titus-larsen-titus-1981-apple-interfacing

transfers will require more than eight bits of information, so multiple
bytes are transferred, one byte at a time.
There are also cases in which the actual value of the data transferred
is meaningless. The bits may be used to represent individual
two-state conditions that are unrelated to the positional values of
the bits. For example, a number of sensors could be connected to the
Apple indicating conditions such as tank empty-or-full, heater on-oroff,
value open-or-closed, and so on. A PEEK command could be
used to input the status of these indicator bits, through an 8-bit input
port. Thus, the value read from this input port might be 100, but the
port is sensing eight individual on or off (logic one or logic zero)
states, so the value of 10010 is meaningless. The individual binary
bits each represent the state of an individual sensor. In this case:
10010 = 011001002
This indicates that three of the sensors are in the logic one state and
five are in the logic zero state.
The POKE and PEEK commands can also be used in a similar
manner to turn a device on or to turn a device off, based upon the
state of the individual bits that have been sensed elsewhere in a control
program. In fact, many of the I/0 addresses used by the Apple
are assigned to simple on/ off devices such as the speaker. Thus, a
simple command:
A =
PEEK(49200)
will generate a 'blip" on the speaker in the Apple. You should understand
that the variable, A, is a "dummy," and its final value is not important,
since the net effect of the simple BASIC statement is to
pulse the speaker once. The speaker control command may be used
in a loop to generate a low buzz from the speaker. This is shown
'
in
kU
l 0 A = PEEK(49200)
20 GOTO 10
Example 1-4. A Simple Speaker Control Program
The important point to remember here is that the PEEK and
POKE instructions are not limited simply to controlling the transfer
of information on the data bus. They may also be used for specific
control functions, such as pulsing a counter, turning on a pump, or
tilting a solar collector.
Assembly Language and BASIC
The BASIC-language programs that you write on your Apple computer
bear very little relationship to the actual instructions that the
6502 microprocessor chip can actually execute. Each of your BASIC
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