Common Bus System

We shall study the common bus system of a very basic computer in this article. A basic computer has 8 registers, memory unit and a control unit. The diagram of the common bus system is as shown below.

Connections:
The outputs of all the registers except the OUTR (output register) are connected to the common bus. The output selected depends upon the binary value of variables S2, S1 and S0. The lines from common bus are connected to the inputs of the registers and memory. A register receives the information from the bus when its LD (load) input is activated while in case of memory the Write input must be enabled to receive the information. The contents of memory are placed onto the bus when its Read input is activated.

Various Registers:
4 registers DR, AC, IR and TR have 16 bits and 2 registers AR and PC have 12 bits. The INPR and OUTR have 8 bits each. The INPR receives character from input device and delivers it to the AC while the OUTR receives character from AC and transfers it to the output device. 5 registers have 3 control inputs LD (load), INR (increment) and CLR (clear). These types of registers are similar to a binary counter.



Abbreviation Register name
OUTR Output register
TR Temporary register
IR Instruction register
INPR Input register
AC Accumulator
DR Data register
PC Program counter
AR Address register

Adder and logic circuit:
The adder and logic circuit provides the 16 inputs of AC. This circuit has 3 sets of inputs. One set comes from the outputs of AC which implements register micro operations. The other set comes from the DR (data register) which are used to perform arithmetic and logic micro operations. The result of these operations is sent to AC while the end around carry is stored in E as shown in diagram. The third set of inputs is from INPR.

Note:
The content of any register can be placed on the common bus and an operation can be performed in the adder and logic circuit during the same clock cycle.

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