The Disk Program; At A Time - Tandy TRS-80 Owner Manual & Programming Manual

ONE
THING AT
A TIME
Note: Chapter
2
shows
how
to
format a
di-sk.
(Type
DIB
JN.TEE- if
you
can't
remember whether a
disk
is
for-
matted.)
Chapter
1
explains the drive
numbers.
Then
type:
10
OPEN "0"t
*lr
"CHECKS/DAT"
20
WRITE
»1
t
"DR.
HORN"
38
CLOSE
»1
RUN
the
program.
You'll
hear
the
motor
of the
disk drive
and
see
the red
light.
The Computer
is
at work, doing several tasks.
First,
it
OPENs
communication
to
the disk so
you
can send your checks
out
to
it.
Then,
it
finds
an
empty
location
to
store
the checks
and
notes the
beginning location of that disk
file
in
the
directory.
All of this
happens
in line 10.
Notice the
meaning
of
the "O", #1,
and
"CHECKS/DAT":
1.
#1
is
a special "buffer"
area
in
memory
called
buffer
#1.
It
communicates
with
the disk
drive.
Line 10
OPENs
this buffer.
(If
you've been
using
tape,
you might
remember
that buffer
•
-
1
communicates
with the tape
recorder.)
2.
"0"
is
the letter
"O"
not a
zero.
It
stands
for out-
put.
It
tells
the
Computer
that buffer
#1
will
be
sending out data
to
the
disk.
3.
"CHECKS/DAT"
is
the
name
of
the disk
file.
The
disk's
directory
uses
this
name
to
index
its
beginning
and ending
locations.
In
line 20,
the
Computer
sends out the
words "DR.
HORN"
to
buffer
#1
which
WRITEs
it
on
the
disk.
Then,
in line 30,
the
Computer
CLOSEs
commu-
nication
with
buffer
#1. In doing
this,
it:
•
sends out
all
the
data remaining
in buffer
#1
to
the disk
file.
•
notes in
the
disk's
directory
where
"CHECKS/
DAT"
ends.
Note:
A
buffer
temporarily
stores
data so the
Com-
puter can input
and
output data
to
the disk
in
Mocks
of
249
characters
I
bytes)
.
Since
buffer
#1
only contains
8
characters
("DR
.
HORN"),
they
would
not be sent out
to
the
dish without closing the
file.
It is
very important that
you
CLOSE
communi-
cation
with
buffer #1.
Why?
Well,
let's
leave
buffer
#1 OPEN.
Delete line
30
and
RUN
the pro-
gram
several times.
The program
appears
to
work
the
same
every time
you
RUN
it.
This
is
because every time
you
RUN
(or
LOAD)
a
program,
the
Computer
will
auto-
matically
CLOSE
communication with any
buff-
ers
you've
left
OPENed.
Now,
let's
assume you
switch
disks
and
RUN
or
LOAD
a program.
The Computer
will
automati-
cally
CLOSE
communication
with buffer #1. In
doing
this,
it
will
send out
its
closing
information
to
the
new
disk (thinking
it's
the
old one).
This
will
very possibly garble the contents
of
both
disks.
Now
that we've
warned you
of
the importance
of
line 30, re-insert this
line in
your
program and
RUN
it
again.
This
is
what
the
program
writes
on
your
disk:
<cne
V
y
s
*»r-
"DR.
HORN"
Vote:
Like our
drawing
of
the disk?
The
en
"CHECKS/DAT"
file
consists of the
words
"DR
HORN''
The
disk's directory notes the beginning
and
ending
locations
of this
file.
You can
verify
that the
Computer
has done
this
by
checking the
disk's directory.
You remember
how
to
do
that.
(Type
D
I
R (ENTER))
Because
this
program
sends your data
out
to
the
disk
file,
we'll call
it
an
output program.
READING THE
DISK
FILE
To get the
Computer
to
read
this
data from
the
disk
back
into
its
memory, you need an
input
pro-
gram. Erase the output
program you
now
have
in
26