Tandy 1000 Basic Reference Manual page 335

Chapter 10 I
BASIC
Keywords
USR
Function
USR[ nu m ber] (argument)
Calls a user's assembly-language subroutine identified with
number
and passes
argument to
that subroutine.
The
number
you specify must be the same as the corresponding
DEF
USR
statement for that routine. If you omit
number,
BASIC
assumes zero.
USR
lets you call as many as 10 assembly-language subroutines
and then continue execution of your BASIC program.
Before you can execute a
USR
function call, you must define the
subroutine's address in a DEF SEG and DEF
USR
statement.
The DEF SEG defines the address of the segment containing the
subroutine. The DEF
USR
statement defines the subroutine
being called and its offset from the beginning of the segment set
by DEF SEG. See DEF SEG, DEF
USR,
and the section "Inter-
facing With Assembly-Language Subroutines" in Chapter 11.
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