Radio Shack TRS-80 Trsdos & Disk Basic Reference Manual page 89

DISK BASIC
Note:
DISK BASIC
automatically creates a
buffer
for loading, saving
and merging
BASIC
programs. This buffer exists in
RAM
below
any data
file
buffers
you
may
request.
It
is
always available for
program I/O, regardless
of
how
you answer
the
FILES?
question.
Suppose you're going to
be using 2
files:
1
for
inputting
data,
1
for
outputting data.
Then you might answer
2 to the
FILES?
question.
However,
if
only
1
of these
files
will
be
open
at
once,
then
you
really
only need
to reserve
1
file
buffer/control block.
Examples:
HOW MRNY FILES?
8
MJH3H
causes
BASIC
to set aside
zero buffers for
I/O
to disk
files.
You
won't be able to
open
files,
but
you
will
have the
maximum
amount
of
RAM
for use
by your
program,
HOW MANY FILES? 15 IdflHdsl
tells
BASIC
to create 15 I/O buffers
and
control blocks;
you
will
then be
able to
have 15
files
open
at
once;
however,
this will
reduce
your
available
memory by 15*290 =
4350
bytes.
HOW MANY FILES?
tells
BASIC
to
use
a default
of
3 for
the
number
of
files
to
be
in
use
at
once.
After
you
answer the
FILES
question,
BASIC
will ask:
MEMORY
SIZE?_
You
respond with the highest
memory
address
(in
decimal form)
you
want
BASIC
to use for storing
and
executing
your
BASIC
programs.
Addresses above the
number you
specify will
then be protected
from use
by BASIC.
Here's
why you might want
to
protect
memory:
You
can load
machine-language programs or routines into high
memory,
and then access these routines
from
DISK BASIC
via
specially
defined
USRn
functions, or via
the
SYSTEM
command.
These machine
language routines
may
be loaded from tape using the
SYSTEM
command,
LOADed
in
the
DOS
READY
mode,
or placed
in
memory
one byte
at
a time using either
DEBUG
or
BASIC
POKE
commands.
If
you
do not
reserve
memory,
such routines
will
be
destroyed during execution of
BASIC
statements.
7-3