Intel MCS48 User Manual page 84

INSTRUCTION SET
point
for
all
data
transfers within the 8048.
Data can
be
transferred
between
the 8
registers of
each working
register
bank and
the
accumulator
directly,
i.e.
the
source
or
destination
register
is
specified
by
the
instruction.
The
remaining
locations of
the
internal
RAM
array are referred
to
as
Data
Memory
and
are
addressed
indirectly via
an
address stored
in
either
R0
or
R1
of
the
active
working
register
bank.
R0 and
R1 are also
used
to indirectly
address
external data
memory when
it
is
present. Transfers
to
and
from
internal
RAM
require
one
cycle while
transfers
to
external
RAM
require two.
Constants
stored
in
Program
Memory
can be
loaded
directly to
the accumulator and
to
the
8
working
registers.
Data can
also
be
transfered
directly
between
the
accumulator
and
the on-board timer/counter
or
the
accumulator and
the
Program
Status
word
(PSW).
Writing
to
the
PSW
alters
machine
status accordingly
and
provides a
means
of
restoring status
after
an
interrupt
or of
altering
the stack pointer
if
necessary.
Accumulator
Operations
Immediate
data,
data
memory,
or the
working
registers
can be
added
with or
without carry to the accumulator.
These
sources can
also
be
ANDed,
ORed,
or
Exclusive
ORed
to
the accumulator. Data
may
be
moved
to or
from
the
accumulator
and working
registers or data
memory. The
two
values can
also
be
exchanged
in
a
single
operation.
In
addition,
the lower 4
bits
of
the
accumulator can be exchanged
with the
lower
4-bits of
any
of
the
internal
RAM
locations.
This
instruction,
along with
an
instruction
which
swaps
the
upper and
lower
4-bit
halves
of
the accumulator, provides
for
easy handling
of 4-bit quantities,
including
BCD
numbers.
To
facilitate
BCD
arithmetic,
a
Decimal
Adjust
instruction
is
included.
This
instruction
is
used
to correct
the
result of
the
binary addition of
two
two-digit
BCD
numbers. Performing a decimal
adjust
on
the
result in
the
accumulator produces
the
required
BCD
result.
Finally,
the
accumulator can
be: incre-
mented, decremented,
cleared, or
comple-
mented and
can be
rotated
left
or right
1-bit at
a time with
or without
carry.
Although
there
is
no
subtract
instruction
in
the
8048,
this
operation
can be
easily
implemented
with three single-byte single-
cycle
instructions.
A
value
may
be
subtracted
from the
accumulator
with the
result
in
the
accumu-
lator by:
Complementing
the
accumulator
Adding
the value
to
the
accumulator
Complementing
the accumulator.
Register
Operations
The
working
registers
can be accessed
via
the accumulator as explained above, or
can
be loaded immediate
with
constants from
program memory.
In
addition,
they
can be
incremented
or
decremented
or
used as
loop counters using the
decrement and
jump,
if
not zero
instruction,
as explained
under branch
instructions.
All
Data
Memory
including
working
registers
can be accessed
with
indirect instructions via
R0
and R1 and can be
incremented.
Flags
There
are four user accessible
flags
in
the
8048: Carry, Auxiliary Carry,
F0,
and
F1.
Carry
indicates
overflow
of
the
accumulator,
and
Auxiliary
Carry
is
used
to
indicate
overflow
between
BCD
digits
and
is
used
during decimal
adjust operation.
Both Carry
and
Auxiliary
Carry
are accessible as part of
the
program
status
word and
are stored
on
the stack during subroutines.
F0 and
F1 are
undedicated
general
purpose
flags to
be
used
as the
programmer
desires.
Both
flags
can be
cleared or
complemented and
tested
by
conditional
jump
instructions.
F0
is
also
accessible
via
the
Program
Status
word and
is
stored
on
the stack with the carry
flags.
Branch
Instructions
The
unconditional
jump
instruction
is
two
bytes
and
allows
jumps anywhere
in
the
first
4-2