VECTOR
GRAPmC,
INC.
Vll-14
Extent
(128 Records)
Allocation
. Units
of
Same File
. I File
Cont~1
Block
I File Control Block
File
Control Block
allocation units
extents
records
-FILENAME.EXT
Directory
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FIGURE 3 - VECTOR 4 CP/M'S FILB STORAGE
AND
ACCESS METHODS
As you
enter
data
at
the
terminal, CP/M counts each 128 bytes as a
recorg.
It
counts
128 records
entered
to
the
same file
as
an
extent.
When you copy
to
disk,
the
Basic Disk Operating System sends
the
file
to
an
allocation
unit,
which
is
a
"geographic"
section
of
the
disk
surface.
This way,
the
BDOS
needn't
move information already on
the
disk.
The
BDOS
keeps
track
of allocation units (the
wayan
office manager
might
keep
track
of
the
paperwork
in
several
drawers)
in a
File
Control
Block or FCB.
When
you
call
the
file,
the
BDOS
looks up
the
FCB for each file and copies
the
file into the Transient Program Area.
When you
type
"DIRn
for
a directory,
the
Console Command Processor
simply reads
the
File Control
Blocks
for
the
disk,
then
displays
the
file
names
on
screen.
Up
to
128 FCBs
can
be
held in memory
for
each disk in use, making file access even
faster.
7100-0001 9-01-82 - Rev. A
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