12.2 Other Ways to Connect Ports to the System

The examples given thus far may leave you with the impression that the CPU always reads and writes peripheral data using the data bus. However, while the CPU generally transfers the data it has read from input ports across the data bus, it does not always use the data bus when writing data to output ports. In fact, a very common output method is to simply access a port's address directly without writing any data to it. Figure 12-4 illustrates a very simple example of this technique using a set/reset (S/R) flip-flop. In this circuit, an address decoder decodes two separate addresses. Any read or write access to the first address sets the output line high; any read or write access to the second address clears the output line. This circuit ignores the data on the CPU's data lines, and it also ignores the status of the read and write lines. The only thing that matters is that the CPU accesses one of these two addresses.

Figure 12-4: Outputting data to a port by simply accessing that port

Another possible way to connect an output port to a system is to connect the read/write status lines to the data input of a D flip-flop. Figure 12-5 shows how you could design such a device. In this diagram, any read of the port sets the output bit to zero, while any write to this port sets the output bit to one.

Figure 12-5: Outputting data using the read/write control as the data to output

These examples of connecting peripheral devices directly to the CPU are only two of an amazing number of different designs that engineers have devised to avoid using the data bus. However, unless otherwise noted, the remaining examples in this chapter presume that the CPU reads and writes data to an external device using the data bus.