Computer Hardware User's Guide
Indirect Addressing
6-12
Example 6–9. Indirect Addressing With Postdisplacement Add and Circular 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
contents of the auxiliary register using circular addressing. This result is used
to update 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
= circ(AR
n
+
disp
)
Assembler Syntax: *AR
n
++(
disp
)%
Modification Field: 00110
(+)
0
031 24 23
Addressxx
AR
n
31
disp
00
0
31
87
Integer
Operand
0...0
(%)
Example 6–10. Indirect Addressing With Postdisplacement Subtract and Circular 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 contents of the auxiliary register using circular addressing. This result is
used to update 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
= circ(AR
n
–
disp
)
Assembler Syntax: *AR
n
––(
disp
)%
Modification Field: 00111
(–)
0
0
31 24 23
Addressxx
AR
n
31
disp
00
0
31
87
Integer
Operand
0...0
(%)