2CEIT507 :COMPUTER ARCHITECTURE & ORGANIZATION Practical-2
Practical: 2
Aim: Implement common bus system of four 4-bit register using
multiplexer.
Theory:
Way of constructing a common bus system is with multiplexers. The multiplexers select the
source register whose binary information is then placed on the bus. The construction of a bus
system for four registers is shown in Figure. Each register has four bits, numbered 0 through
3. The bus consists of four 4x1 multiplexers each having four data inputs, 0 through 3, and
two selection inputs, S1 and S0 . In order not to complicate the diagram with 16 lines
crossing each other, we use labels to show the connections from the outputs of the registers to
the inputs of the multiplexers. For example, output 1 of register A is connected to input 0 of
MUX 1 because this input is labeled A1 .The diagram shows that the bits in the same
significant position in each register are connected to the data inputs of one multiplexer to
form one line of the bus. Thus MUX 0 multiplexes the four 0 bits of the registers, MUX 1
multiplexes the four 1 bits of the registers, and similarly for the other two bits.
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. 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. Below table shows the
register that is selected by the bus for each of the four possible binary values of the selection
lines.
S1 S0 Register selected
0 0 A
0 1 B
1 0 C
1 1 D
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.
2CEIT507 :COMPUTER ARCHITECTURE & ORGANIZATION Practical-2
Block Diagram:
Circuit:
