Computer Hardware User's Guide
Indirect Addressing
6-10
Example 6–5. Indirect Addressing With Predisplacement Add and Modify
The address of the operand to fetch is the sum of an auxiliary register (AR
n
) and
the displacement (
disp
). The displacement is either an 8-bit unsigned integer
contained in the instruction word or an implied value of 1. After the data is
fetched, the auxiliary register is updated with the address generated.
Operation: operand address = AR
n
+
disp
AR
n
= AR
n
+
disp
Assembler Syntax: *++AR
n
(
disp
)
Modification Field: 00010
(+)
0
0
31 24 23
Addressxx
AR
n
31
disp
00
0
31
87
Integer
Operand
0...0
Example 6–6. Indirect Addressing With Predisplacement Subtract and Modify
The address of the operand to fetch is the contents of an auxiliary register
(AR
n
) minus the displacement (
disp
). The displacement is either an 8-bit
unsigned integer contained in the instruction word or an implied value of 1.
After the data is fetched, the auxiliary register is updated with the address
generated.
Operation: operand address = AR
n
–
disp
AR
n
= AR
n
–
disp
Assembler Syntax: *––AR
n
(
disp
)
Modification Field: 00011
(–)
0
31 24 23
Addressxx
AR
n
31
disp
00
0
31
87
Integer
Operand
0...0
0