1. A method for USING range validation during assembly of an assembly language program, the method comprising: parsing assembler language source code loaded for assembly in an assembler executing in memory of a computer; identifying a USING statement in the source code establishing a base address as the content of a register; responsive to additionally identifying a subsequent statement in the source code modifying the content of the register, generating a warning of an invalid USING range, determining that the subsequent statement modifies the content of the register to a value consistent with an address but an address that differs from the base address identified by the USING statement, and including in the warning text that the register had been the subject of a USING statement and has been directed to hold a value that is a different address than that specified by the USING statement.