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communications take place (protocol) is also indicated. This protocol has to be pro-
grammed in the form of routines that run under the control of the CPU. Consider, for
example, an input operation from Device 6 (could be the keyboard) in the case of
shared I
O arrangement. Let us also assume that there are eight different I
O
/
/
devices connected to the processor in this case (see Fig. 8.4).
The following protocol steps (program) have to be followed:
1. The processor executes an input instruction from device 6, for example,
INPUT 6. The effect of executing this instruction is to send the device
number to the address decoder circuitry in each input device in order to ident-
ify the specific input device to be involved. In this case, the output of the deco-
der in Device #6 will be enabled, while the outputs of all other decoders will
be disabled.
2. The buffers (in the figure we assumed that there are eight such buffers) holding
the data in the specified input device (Device #6) will be enabled by the output
of the address decoder circuitry.
3. The data output of the enabled buffers will be available on the data bus.
Figure 8.4 Example eight-I / O device connection to a processor
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