Apple IIe - Page 260

260 Apple
lie
Users
Handbook
The
checksum
is
a
number
which
the
Apple
calculates
from
the
data
when
it
is
saved. This
number
is
unique
for
any
specific
data.
Therefore,
if
the
checksum
does
not
verify
when
it
is
read, an
error
most
likely
occurred
when
the
data was
written
to
the
cassette.
Saving and Retrieving Data from Disk
Unfortunately,
the
cassette
is
slow
and
very
prone
to
errors.
Therefore
most
users
prefer
a disk
drive
for
data storage.
How-
ever
,
the
monitor
does
not
include
commands
for
reading
or
writing
to
diskette
. In
order
to
save
assembly
code
on
disk,
the
user
must
first
activate
BASIC,
Then
,
the
binary
save (BSA
VE)
or
binary
load
(BLOAD)
commands
must
be
executed
in
order
to
save
or
read
the
data.
To save
the
same
memory
addresses
as
were
saved
in
the
cassette
example
(1000-1040),
the
following
BASIC
command
would
be
required:
*CTRL-C -
into
BASIC
]
or
>
BSA
VE
TXT
A,A$1000,L$41
TXT A
is
the
name
of
the
disk
file
. A stands
for
at
memory
loca-
tion.
L stands
for
length
of
memory
block
.
In
the
preceding
example,
the
block
of
data
beginning
at address $1000
and
extending
to
$1040
would
be
saved
on
disk
with
the
filename
,
TXTA.
To
recall
this
block
of
data
from
disk,
type
:
]
or
>
BLOAD
TXT
A,A$2000
This
command
will
load
the
file
at
locations
2000
to
2040.
If
"A$2000"
had
not
been
entered
on
this
line,
the
binary
file
would
have
been
loaded
at
the
same
locations
from
which
it
had
been
saved
(1000-1040).
It
is
good
practice
to
verify
that
data
written
to
the
cassette
unit
has
been
saved
accurately.
Although
errors
are
encountered