titus-larsen-titus-1981-apple-interfacing

Gei'!eral-fi!Jrp@S('; I/ 0 Commaru:li:>
Although there are some general-purpose I/0 commands in the
INTEGER BASIC interpreter program for the Apple computer, for
this book we have chosen to use what we consider to be the more
flexible APPLESOFT BASIC interpreter program If you wish to
convert your Apple computer to this program, a local Apple computer
dealer can assist you.
The two If 0 device commands are PEEK and POKE. They are
used to transfer data to an external device from the computer
(POKE), and to the computer from an external device (PEEK).
There is a specific format for these instrnctions that must be used if
the instructions are to operate properly.
Input and output devices will be referred to as ports. Thus, an output
device will be an output port and an input device will be an inport.
This is standard nomenclature used throughout the microcomputer
industry.
The output instruction, POKE, must specify the address of the
I/0 device that is to be involved in the transfer of data and also the
value that is to be transferred to the addressed device. The actual
format for the POKE instruction is POKE, x,y, where the x value
represents the decimal address of the output device that is to receive
the data value, y. The data, y, must also be a decimal number. Since
the 6502 microprocessor chip can address 65536 memory locations,
the address must be within the range of 0 to 65535, inclusive. The
data value must be within the range of 0 to 255, inclusive, since the
computer uses an 8-bit data bus for all data transfers, and the largest
number that can be transferred on such a bus is 255.
The value 215 is sent to output port 12684 in the following statement:
POKE 12684,215.
The input instruction, PEEK, is similar to the POKE instruction,
except that no data value is incorporated into the command. VVe are
interested in determining the value present at the specific input device,
so only the decimal address of the input device is specified;
PEEK ( x), where x is the decimal address of the input device.
It does little good to input a value without doing something with
it, so the input command is always incorporated in a complete statement,
rather than being a statement by itself. An example of this is
Q= PEEK ( 34579) .
In this case, the variable, Q, is assigned the decimal value that has
been input from device 34579. It is important that you remember to
enclose the address of the input device in parentheses.
Whenever a PEEK command is used, the value that is input will be
between 0 and 255, :inclusive. Again, this is due to the limitation of
8-bit transfers.
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