Tandy 600 Programmer's Reference Manual page 56

Internal files
under
HH
0/S have a
file
size
limit.
Files
are
not allowed
to
grow
larger
than the
current size
limit
on
that
file.
The
default size
limit
for
an
internal
file is
FFFFh
bytes (64k-1).
By use
of
function
E1h,
it
is
possible
to set
smaller size
limits.
An expand
file
request
which
would cause
the
file to
exceed the current size
limit
on
that
file
will
fail.
The
value returned
in
CX
will
be
the size
of
the largest
possible block that
could
be added before the current size
limit is
exceeded.
If
methods
1,
or 2 are
requested, the current
file
data
will
be
moved
to
form a hole of the
requested
size.
For
method
2,
the byte
at
the current
file
position
will
be
moved
to
become
the
next byte following the insertion.
Following the
operation, the current
file
position
will
be
set to
point to
the
beginning
of
the inserted block.
Error
Returns:
51