4.2 The RS flip-flop, although simple in operation, is adequate for all purposes and is a basic flip-flop circuit. Let us examine the operation of this flip-flop in a configuration called a transfer circuit. Figure 4.3 shoes two sets of flip-flips named X1, X2 and X3 and Y1, Y2, and Y3. The function of this configuration is to transfer the states, or contents of Y1 into X1, Y2 into X2, and Y3 into X3 upon the TRANSFER command which consists of a 1 on the TRANSFER line.
Assume that Y1, Y2, and Y3 have been set to some states that we want to remember, or store, in X1, X2 and X3, while the Y flip-flops are used for further calculations. Placing a 1 on the TRANSFER line will cause this desired transfer of information, Understanding the transfer of the state of Y1 into X1, depends on seeing that if Y1 is in the 0 state, the Y1 output line has a 0 on it, and so the input line connected to the AND gate will place a 0 on the S input line of X1, while the Y′1 output from Y1 will be a 1, causing, in the presence of a 1 on the TRANSFER line, a 1 on the R input of X1. Similar reasoning
will show that a 1 in Y1 will cause a 1 to be placed in the X1 in the presence of a 1 on the TRANSFER line. As long as the TRANSFER line is a 0, both inputs to the X flip-flops will be 0s, and the flip-flop will remain in the last state it assumed.
The above simple operation, the transfer operation, is quite important. Related sets of flip-flops in a computer are called registers, and the three flip-flops, Y1, Y2, and Y3 would be called simply register Y, and the three flip-flops, X1, X2, and X3 would be called register X. Then a 1 on the TRANSFER line would transfer the contents of register Y into register X. That is an important concept.
4.3 A very important face about digital computers is that they are clocked. This means that there is some “master clock” somewhere sending out signals which are carefully regulated in time. These signals initiate...