titus-larsen-titus-1981-apple-interfacing

CHAPTER 4
Flags and Decisions
n almost all of the previous examples, we have assumed that there
•ttle synchronization required between the computer and the exnal
1/0 devices. Thus, output ports have been assumed to always
ready for more data to be transferred to them. In the case of input
ts, we have assumed that the data values are present and ready
transfer to the computer, when the computer reaches a PEEK
mand in a program. This may not always be the case. We must
en deal with 1/0 devices that are slower than the computer.
1/0 DEVICE SYNCHRONIZATION
Since not all I/ 0 devices may be ready for the computer at all
es, a means of synchronizing the computer and the 1/0 devices
equired. The synchronization generally involves the use of signals
t are called -flags. These signals are used to indicate that various
·ces are busy or not busy, ready or not ready, converting or not
verting, and so on. Thus, "flags" indicate the status of devices, and
are often called status fiags.
or illustrative purposes, we will assume that we are required to
rface a device to an Apple computer. The device will provide
it data values to the computer on an irregular basis. In most cases,
h devices also generate a flag signal that indicates that the device
ready to transfer its information to the computer. Such a device is
wn' in Fig. 4-1. Note that a standard three-state input port has
n used to transfer the information to the computer. The READY
presents an interesting problem. How is the computer going to
nitor or check the condition of the READY flag, so that it can
termine when a new data value is ready
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