262
The
VIC
20
User
Guide
DISKETTE
SECTORING
If you manually rotate the diskette inside its jacket, you will find one
or
more small holes in the diskette which come into alignment with the small
hole in the jacket.
If
there
is
only one hole, the diskette
is
soft-sectored. If
there are several holes, the diskette
is
hard-sectored. The holes in the
hard-sectored diskette are used on some disk systems
to
position the sectors
on the diskette. Since the
1540
disk drive provides its own special sectoring,
you should only use soft-sectored diskettes.
DISKlnE
DIRECTORY
Track
18
on
the
1540
diskettes
is
used for the directory. The directory
contains the names, starting sector addresses, and file types
of
all the
files
on
the diskette.
To list a diskette's directory, load it into memory by entering
LORD
"'"
1
8
The name
of
the directory file
is
$ (dollar sign), and
we
are loading it
from device number 8 (the disk drive). To list the diskette directory, simply
type
LIST
and the directory will be displayed on the screen.
THE
BLOCK
AVAIWILITY
MAP
(BAM)
The BAM resides on track
18
of the diskette.
It
contains information
pertaining to memory space allocation on the diskette.
INITUWZATION
Each time the disk drive
is
accessed,
it
cOnipares the identifier number
on the diskette with the
ID
number stored in disk memory.
If
the
ID
numbers match, the disk drive proceeds with the instruction it was given.
If
there
is
a mismatch, the disk drive automatically initializes the diskette.
When the diskette
is
initialized, the contents
of
the BAM are copied
into disk memory. This tells the disk drive which sectors are available to be
written to.
If
the
ID
numbers
of
two diskettes are the same, the disk drive
will not initialize automatically.
It
will not update the BAM with the
allocation information and may write over sections
of
other programs
or
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