HLASM Language Reference

Operand Entries
Relocatability of Addresses
If the value of an address expression changes when the assumed origin of the
program is changed, and changes by the same amount, then the address is simply
relocatable. If the addressing expression does not change when the assumed origin
of the program is changed, then that address is absolute. If the addressing
expression changes by some other amount, the address may be complexly
relocatable.
Addresses in the relative-offset or base-displacement form are relocatable,
because:
Each relocatable address is assembled as a signed relative offset from the
instruction, or as a displacement from a base address and a base register.
The base register contains the base address.
If the object module assembled from your source module is relocated, only the
contents of the base register need reflect this relocation. This means that the
location in virtual storage of your base has changed, and that your base
register must contain this new base address.
Addresses in your program have been assembled as relative to the base
address; therefore, the sum of the displacement and the contents of the base
register point to the correct address after relocation.
Absolute addresses are also assembled in the base-displacement form, but always
indicate a fixed location in virtual storage. This means that the contents of the
base register must always be a fixed absolute address value regardless of
relocation.
Machine or Object Code Format
Addresses assembled into the object code of machine instructions have the format
given in Figure 23 on page 90. Not all of the instruction formats are shown in
Figure 23.
The addresses represented have a value that is the sum of a displacement (see
▌1▐ in Figure 23) and the contents of a base register (see ▌2▐ in Figure 23).
Index Register: In RX-format instructions, the address represented has a value
that is the sum of a displacement, the contents of a base register, and the contents
of an index register (see ▌3▐ in Figure 23).
Chapter 4. Machine Instruction Statements 89