strapped for DD only). If you look at the
spacing between tracks compared with that between bits on a track you would
see that provided the centre hole is rigid enough and the drive has a good
stepper, 80 tracks is easy. It did depend on your HD drive as some do DD
at 360rpm and 300kb/s while others slow down the drive to 300rpm.
I got 640KB on my Hitachi Peach by using NEC FD-1155C drives instead of
the 40 track drives it came with.

disk    speed           data rate       trk     sector  trk/hd
size    rpm     s/rev   kb/s    scheme  kb      cnt*sz  count   KB
--------------------------------------------------------------------------------
5.25"   300     0.200   250     MFM     50.00   9*512   40/2    360     DSDD (3)
5.25"   300     0.200   250     MFM     50.00   9*512   80/2    720     DSDD

5.25"   360     0.167   300     MFM     50.00   9*512   40/2    360     DSDD (4)
5.25"   360     0.167   300     MFM     50.00   9*512   80/2    720     DSDD

5.25"   360     0.167   500     MFM     83.33   15*512  80/2    1200    DSHD (4)

(3)     IBM PC
(4)     IBM PC/AT

>On a PC, I believe double density disks are required if you want to use
>MFM (160K/180K for single sided, 320K/360K for double sided).  A
>theoretical FM formatted 5.25" disk could use single density, and I
>imagine it would hold half the data of an MFM disk (i.e. a maximum of
>90K for a single sided disk).

Seems correct.

>My memory is a bit hazy on 8 inch disks - I used them on a CP/M machine
>for a while.  Assuming I remember the details correctly, a single
>density 8 inch disk was 40 tracks, and a double density one was up to 80
>tracks (our CP/M system used 77).

77 was the maximum on 8" floppies.

disk    speed           data rate       trk     sector  trk/hd
size    rpm     s/rev   kb/s    scheme  kb      cnt*sz  count   KB
--------------------------------------------------------------------------------
8"      360     0.167   500     FM      41.67   26*128  77/1    260     SSSD (8)
8"      360     0.167   500     FM      41.67   26*128  77/2    520     DSSD (8)

8"      360     0.167   500     MFM     83.33   26*256  77/1    520     SSDD (9)
8"      360     0.167   500     MFM     83.33   26*256  77/2    1040    DSDD (9)

(8)     IBM 3740
(9)     IBM System 34
--
David Wilson  School of IT & CS, Uni of Wollongong, Australia