Base address

The term base address is used throughout this manual to mean the location counter value within a control section from which the assembler can compute displacements to locations, or addresses, within the control section. Do not confuse this with the storage address of a control section when it is loaded into storage at execution time.

The USING instruction has three formats:

The first format specifies a base address, an optional range, and one or more base registers. This format of the USING instruction is called an ordinary USING instruction, and is described under Ordinary USING instruction.
The second format specifies a base address, an optional range, one or more base registers, and a USING label which can be used as a symbol qualifier. This format of the USING instruction is called a labeled USING instruction, and is described under Labeled USING instruction.
The third format specifies a base address, an optional range, and a relocatable expression instead of one or more base registers. This format of a USING instruction is called a dependent USING instruction, and is described under Dependent USING instruction. If a USING label is also specified, this format of the USING instruction is called a labeled dependent USING instruction.

Note: The assembler identifies and warns about statements where the implied alignment of an operand does not match the requirements of the instruction. However, if the base for a USING is not aligned on the required boundary, the assembler cannot diagnose a problem. For example:

DS1      DSECT
         DS    H
REGPAIR  DS    2ADL8               Halfword alignment
DS2      DSECT
REGPAIR_ALIGN  DS 2ADL8            Doubleword alignment
         CSECT
         ...
         USING DS1,R1              Ordinary USING
         USING DS2,REGPAIR         Dependent USING
         STPQ  R0,REGPAIR          REGPAIR is not a quadword
         STPQ  R0,REGPAIR_ALIGN    But REGPAIR_ALIGN is

The first STPQ instruction is diagnosed as an alignment error. The second STPQ instruction is not, even though the same storage location is implied by the code.

You must take care to ensure base addresses match the alignment requirements of storage mapped by a USING. For a description of the alignment requirements of instructions, see the relevant z/Architecture Principles of Operation.