BIOS
Disk
Definition Tables
7-6
Position 00
corresponds
to
the high
order
bit
of
the byte labeled ALO,
and
15
corresponds
to the low
order
bit
of
the byte labeled ALl. Each bit
position
reserves a
data
block
for
a
number
of
directory
entries,
thus
allowing a
total
of
16
data
blocks
to
be assigned
for
directory entries. The bits are assigned starting
at
00
and
filled
to
the right until
position
15.
Each directory
entry
occupies 32 bytes, as shown in Table
7-6.
Table 7-6
BLS
and Number
of
Directory Entries
BLS
Directory Entries
1,024 32 times
# bits
2,048 64 times
# bits
4,096
128
times # bits
8,192 256 times
# bits
16,384 512 times
# bits
Thus,
if
DRM
is
127 (128 directory entries),
and
BLS
is
1024, there are 32 directory
entries
per
block, requiring
four
reserved blocks. In this case, the
four
high
order
bits
of
ALO
are set, resulting in the values
ALO
=
OFOH
and
ALl =
OOH.
The CKS value
is
determined as follows .
•
If
the disk drive medium
is
removable, CKS =
(DRM+I)/4,
where
DRM
is
the last directory entry
number
.
•
If
the medium
is
fixed, CKS=O,
and
no
directory records need
to
be checked.
The
OFF
field determines the
number
of
tracks which are skipped
at
the beginning
of
the physical disk. This value
is
automatically
added
whenever
SETTRK
is
called,
and
can
be used as a mechanism
for
skipping reserved
operating
system
tracks,
or
for
partitioning
a large disk into smaller segmented sections.
NOTES
• Several D
PH's
can address the same D PB
if
their drive characteristics are identical.
•
The
DPB
can
be dynamically
changed
when a
new drive
is
addressed
by
changing the
pointer
in
the
DPH.
The
BDOS
copies the
DPB
values
to
a local
area
whenever the
SELDSK
function
is
invoked.
To Next Page
Loading...
