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Yes. Simply reverse the commands at lines 60 and 80. You can easily<br />
"invert" the sense of a flag in software.<br />
In this step, you will use a short program that will count the<br />
number of times that the flag is set. Again, the assembly-language<br />
subroutine will be used. You may wish to add another pulser circuit<br />
to provide the flag-clearing operation to replace the jumper wire<br />
between pin 1 on the SN7474 and +5 volts.<br />
Enter the following program and run it:<br />
POKE 10,76: POKE 11,3: POKE 12,3<br />
POKE 768,l<br />
HOME: C = 0<br />
POKE 769, PEEK(49319)<br />
Z = USR(O)<br />
IF PEEK(770) = 0 THEN 40<br />
C = C+l: HTAB l: VTAB l: PRINT C<br />
GOTO 40<br />
Be sure that the Hip-flop is cleared before you test the program.<br />
With the program running, actuate the pulser and set the Hip-flop.<br />
hat do you observe Is this what you expected<br />
We found that the count started as soon as the flip-Hop was set, and<br />
that it continued for as long as the Hag remained set. Clearing the<br />
'p-flop stopped the count. What we really wanted was one count<br />
.each time the flip-Hop was set.<br />
·· Why didn't this happen as expected The set state of the Hip-flop<br />
eontinued to be tested and detected by the program. We could not<br />
reset the flip-Hop fast enough by hand to stop the counting at one<br />
count per pulser actuation.<br />
In most computer systems, the computer, or the flag-containing<br />
.device clears the flag after it has been detected. To allow your interface<br />
to clear the Hip-flop, add the circuit shown in Fig. 6-22. Y-ou will<br />
need an SN7402 NOR-gate chip. Be sure,that you wire the' +5-volt<br />
ower supply to pin 14, and ground to pin 7, on the SN7402 chip.<br />
Since the NOR-gate circuit will provide the rese t signal for the Hip-<br />
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