Ques. Describe sequential Arithmetic and Logic Unit(ALU) using proper diagrams.
AKTU 2017-18,Marks 07
Answer:
Sequential Arithmetic and Logic Unit(ALU):
ALU is the fundamental unit of any computing system.It is the heart of a processor i.e everything else in the CPU is there to support the ALU. Modern CPUs contain very powerful and complex ALUs. In addition to ALUs, modern CPUs contain a control unit (CU).
Sequential Circuits:
Made up of combinational circuits and memory elements.Output depends on input and previous state,these are memory elements capable of storing 1-bit information.
Sequential circuit has memory so output can vary based on input.
Ques. Write a short note on look ahead carry adders.
Answer:
Advantage over Simple adders:
A carry-lookahead adder (CLA) or fast adder is a type of electronics adder used in digital logic. A carry-lookahead adder improves speed by reducing the amount of time required to determine carry bits.
Carry-lookahead depends on two things:
1. Calculating for each digit position whether that position is going to propagate a carry if one comes in from the right.
2. Combining these calculated values to be able to deduce quickly whether, for each group of digits, that group is going to propagate a carry that comes in from the right.
Look Ahead Carry Adder:
A carry look-ahead adder reduces the propagation delay by introducing more complex hardware.Carry-lookahead logic uses the concepts of generating and propagating carries
For each bit in a binary sequence to be added, the carry-lookahead logic will determine whether that bit pair will generate a carry or propagate a carry.
The logic for the generate (G) and propagate (P) values are given below:
Ci the carry bit of digit i, and Pi and Gi the propagate and generate bits of digit i respectively
Substituting C1 into C2
Substituting C2 into C3
And so on
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