Reference Guide

Chapter 2 35

Program Structure

Operands and Completers

Machine instructions usually require one or more operands.

These operands tell the processor what data to use and where to store

the result. Operands can identify a register, a location in memory, or an

immediate constant (that is, data that is coded into the instruction

itself). The operation code determines how many and what kinds of

operands are required.

Registers used in operands should be either predeﬁned register symbols

(with the % preﬁx) or user-deﬁned register symbols deﬁned with the .REG

directive. They can also be absolute expressions. See “Registers and

The following example shows a few machine instructions with register

operands:

SCRATCH .REG %r18 ;define register SCRATCH

ADD %r3,%r7,%r4 ;r3 + r7 -> r4

OR %r7,%r3,%r8 ;inclusive or of r7,r3 -> r8

COPY SCRATCH,%r7 ;copy r18 to r7

MTCTL %r2,%sar ;set shift amount register (cr11)

MFSP %sr4,%r10 ;fetch contents of sr4

Operands designating memory locations usually consist of an expression

and a general register used as a base register. Some instructions also

require a space register designation. In general, such operands are

written in the form expr(sr,gr) or expr(gr), as in the following

examples:

local_off .EQU -64

LDW 4(%dp),%r2

STW %r0,local_off-4(%sp)

LDW 0(%sr3,%r2),%r9

Notice that the space register can be omitted on instructions that allow

short addressing, as in the STW instruction shown above.

If only one register is given, it is assumed to be the general register, and

the space register ﬁeld in the machine instruction is set to zero, which

indicates short addressing.

The expression in a memory operand is either absolute or relocatable.

Absolute expressions are meaningful when the base register contains the

address of an array, record, or the stack pointer to which a constant offset