Reference Guide

30 Chapter 2
Program Structure
Expressions
Expressions produce either an absolute or a relocatable result. Any
operation involving only absolute terms yields an absolute result.
Relocatable terms are allowed only for the + and - operators. The legal
combinations involving relocatable terms are shown in “Legal
Combinations For Relocatable Terms” on page 30.
Table 2-10 Legal Combinations For Relocatable Terms
For more information on the term relocatable, refer to “Assembler
Features” on page 15.
NOTE The combination “relocatable-relocatable+relocatable” is not permitted.
For example, assume the symbols MONTH and YEAR are relocatable, and
JANUARY and FEBRUARY are absolute. The expressions MONTH+JANUARY
and MONTH+FEBRUARY-4 are relocatable, while the expressions
YEAR-MONTH and FEBRUARY-4 are absolute. The expression
MONTH+JANUARY*4 is also legal and produces a relocatable result,
because JANUARY*4 is evaluated first, producing an absolute
intermediate result that is added to MONTH. The expression MONTH+YEAR
is illegal, because the sum of two relocatable terms is not permitted.
Because all instructions are a single word in length, it is not possible to
form a complete 32-bit address in a single instruction. Therefore, it is
likely that the Assembler (or linker) may not be able to insert the final
address of a symbol into the instruction as desired. For example, to load
the contents of a word into a register, the following instruction could be
used:
LDW START,%r2
Because LDW provides only 14 bits for the address of START, the
Assembler or linker prints an error message if the address of START
requires more than 14 bits. There are two instructions, LDIL and ADDIL,
whose function is to form the left-most 21 bits of a 32-bit address. The
succeeding instruction, by using the target of the LDIL or ADDIL as a
Operation Result
Absolute + Relocatable Relocatable
Relocatable + Absolute Relocatable
Relocatable - Absolute Relocatable
Relocatable - Relocatable (defined locally) Absolute