We may rearrange these two expressions as follows: The simplified expression for S and C are The two outputs are designated by the symbols S for sum and C for carry. The third input Z, represents the carry from the previous lower position. Two of the input variables, denoted by x and y, represent the two bits to be added. It consists of three inputs and two outputs.
WHY STUDY COMPUTER ORGANIZATION AND ARCHITECTURE FULL
Ī full adder is a combinational circuit that forms the arithmetic sum of three bits. This circuit can not handle the carry input, so it is termed as half adder.The circuit diagram and block diagram of Half Adder is shown in Figure. The simplified sum of products expressions areįigure : Circuit diagram and Block diagram of Half Adder The binary addition operation of single bit is shown in the truth table The input variables designate the augends and addend bits The output variables produce the sum and carry. In general, the adder circuit needs two binary inputs and two binary outputs. If we want to construct a circuit which will perform the AND operation on two 4-bit number, the implementation of the 4-bit AND operation is shown in the Figure below.īinary adder is used to add two binary numbers. The EX-OR gate and its truth table is given in Figure below. The OR gate and its truth table is shown in Figure below.ĮX-OR gate: The output is high if either of the input is high. OR gate: The output is high if any one of the input is high. The AND gate and its truth table is shown in Figure below Some of the common logic gates are mentioned here.ĪND gate: The output is high if both the inputs are high. These logic gates can be used to implement the logical operation. There are several logic gates exists in digital logic circuit. These basic operations can be used to implement some complicated operations which are not feasible to implement directly in hardware. We should have some hardware implementations for basic operations. After the completion of the operation, the result is stored in register C. The input data are stored in A and B, and according to the operation specified in the control lines, the ALU perform the operation and put the result in register C.Īs for example, if the contents of controls lines are, 000, then the decoder enables the addition operation and it activates the adder circuit and the addition operation is performed on the data that are available in storage register A and B. The ALU has got two input registers named as A and B and one output storage register, named as C. The block diagram of the ALU is shown in figure below. One possible combination is given here.Ī decode is used to decode the instruction. The input combination of these control lines are shown below:Ĭontrol line is used to identify the group: logical or arithmetic, ie: arithmetic operation : logical operation.Ĭontrol lines and are used to identify any one of the four operations in a group. We need three control lines to identify any one of these operations. Also consider that the ALU is having four logical operations: OR, AND, NOT & EX-OR. Addition, subtraction, multiplication and division. So, with the help of three control lines, any one of these eight operations can be identified.Ĭonsider an ALU is having four arithmetic operations. Also to identify the any one of these two groups- arithmetic or logical, another control line is needed. To identify any one of these four logical operations or four arithmetic operations, two control lines are needed. ALU is having collection of two types of operations:Ĭonsider an ALU having 4 arithmetic operations and 4 logical operation. The basic operations are implemented in hardware level. ALU is responsible to perform the operation in the computer.
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