HLASM Language Reference

Addressing
| The System/390® and z/Architecture architectures have two ways of resolving
addresses in your program, depending on the machine instruction type:
base-displacement, where the address is computed by adding the
displacement to the contents of a base register.
| relative-immediate, where the address is computed by adding 2 × the signed
immediate operand field to the instruction's address (refer to “RI Format” on
page 91 and “RSI Format” on page 95).
Addressing within Source Modules: Establishing Addressability
You can use symbolic addresses in machine instructions and certain assembler
instructions. This is much easier than explicitly coding the addresses in the form
required by the hardware. Symbolic addresses you code in the instruction
operands are implicit addresses, and addresses in which you specify the
base-displacement or intermediate form are explicit addresses.
The assembler converts your implicit addresses into the explicit addresses required
for the assembled object code of the machine instruction. However, for
base-displacement operands, you must first establish addressability, as described
below.
Base Address Definition: The term base address is used throughout this manual
to mean the location counter value within a control section, element, or part from
which the assembler can compute displacements to locations, or addresses. The
base address need not always be the storage address of a control section,
element, or part when it is loaded into storage at execution time.
How to Establish Addressability
To establish the addressability of a control section, element, or part (see “Sections,
Elements, and Parts” on page 52), you must:
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.
The following example shows the base address at MYPROG, that is assigned by
register 12. Register 12 is loaded with the value in register 15, which by convention
usually 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
Similarly, you can use a BASR or similar instruction to put the address of the
following instruction into register 12.
BASR 12,
USING ,12
The USING instruction indicates that register 12 may be used as a base register
containing that address.
64 HLASM V1R5 Language Reference