Chapter
3.
Instruction
Set
DCX
DI
3·22
DECREMENT
REGISTER
PAIR
OCX
decrements
the
contents
of
the specified register pair
qy
one.
OCX
affects none
of
the condition flags.
Because
OCX
preserves
all
the flags, it can
be
used for address modification
in
any instruction sequence
that
relies on the passing of the flags.
Op
co de
Operand
OCX
OCX
may
decrement
only the
B&C,
O&E, H&L, or the
SP
(Stack Pointer) register pairs. Notice
that
the letter
H must
be
used
to
specify the
Hand
L pair.
Exercise care when decrementing the stack pointer as this causes a loss
of
synchronization between the pointer
and
the
actual
contents
of
the stack.
Example:
~J
__
O----'-_R
__
P----'-
__
O_
1 3
Cycles:
States:
Addressing:
Flags:
5
(6
on 8085)
register·
none
A,sume
that
the
Hand
L registers contain the address 9800H when the instruction
OCX
H
is
executed.
OCX
considers the
contents
of
the two registers to
be
a single 16-bit value and therefore performs a borrow from the
H register to produce the
value
97FFH.
DISABLE
INTERRUPTS
The inter·rupt sy,tem
is
disabled when the processor recognizes an interrupt or immediately following execution
of
a
01
instruction.
In
applications
that
use
interrupts, the
01
instruction
is
commonly used only when a code sequence must
not
be
interrupted. For example, time-dependent code sequences become inaccurate when interrupted. You can disable
the
interrupt
system by including a
01
imtruction
at the beginning
of
the code sequence. Because you
cannot
predict the occurrence
of
an
IIlterrupt, include an E I instruction at the end
of
the
time·dependent
code sequence.
Opcode
Operand
01
Operands are not permitted with the
01
instruction.
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