To specify the starting address of an assembly language subroutine to be called from memory by the USR function.
n may be any digit from 0 to 9. The digit corresponds to the USR routine address being specified. If n is omitted, DEF USR0 is assumed.
integer is the offset address of the USR routine. If more than 10 USR routines are required, DEF USR[n] may appear in the program as many times as necessary to redefine the USR[n] starting address.
Add the current segment value to the integer to get the starting address of the user routine.
When an Assembly Language Subroutine is called, the GW-BASIC program execution is paused, and control is transferred to the Assembly Language program. When that program is executed, control is returned to the GW-BASIC program at the point of interruption.
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