VECTOR
..
USER'S MANUAL
INTRODUCTION TO VECTOR .. CP/M
Consider an office
clerical
system. One simple-seeming way
to
file
business
documents
might
be
to
keep
them in one cabinet, with each new
entry
being
filed
in
front
of the one filed
last.
With a few dozen
entries,
this
might
be
practical.
But
raise
the
number
to
several
hundred pieces of paper (using a
new
piece
of
paper
for
every
128
characters),
and you
can
imagine
the
difficulty.
No one
could
ever
find
the
file he or she wanted, and business
would grind
to
a
halt
each time you needed a
document
other
than
the
last
one filed away.
But
if
you
break
the
file organization into larger chunks, you
get
a much more
practical
system.
You
can
file
documents
alphabetically,
by
month,
by
accounts,
or
in
almost
any
way
that
makes
sense
to
you
or
your company.
This
eliminates
all
but
a limited
area
of
the filing cabinet, leaving you
free
to
search
there,
rather
than look for one piece of paper
out
of
all
the
hundreds
which
may
be anywhere inside. And
the
way
that
the computer
arranges
this
higher-level organization is what gives Vector 4 CP/M its speed and flexibility.
Vector 4 CP/M counts every 128 records, and puts them in an
"extent".
Each
fUe is considered
to
have
at
least
one
extent
(whether
it
actually contains 128
records
or
fewer). Furthermore, CP/M
divides
the
surface
of
the
disk
into
"allocation
units". The
size
of
an allocation
unit
can range, depending on
the
disk configuration, from 2,000
to
8,000 bytes.
Make
Room
for
Data
If
the
program must
~hange
any information already stored on disk
(the
wayan
accounting program might
update
a
total
of
accounts
receivable),
the
Basic
Disk
Operating
System
has
another
job
as
well:
to
find
the
data
to
be
updated, then
to
remove, add, or change information as the program
directs
it.
But
the
BDOS
can't
"squeeze"
new
data
into
the
existing fUes,
the
way you
can
in a file folder. The computer must
store
where
it
has space on
the
disk
surface, then remember where each
file
is "continued".
If
a file
needs
more
space
for
new
records,
a
full
new
allocation
unit
is
assigned,
even
if
only one record (128 bytes) is used. Each
extent
can
have
up to eight
allocation
units
assigned
to
it.
All
this
location
or
"address"
information
(disk
drive,
file
name,
extent,
and allocation unit) is
stored
in a
File Control Block
(FCB) for each file. CP/M keeps a
master
list
in memory
of
all
allocation
units
currently
assigned to each file, along with
all
unused
units remaining on
the
disk.
As new
records
are
entered
or
created,
CP/M
updates
its
master
list
and
assigns
new
allocation units where
it
"knows"
it
has room.
The
File
Control
Block
(a
copy
of
which is
stored
in
the
Transient Program Area for
the
file
being accessed) is also updated
to
include the "address" of
the
new
allocation
units.
This
way,
the
operating system keeps
track
of all
the
various
parts
of
the
fUe so
that
you need only
refer
to
it
by
~.
Rev. A - 9-01-82
7100-0001
Vll-13
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