VECTOR 4 USER'S
MANUAL
INTRODUCTION
TO
VECTOR 4 CP/M
As
a file is erased or made smaller, the master
list
(and
of
course
the
file's
FCB) is
corrected
to
show
the
new
free
disk space, which means
that
new
information
can
be
written
there.
This
technique,
called
"dynam
ic
file
management", is one of
the
reasons
why
the CP/M operating system has proven
so popular since its introduction.
Because
of
this
organization,
the
BDOS
can
eliminate
all
but
the
desired
allocation unit and begin searching there
rather
than having
to
read the
entire
disk.
This is similar
to
the
way you
may
play only one
track
on a phonograph
album, by moving the
tone
arm
where
you
want
to
begin.
But
due
to
the
speed
with
which
the
computer
can
operate, the
exact
location on the
surface
of
the
disk where the
BDOS
writes
data
is of
no
consequence.
Rather
than
having
to
follow a sequence, such as
the
grooves in a phonograph record, the
computer
can
move
the
disk and read-write head
to
any
record
position
with
very
little
search time.
How
You See
This
The
final
step
in
CP/M's
storage
technique is forming
the
disk directory. At
the beginning of the
data
storage
area
(that
is,
after
the
system
tracks)
of
each
disk
or
diskette, CP/M writes an entry for each
extent
of each fUe. The
first
entry for
each
file
is
displayed
when you
enter
the
"DIR" command.
(The
other
entries
simply
tell
the
computer where
to
go when
it
reaches
the
end
of
the
first
extent.) This is
possible
because
the
directory
you
see
is
acopy
of
the
first
32
bytes
of
the
File Control Block for each document, and
the
FCB is automatically updated when any change is made
to
the file.
When you
first
access
a
disk
or
diskette,
Vector
4 CP/M copies up
to
128
FCBs from the directory into memory. This way, the memory already "knows
ll
where
to
look on
the
disk
for
any
file
for
which
it
pulls an FCB. For a
floppy, this means
that
the
entire
directory
is
copied
(since
a
double-sided
floppy
diskette
can
hold 128
entries,
while
a single-sided holds 64). For a
hard disk, you
may
get
only a portion of the fUes actually on record.
Since
you
can configur'e your hard disk
to
hold up
to
1,024
separate
files, the
directory in memory will obviously not manage all
of
them
at
once.
But
for
smaller
directories,
you
can
still
get
the
entire
contents into memory.
In
any
case, this means
that
the Basic Disk Operating System
doesn't
need to
look
at
the
disk
to
find out where
it
has to look
on
the
disk, and this means a
faster
operation
to
you.
Rev. A - 9-01-82
7100-0001
VII-1S
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