How to establish addressability
- Specify a base address from which the assembler can compute displacements to the addresses within the control section, element, or part.
- Assign the base registers to contain the base addresses.
- Write the instructions that load the base registers with the base addresses.
MYPROG
,
that is assigned by register 12
. Register 12
is
loaded with the value in register 15
. By convention,
register 15
contains the storage address (set by
the operating system) of the control section (CSECT) when the program
is loaded into storage at execution time. MYPROG CSECT The base address
USING MYPROG,12 Assign the base register
LR 12,15 Load the base address
12
.
BASR 12,0
USING *,12
The USING instruction indicates that
register 12
can be used as a base register containing
that address.During assembly, the implicit addresses you code are converted into their explicit base-displacement form; then, they are assembled into the object code of the machine instructions in which they have been coded.
During execution, the base address is loaded into the base register.
- z/VM® and z/OS®
- If you specify multiple classes, you must provide addressability for each element. For example, suppose you define two classes that must reference positions in the other:
MYPROG CSECT , CLASS_A CATTR RMODE(24) Define class CLASS_A BASR 12,0 Local base register USING *,12 Addressability for this element - - - L 1,Addr_B Address of BDATA in CLASS_B USING BDATA,1 - - - ADATA DS F Data in CLASS_A Addr_B DC A(BDATA) - - - CLASS_B CATTR RMODE(31) Define class CLASS_B BASR 11,0 Local base register USING *,11 Addressability for this element - - - L 2,Addr_A Address of ADATA in CLASS_A USING ADATA,2 - - - BDATA DS D Data in CLASS_B Addr_A DC A(ADATA)
A class specifying the
deferred load
(DEFLOAD) attribute on its defining CATTR statement cannot be referenced from other classes using A-type or V-type address constants. However, A-type and V-type address constants can be used within a deferred-load class to refer to locations within that class or within any default_load (LOAD) class.The loading service for deferred-load classes provides the origin address of the deferred-load segment containing the classes. You can then use Q-type address constants in other classes to calculate the addresses of items in the loaded classes. For example:MYPROG CSECT , CLASS_A CATTR RMODE(31) BASR 12,0 Set base register USING *,12 Addressability for this element - - - * Address of CLASS_B segment assumed to be returned in register 8 - - - A 8,BDATAOff Add offset of BDATA in CLASS_B USING BDATA,8 - - - BDATAOff DC Q(BDATA) Offset of BDATA - - - CLASS_B CATTR DEFLOAD,RMODE(ANY) Define deferred-load class - - - BDATA DS F Data in deferred-load class
Parts must always be referenced from LOAD classes using Q-type address constants using the techniques shown in this example, whether or not they reside in deferred load classes. This is because parts are subject to reordering during binding. As noted above, parts can reference other parts in the same class using A-type and V-type address constants.