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SYSTEM DESCRIPTION
24. 1 -- Diskette Structure
Diskette
Identi~ication
Block has the rollowing format:
Bytes
Size
Contents
-----
--------
0-7
8
Diskette ID
8-9
2
Version number
$A-B
2
Revision number
$C-l1
6
Generation date
$12-25
$14
User name
$26-39
$14
MDOS overlay
RIB
addresses
$3A-$7F
$46
Zeroes
24.1.2
Cluster Allocation Table
The Cluster- Allocation Table (CAT) contains a bit map
Or
the
areas
on
the
disk~tte
that are available for new space
allocation.
Each
bit
in
the
CAT
represents
a
physical
cluster of diskette stDrage.
The rirst bit of the first byte
of the CAT (bit 7 of
byte
0)
represents
cluster
O.
The
subsequent
bits represent subsequent clusters.
A bit set to
one indicates that the cluster is allocated.
If
a bit is set
to
zero,
it
indicates
that
the
corresponding cluster is
available for allocation.
Since not all
128
bytes
o~
the CAT
correspond
to
physical
clusters)
the parts of the CAT that
~epresent
clusters beyond the physical end
of
the
diskette
are
marked
as
allocated so that they cannot
be
used
by
any
MDOS functions.
an
single-Sided
diskettes}
bytes
Q-$3E
of
the
CAT
correspond
to
the
physical
locations
on
the
diskettei
however,
in
byte
$3EJ
bits
0-3
are
set
to
one
since
no
physical
sectors correspond to·those cluster numbers.
Bytes
$3F-7F are
set
to
all
ones.
The
cluster
division
for
allocation only includes
2000
(decimal) sectors.
Since there
are
2002
sectors,
the
last
two
phYSical
sectors
or
a
Single-sided
diskette
are
not
available
for
allocation
($7DO-7D1L
On double-sided diskettes, bytes O-$7D correspond to the
physical
locations
on
the
diskette; however,
in byte $7D,
bits
0-6
are set to one since no physical sectors
correspond
to
those
cluster numbers.
Bytes $7E and $7F are set to all
ones.
The
cluster
division
for
allocation
includes
all
physical
sectors
(4004.
decimal>.
There
are
no
unused
sectors
on
a double-sided diskette.
24.1.3
Lockout Cluster Allocation Table
The
Lockout 'Cluster
Allocation
Table,
or
LCAT,
is
similar
to
the
CAT
in structure; however,
it is only used
during the DOSGEN and REPAIR processes.
The LCAT provides
a
Page
24-03
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