Bus and Memory Transfers -2
The two selection lines S1 and S0 are connected to the selection inputs of all four multiplexers. The selection lines choose the four bits of one register and transfer them into the four-line common bus. When S1S0 = 00, the 0 data inputs of all four multiplexers are selected and applied to the outputs that form the bus.s. This causes the bus lines to receive the content of register A since the outputs of this register are connected to the 0 data inputs of the multiplexers. Similarly, register B is selected if S1S0 = 01, and so on. Table 4-2 shows the register that is selected by the bus for each of the four possible binary value of the selection lines.
In general, a bus system will multiplex k registers of n bits each to produce an n-line common bus. The number of multiplexers needed to construct the bus is equal to n, the number of bits in each register. The size of each multiplexer must be k x 1 since it multiplexes k data lines. For example, a common bus for eight registers of 16 bits each requires 16 multiplexers, one for each line in the bus. Each multiplexer must have eight data input lines and three selection lines to multiplex one significant bit in the eight registers.
The transfer of information from a bus into one of many destination registers can be accomplished by connecting the bus lines to the inputs of all destination registers and activating the load control of the particular destination register selected. The symbolic statement for a bus transfer may mention the bus or its presence may be implied in the statement. When the bus is includes in the statement, the register transfer is symbolized as follows:
BUS +- C, R1 +- BUS
The content of register C is placed on the bus, and the content of the bus is loaded into register R 1 by activating its load control input. If the bus is known to exist in the system, it may be convenient just to show the direct transfer.
R1 +- C
From this statement the designer knows which control signals must be activated to produce the transfer through the bus.