Computer Hardware User's Guide

Indirect Addressing
6-15 Addressing Modes
Example 6–15. Indirect Addressing With Postindex Add and Modify
The address of the operand to fetch is the contents of an auxiliary register
(AR n ). After the operand is fetched, the index register (IR0 or IR1) is added
to the auxiliary register.
Operation: operand address = AR n
AR n = AR n + IR m
Assembler Syntax: *AR n ++(IR m )
Modification Field: 01100 if
m = 0
10100 if m = 1
Index
(+)
0
0
31 24 23
Addressxx
AR n
31
xx
0
31
24 23
Operand
IR m
Example 6–16. Indirect Addressing With Postindex Subtract and Modify
The address of the operand to fetch is the contents of an auxiliary register (AR n ).
After the operand is fetched, the index register (IR0 or IR1) is subtracted from
the auxiliary register.
Operation: operand address = AR
n
AR n = AR n – IR m
Assembler Syntax: *AR n ––(IR m )
Modification Field: 01101 if m = 0
10101 if m = 1
(–)
0
0
31 24 23
Addressxx
AR n
31
IR
m
xx
0
31
24 23
Index
Operand