Atari 400 User Manual

Alari 810 Di sk Driv e 2 09
Ea ch individu al sec tor hold s 128 byt es of data. Wh en DOS has
access
to the tr ack and sec tor where a particular data item is
being stored, it wi ll on ly have to sea rch 1 28 bytes to find that
item.
Th e result of dividing th e diskette surface into trac ks and
sectors is that access tim e is greatly decreased.
Hard and Soft Sectors
Locating a pa rticular tr ack on the disk surface is a relatively
uncompli ca ted matte r. Th e drive merely moves the h ea d to th e
position on the diskette where the specified track is l ocated,
much lik e the needle on a phonograph is positi oned to the
lo ca tion of a spec ific so ng on a r eco rd album.
However , lo cat in g a particular sector is a m ore diffi cu lt process.
Two di ffere nt me thods are u se d to locate secto rs on a disk; hard
sectoring and soft sectoring.
Both the hard and soft sector metho ds inv olve the use of an
ind ex hole. Th e in dex hole is sh ow n in Illu s tration 7-2. It is
l ocated just to the right of the large hole in the middle of the 5 114
inch diskette.
The
index hole , as shown in Illustration 7-2, is a hole on ly in the
di s kett e's prot ect ive cove ring. Another ind ex hole is l ocated on
th e actual di skette surface inside the envel ope. As the diskette
spins, the ind ex hol e (or hol es) on the di skette surface passes
und e rneath the hole in the prot ective enve l ope .
A light source inside the disk dri ve s hin es lig ht onto the area of
the diskette contai nin g th e ind ex hol e. When an ind ex hole on
the di sk s urf ace is aligned with the inde x hol e on the protective
e nvelop e, th e lig ht will s hin e throu gh to a sensor. The sensor wi ll
relay
informati on on the l ocation of the ind ex hol es, which can
be us ed to ca lculate th e various sector lo ca tions .
Now that we have discussed the co ncepts of loca ting sectors, we
wil l di sc uss the difference between hard and soft sectored
di ske tt es. A hard sectored d iskette co ntain s a number of holes ,
each of which indicat es the lo ca tion of a sector. An extra hole is
used to indi cate the lo cation of the first sector . The location of

Atari 400 Specifications

General

Manufacturer	Atari
Model	400
Type	Home Computer
Release Year	1979
CPU	MOS Technology 6502
CPU Speed	1.79 MHz
ROM	10 KB
Operating System	Atari OS
Keyboard	Membrane keyboard
RAM	8 KB (expandable to 48 KB)
Graphics	ANTIC and GTIA chips
Sound	4 channels
Display	RF output for connection to TV
Storage	Optional cassette tape drive or floppy disk drive
Ports	Cartridge
Display Resolution	320x192 (16 colors)