Tandy TRS-80 Owner Manual & Programming Manual page 44

A MORE
DIRECT
APPR O AC
H
>*
"MARIE
ALEXANDER"
record
1
" J
.
DOE"
record 2
"MARK
JONES"
record
3
"BILL
SMITH"
record 4
//
With
each record the
same
length
(the
length
of
a
sector),
the
Computer
can go
directly to
"J.
DOE!'
All
it
has
to
do
is
count
down
to
the second
record.
We
call
this
a
"direct access"
file.
By
direct access,
we mean
you can
directly access
any
record
you
want
in the
file.
A
direct
access
file
has one shortcoming.
Each
record
is
the
size
of
a
sector
—
256
bytes.
Since one
of
these bytes holds
one
character of data,
each
record
is
large
enough
to
hold
256
characters.
This
means
that our
drawing above
is
a
little
mis-
leading.
If
we
illustrated
all
the
empty
space
in
each
record,
they would each have
to
be nearly ten
times as
long.
We
simply
don't
have enough room
on
the page.
If
you're
a
beginner,
all
this
empty
space probably
won't bother you.
An
empty
disk
can
hold
up
to
612
records
—
each 256
bytes
long.
Later
on,
when
you
become more
comfortable with programming,
you'll
probably
want
to
pack
more
records
into a
disk
file.
You can then
progress
to
Chapter
9,
where
we
will
demonstrate
how
to
make
smaller
records.
PUTTING
A RECORD ON
DISK
Enough
theory! Let's
put one
record
in
a disk
file.
Since
it'll
be a
direct access
file,
we
don't
have
to
start
with
the
first.
We'll start
with
the second.
Erase
memory
and
type:
10
OPEN
"D"
t
til,
"NAMES/DAT"
20
WRITE
*1
t
"J.
DOE"
30 PUT *1
.
2
40
CLOSE
*1
The program
looks familiar
.
.
.
except
for
the
word
PUT
in line
30
and
the
"D"
in line 10.
More
on
that
later
. .
Now
let's
add
some
lines so the
Computer
will
read
this
record
back
into
its
main memory.
Type:
34
GET
81
t
2
3G INPUT
#1
,
A*
38 PRINT A$
Note
that line
34 uses another
new
word
—
GET
Hmmm
. .
.
any
ideas?
Let's
look
at
the entire
program:
10
OPEN
"D"i
»1
"NAMES/DAT"
2(3
WRITE »1
,
"J.
DOE"
30 PUT
*1
.
2
3 a
GET «1
i
2
36 INPUT »1
p
A$
38
PRINT
A$
40
CLOSE
*1
RUN
it
. . .
You'll
hear the
now
familiar
sound from
your disk
drive.
The Computer
is
writing
"J.
DOE"
in
the
disk
file
and
then reading
it
back
into
memory.
Here's
how
. .
Line 10
OPENs
buffer
#1
which
will
communi-
cate
with a
disk
file
named "NAMES/DAT." As we
said in
the
last
two
chapters, buffer
#1
is
one
of
the 15
"buffer"
areas
which can communicate
with
your
disk.
Communication
is
being
OPENed
for "D."
"D"
stands
for direct access.
Unlike
sequential
access,
you
don't
have
to specify
whether
you're
OPENing
34