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Passes a variable to a previously subroutine in machine language. The location of the subroutine has to be defined first, using the DEF USR instruction.

Syntax
USR[<number>](<variable>)
[edit] Parameters

<number> can range from 0 to 9. When omitted, 0 is assumed.

<variable> is a variable or a string can be retrieved by the machine subroutine.
[edit] Example 

https://www.msx.org/wiki/USR