10.1
BIT MANIPULATION
SECTION
10
SOFTWARE
The microcomputers have the ability to set or clear any single RAM or
liD
bit (except the data direc-
tion registers) with a single instruction (BSET,
BCLR)
(see
Caution below). Any bit
in
page
zero
can
be
tested using the
BRSET
and
BRCLR
instructions
and
the program branches
as
a result of its
state. The carry bit equals the
value of the bit references by BRSET or BRCLR. The capability of
working with any bit
in
RAM, ROM, or
liD
allows the user to have individual flags
in
RAM or to
handle single
liD
bits
as
control lines.
CAUTION
The corresponding data direction registers for ports A,
B,
and
C
are
write-only registers
(locations
$004,
$005,
and
$006),
A
read
operation
on
these registers
is
undefined. Since
BSET
and
BCLR
are
read-modify-write functions, they cannot
be
used
to set a data direc-
tion regiser bit
(all
"unaffected" bits would
be
set). It
is
recommended that
all
data direc-
tion register bits
in
a port
be
written using a single-store instruction.
The coding examples shown
in
Figure
10-1
illustrate the usefulness of the bit manipulation
and
test
instruction. Assume that the microcomputer
is
to
communicate
with
an
external serial device. The
external device
has
a data ready signal, a data output line, and a clock line to clock data one bit at a
time, least significant bit first out
of
the device. The microcomputer waits until the data
is
ready,
clocks the external device, picks up the data
in
the carry flag, clears the clock line, and finally
accumulates the data bit
in
a random-access memory location.
READY
Serial
Clock
Device
Data
I---
2 p
,
~
T
OA
-
MCU
SELF
BRSET
2,PORTA,SELF
BSET
1
,PORTA
BRCLR
O,PORTA,CONT
CONT
BCLR
I,PORTA
ROR
RAMLOC
Figure
10-1.
Bit
Manipulation
Examples
10-1
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