Tandy 600 Programmer's Reference Manual page 39

FUNCTH
AX
BX
CX
DX
XXXXXXXX XXXXXXXX
XXXXXXXX XXXXXXXX
XXXXXXXX
XXXXXXXX
XXXXXXXX XXXXXXXX
Entry
AH =
42h
(function
code)
AL =
Method
BX =
File
handle
CX:DX
=
Offset
SP:
BP:
SI:
Dl;
IP:
FLG
CS
DS
ss
ES
Exit
AX
=
status
code
Function
42h
moves
the pointer
to
the current position
in
the specified
file.
On
entry to
the
function,
BX
specifies the
handle
of
a currently
open
file,
CX:DX
specifies the offset
to
use
in
moving
the
file
pointer,
and
AL
specifies the
method
to
employ.
The
following
methods
are
defined:
Method
1
2
Description
Move
the pointer
to
CX
bytes from the beginning
of
the
file
Move
the pointer
to
CX
bytes
from the current
position
Move
the pointer
to
CX
bytes
after
the current
end
of
the
file
The
value
in
CX:DX
should
be
regarded as a 32
bit
integer with
the
most
significant
1
6
bits
in
CX.
Internal files
are
restricted to
being
no
larger
than
64k
bytes
in
size.
For internal
files
if
CX
is
not
0,
an
error
will
be
returned.
With
any
of
the
above methods,
it
is
possible
to
specify
a location past the current
end
of
the
file.
If
this
occurs, the
file
will
be extended as required
to
allow the operation
to
succeed.
An
error
will
result
if
an attempt
is
made
to
position
the
file
pointer
beyond
the
64k
byte
limit
on
the size
of
an
internal
file.
Error
Returns:
34