ADDRESSING
MOTOROLA
ADDRESS GENERATION UNIT 4 - 13
an extra instruction cycle, cannot be used for making XY: memory references.
4.4.1.7 Predecrement By 1
The address of the operand is the contents of the address register, Rn, decremented by
1 before the operand address is used (see Table 4-1 and Figure 4-10). The contents of
Rn are decremented and stored in the same address register. This addressing mode re-
quires an extra instruction cycle. This mode cannot be used for making XY: memory
references, nor can it be used for modifying the contents of Rn without an associated data
A5B4C 6 00000 1
15 0
15 0
15 0
EXAMPLE: MOVE X1,X: (R2)+N2
BEFORE EXECUTION AFTER EXECUTION
X1 X0
47 24 23 0
23 0 23 0
X MEMORY
23 0
XXXXXX
$3200
X MEMORY
23 0
$3200
$3200
$FFFF
R2
N2
M2
15 0
15 0
15 0
$3204
$FFFF
R2
N2
M2
Assembler Syntax: (Rn)+Nn
Memory Spaces: P:, X:, Y:, XY:, L:
Additional Instruction Execution Time (Clocks): 0
Additional Effective Address Words: 0
XXXXXX
$3204 $3204 XXXXXX
A5B4C6 000001
X1 X0
47 24 23 0
23 0 23 0
$0004 $0004
$A5B4C6
Figure 4-7 Address Register Indirect — Postincrement by Offset Nn