DEF USR¶

Syntax¶

DEF USR[<digit>]=<integer expression>

Versions: Extended, Disk

Purpose¶

To specify the starting address of an assembly language subroutine.

Remarks¶

<digit> may be any digit from 0 to 9. The digit corresponds to the number of the USR routine whose address is being specified.
If <digit> is omitted, DEF USR0 is assumed.
The value of <integer expression> is the starting address of the USR routine.
Any number of DEF USR statements may appear in a program to redefine subroutine starting addresses, thus allowing access to as many subroutines as necessary.
See Appendix C, Assembly Language Subroutines, in the original MBASIC documentation for details on writing assembly language routines.

Example¶

200 DEF USR0=24000
210 X=USR0(Y^2/2.89)

This example defines USR routine 0 to start at memory location 24000, then calls it with a calculated parameter.

Implementation Note¶

⚠️ Not Implemented: This feature defines the starting address of assembly language subroutines and is not implemented in this Python-based interpreter.

Behavior: Statement is parsed but no operation is performed

Why: Cannot execute machine code from a Python interpreter. DEF USR was used to specify memory addresses where hand-written assembly language routines were loaded for performance-critical operations or hardware access.

Alternative: Use DEF FN to define custom functions in BASIC instead of assembly language subroutines.

Historical Reference: The documentation above is preserved from the original MBASIC 5.21 manual for historical reference.