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        riddles >> cs >> gate question
        
(Message started by: kirru on Dec 5^th, 2012, 11:26pm)

Title: gate question
Post by kirru on Dec 5^th, 2012, 11:26pm

A micro program control unit is required to generate a total of 25 control signals. Assume that during any microinstruction, at most two control signals are active. Minimum number of bits required in the control word to generate the required control signals will be
(a) 2 (b) 2.5 (c) 10 (d) 12

Title: Re: gate question
Post by Grimbal on Dec 6^th, 2012, 5:05am

The best sense I can make of that question is that you want to transmit up to 2 simultaneous signals out of a choice of 25, and ask how many bits you need for that.

One solution would be to have 2 groups of 5 bits, each can send one of 31 signals (or the absence of signal).

But it is not optimal. The number of different states is 1 (no signal) + 25 (one signal) + (25*24/2) (2 signals) = 326 states. You can transmit any of these states over 9 bits. But it is more complex to encode/decode, adding an extra bit would probably cost less.