Computer Hardware User's Guide

Indirect Addressing
6-11 Addressing Modes
Example 6–7. Indirect Addressing With Postdisplacement 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 displacement ( disp ) is added to the auxiliary
register. The displacement is either an 8-bit unsigned integer contained in the
instruction word or an implied value of 1.
Operation: operand address = AR
n
AR n = AR n + disp
Assembler Syntax: *AR n ++( disp )
Modification Field: 00100
Integer
(+)
0
0
31 24 23
Addressxx
31
disp 00
0
31
87
Operand
0...0
AR n
Example 6–8. Indirect Addressing With Postdisplacement 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 displacement ( disp ) is subtracted from
the auxiliary register. The displacement is either an 8-bit unsigned integer
contained in the instruction word or an implied value of 1.
Operation: operand address = AR n
AR n = AR n – disp
Assembler Syntax: *AR n ––( disp )
Modification Field: 00101
Integer
(–)
0
0
31 24 23
Addressxx
AR
n
31
disp 00
0
31
87
Operand
0...0