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DEC
(M) – $01 ⇒ M
Operation:
Description:
Subtract one from the content of memory location M.
The N, Z and V status bits are set or cleared according to the results of
the operation. The C status bit is not affected by the operation, thus al-
lowing the DEC instruction to be used as a loop counter in multiple-pre-
cision computations.
Condition Codes and Boolean Formulas:
S
–
N:
Z:
V:
Addressing Modes, Machine Code, and Execution Times:
Source Form
DEC opr16a
DEC oprx0_xysp
DEC oprx9,xysp
DEC oprx16,xysp
DEC [D ,xysp ]
DEC [ oprx16,xysp ]
MOTOROLA
6-72
Decrement Memory
X
H
I
N
Z
∆
∆
–
–
–
Set if MSB of result is set; cleared otherwise.
Set if result is $00; cleared otherwise.
Set if there was a two's complement overflow as a result of the operation;
cleared otherwise. Two's complement overflow occurs if and only if (M)
was $80 before the operation.
Address Mode
EXT
IDX
IDX1
IDX2
[D,IDX]
[IDX2]
INSTRUCTION GLOSSARY
V
C
∆
–
Object Code
73 hh ll
63 xb
63 xb ff
63 xb ee ff
63 xb
63 xb ee ff
DEC
Cycles
Access Detail
4
rOPw
3
rPw
4
rPOw
5
frPPw
6
fIfrPw
6
fIPrPw
CPU12
REFERENCE MANUAL
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