Tandy 1000 Basic Reference Manual page 142

Chapter 10 I BASIC Keywords
DEF USR
Statement
DEF USR[number]
=
offset
Defines the user number and segment offset of
a
subroutine to
be called by the USR function.
Number
may be an integer in the range 0 to 9. If you omit num-
ber,
BASIC assumes USR0.
Offset
is an integer in the range 0 to 65535. BASIC computes
where the subroutine begins in memory by adding the
offset
to
the current segment address as set by DEF SEG. BASIC trans-
fers control to this address when you execute the USR function.
If the subroutine is not in BASIC's data segment, you must exe-
cute a DEF SEG statement before the USR function.
A program may contain any number of DEF USR statements, al-
lowing access to as many subroutines as necessary. However,
only 10 definitions may be in effect at one time.
See the section "Interfacing With Assembly-Language Subrou-
tines" in Chapter 11 and USR in this chapter for more details.
Examples
DEF
U S R 3
=
6 H 0 0 2 0
DEF
SEG
=
& H I 7 0 0
USR3 begins at 20H bytes into the current data segment which
is set at 1700 hexadecimal. When your program calls USR3, con-
trol branches to your subroutine beginning
at
absolute address
17020. (1700*10
+
20).
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