titus-larsen-titus-1981-apple-interfacing

POKE 45124,98
POKE N,120
POKE 45124,X
POKE X,M
l = PEEK (23109)
l = PEEK (Q)
The input and output commands may have variables specified
within them, rather than specific values for port addresses, and in the
case of the POKE command, data values. Thus, all of the PEEK and
POKE commands shown in Table 1-2 are valid. We have assumed,
of course, that the values for the variables, N, M, X and Q have been
specified somewhere in the program prior to the use of the instructions
shown in Table 1-2.
Input and output commands in which the address values exceed
65535 will generate an ILLEGAL QUANTITY ERROR in the Apple
computer. An attempt to output a numerical data value that exceeds
255 will also generate an ILLEGAL QUANTITY ERROR.
We have provided some examples that show the use of the POKE
and PEEK commands. 'While the programs shown in Example 1-3
can be executed, they will not do anything useful, since you do not
have any external I/0 connected to your computer, at present.
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10 lf\IPUT "OUTPUT PORT # =";P
20 lf\IPUT "VALUE FOR OUTPUT"; V
30 POKE P,V
40 GOTO 10
10 lf\IPUT "INPUT PORT # = "; M
20 _ PRl1'1T "VALUE AT PORT ="; PEEK-(M),
30 GOTO 10
Since 6502-based computers cannot
between memory
locations that are used for the temporary storage of programs and
data, and those that are being used for I/ 0 ports, the PEEK and
POKE instructions are frequently used to examine and alter the contents
of various memory locations within the Apple. If you POKE
information into read/write memory in an indiscriminate fashion,
you may "write over" important parts of your program, or information
that has been tempornrily stored by the BASIC interpreter. The
net effect is a "crash" of the computer system, in which your program
and data 'Nill be lost or significantly altered. It is probably not a good
idea to randomly POKE information into various addresses, until
some specific guidelines are provided. Of course, you can use the
PEEK command to examine the contents of a memory location
v1henever you since this command will not alter the contents