titus-larsen-titus-1981-apple-interfacing

None of the outputs should be active, since device address 49325 has
not been implemented in the circuit. Furthermore, address 49325 is
not readily available on the breadboard. Of the addresses in the
block 49312 through 49327, only addresses 49312 through 49319 are
available at the ADDRESS socket.
Change line 10 in the program so that it is now
10 A=PEEK(49318):B=PEEK(49319)
Where do you observe the pulses in the circuit when you run the
· odified program
u should find that outputs A and B are active. Output C is not ace
since it is an output control pulse, and there are no output
OKE) commands in the program.
Make another modification to your program. Change line 10 so
at you can control output device 49318. Your statement at line 10
ould look like this:
u can use any data value that is between 0 and 255, inclusive. Now
your program and test outputs A, B, and C. Which output do you
pect to be active Is this what you found
tput C is active, since the POKE command is an output-type comnd,
and the address, 49318, corresponds to the "6" output pin from
decoder. You are probably surprised to see that the B output is
active. When a POKE instruction is executed by the BASIC inreter
in the Apple, the computer system does a read-before-write
!')ration, so that the selected address is read from, before being
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