titus-larsen-titus-1981-apple-interfacing

VALUE 00111010 00011010 11110000 00011111
MASK 00100000 00100000 00100000 00100000
RESULT 00100000 00000000 00100000 00000000
Fig. 4-3. Example of AND operation in which eight bits of information are
operated on.
"mask," and the "B" bit as information or data. When the mask is
zero, the result is a zero. When the mask is a one, the data is passed
ough the gate. In this way, selected bits may be masked, while
hers are "passed through" the mask. If, for example, we wished to
eek the state of bit D5 in the data. word 00111010, a mask of
100000 could be used. The mask is '}NOOd with the data word, as
own in Fig. 4-3, for several different data words. In all cases, the
gic state of D5 was passed through to bit D5 in the result. All of
e other bits were masked, or set to zero. In this way, the total re­
. lt was zero when bit D5 was zero, and the result was nonzero when
't D5 was a one. This could be used as the basis for decision making
ps in a program. You must remember to convert the masks to their
.eimal equivalent before trying to use them in a BASIC program .
. the case of bit D5, the mask would be converted to 32.
FLAG-DETECTING SOFTWARE
.Once an interface has been constructed so that the states of the
ious flags may be detected, as shown in Fig. 4-4, software may be
d to make decisions based upon the states of the flags.
In some dialects of BASIC, there are logical operations that will
rform bit-by-bit AND operations, such as the ones shown in Fig .
. In these cases, simple expressions may be used in BASIC pro-
INPUT DEVICE
ONE·BIT INPUT PORT .---- RD 49321
J"L
..
READY/BUSY 1-----t >-----.
D7
t---+--D7
'TO DATA
BUS
DO t----t
1---DO
.__..n... _ __ RD 49320
Fig. 4-4. Complete interface in which the flag is detected by software.
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