Chapter
3.
Instruction
Set
DAD
OCR
3-20
DOUBLE
REGISTER
ADD
DAD adds
the
16·bit
value
in
the specified register pair
to
the
contents
of
the
Hand
L regi.,ter pair. The result
is
stored
in
Hand
L.
Op co de Operand
DAD
DAD
may add only the
contents
of
the B&C, D&E,
H&
L,
or the
SP
(Stack Pointer) register pairs
to
the
contents
of H&L. Notice
that
the
letter
H must be used
to
specify thdt the H&L register pair
is
to
b~
added to itself.
DAD sets the
carry flag
ON
if there
is
a carlY
out
of
the
Hand
L registers. DAD ,tffects none
of
the
condition
flags
other
than carry.
[ 0 0 R P
Cycles:
States:
Addressing:
Flags:
Examples:
3
10
register
CY
The DAD instruction provides a
meam
for
'>dving
the
current
contents
of
the stack pointer.
LXI
H,OOH
DAD
SP
SHLD SAVSP
;CLEAR
H&
L TO ZEROS
;GET
SP
INTO
H&L
;STORE
SP
IN
MEMORY
The
instruction DAD H doubles the number
in
the
Hand
L registers
except
when the opeldtion
Cduses
a carry
out
of
the
H register.
DECREMENT
DCR subtracts one from the
contents
of
the specified byte. DCR dffects
all
the condition flags except the cdrry
f1dg.
Because DCR preserves the carry
f1dg,
it
Cdn
be used within
multi·byte
arithmetic routines for decrementing
chardcter
counts
and similar purposes.
Decrement Register
Op
co de Operand
DCR
reg
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