Computer Hardware User's Guide

Indirect Addressing
6-8
Table 6–2. Indirect Addressing (Continued)
(c) Indirect addressing with index register IR1
Mod Field Syntax Operation Description
10000 *+AR n (IR1) addr = AR n + IR1 With preindex (IR1) add
10001 *–AR n (IR1) addr = AR n – IR1 With preindex (IR1) subtract
10010 *++AR
n (IR1) addr = AR n + IR1
AR
n = AR n + IR1
With preindex (IR1) add and modify
10011 *––AR
n (IR1) addr = AR n – IR1
AR
n = AR n – IR1
With preindex (IR1) subtract and modify
10100 *AR
n ++(IR1) addr = AR n
AR n = AR n + IR1
With postindex (IR1) add and modify
10101 *AR
n ––(IR1) addr = AR n
AR n = AR n – IR1
With postindex (IR1) subtract and modify
10110 *AR
n ++(IR1)% addr = AR n
AR n = circ(AR n + IR1)
With postindex (IR1) add and circular modify
10111
*AR n ––(IR1)% addr = AR n
AR n = circ(AR n – IR1)
With postindex (IR1) subtract and circular modify
(d) Indirect addressing (special cases)
Mod Field Syntax Operation Description
11000 *AR n addr = AR n Indirect
11001 *AR n ++(IR0)B addr = AR n
AR n = B(AR n + IR0)
With postindex (IR0) add
and bit-reversed modify
Legend: addr memory address circ() address in circular addressing
AR
n auxiliary registers AR0–AR7 ++ add and modify by 1
B where bit-reversed addressing is performed –– subtract and modify by 1
B() bit-reversed address % where circular addressing is performed
IR
n index register IR0 or IR1
Example 6–3 through Example 6–19 show the operation for each type of indirect
addressing.