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        ABOUT THE DISK FORMAT

            by nir darey



A diskette is composed of a thin
magnetic disk covered by thin plastic
usually black. The cover has an open
area on both sides exposing the
magnetic disk surface to the drive
for reading or writing.
As the diskette spins in the drive,
the read/write head is actually
over the opening, reading/writing the
disk surface like a cassette recorder
would.

The diskette is divided into tracks.
A track is a ring about the center
of the diskette.
The drive head can be positioned
over any one of the tracks, and data
can be readed from the surface.

A disk can be formated in different
formats. Single Density format is
dividing the disk into 40 tracks of
18 sectors each with 128 bytes per
sector, total of 720 sectors on the
disk, that give us 90 kilobytes on
the disk.
Dual Density or as most known as
1050 Density, the disk is divided
to 40 tracks of 26 sectors with 128
byte each, that give us 130 kb. on
a disk.
Double Density format is dividing the
disk to 40 tracks of 18 sectors each
with 256 bytes per sector for a total
of 720 sector, which give us 180 kb.

now we will talk about how data is
transferred from the diskette into
the computer.
A sector data is a magnetic fields
that being converted into electric
pulses which are fed to the floppy
disk controller.
The floppy disk controller is the
interface between the read and write
head and the drive microprocessor.
the floppy disk controller performs
all sector searches and uses to
transfer data between the
microprocessor and the physical disk.

The disk drives processor receives
a full sector of data every 1/18 of
a disk spin.
This is about 0.0115 seconds.

The format is controlled by the
ATARI ROM C.
sector sequence per track is:
(the track is circular)
18, 1, 3, 5, 7, 9, 11, 13, 15,
17, 2, 4, 6, 8, 10, 12, 14, 16

However, the serial transfer between
the computer and disk drive is
slower, about 0.09 seconds.
Since the diskette is turning at
288 RPM, you will find out that two
sectors can be readed in one disk
spin. This is the concept when
creating fast format.

About the Disk Operating System
format.
DOS 2.0 format the disk at Single
Density total of 720 sectors.
DOS 2.5 can format the disk in Dual
Density total of 1040 sectors.
both formats uses sectors which
contains 128 bytes of data.
in DOS 2.0 there are 707 sectors
free for files, and in DOS 2.5 there
are 1011 free sectors.
you probably ask what happened to the
missing sectors ?, well the DOS uses
most of those sectors as information
about the disk and the files that
are on the disk.

The disk sector map for DOS 2.0:

sectors :   contents       :
----------------------------
1-3      boot information
4-369    free for files
360      VTOC
361-368  directory information
369-719  free for files
720      not used

The disk sector map for DOS 2.5 is
the same as DOS 2.0 with the
additions of the following:

sectors  :   contents       :
-----------------------------
721-1023  free for files
1024      extended VTOC
1025-1040 not used

well that's all what i can say about
the format of the diskette.