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An instruction set of a processor has 125 signals which can be divided into 5 groups of mutually exclusive signals as follows:

Group 1 : 20 signals, Group 2 : 70 signals, Group 3 : 2 signals, Group 4 : 10 signals, Group 5 : 23 signals.

How many bits of the control words can be saved by using vertical microprogramming over horizontal microprogramming?

1. 0
2. 103
3. 22
4. 55
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In case of vertical microprogramming we can use 3bits for identifying any group out of 5 and 7 bits for control signals since max no of control signals required is 70. Why won't this method work?

In horizontal microprogramming we need 1 bit for every control word, therefore total bits in

Horizontal Microprogramming=20+70+2+10+23=125

Now lets consider vertical microprogramming, In vertical microprogramming we use Decoder (n to 2n ) and output lines are equal to number of control words . A input is given according to what control word we have to select.

Now in this question these 5 groups contains mutually exclusive signals, i.e, they can be activated one at a time for a given group, we can safely use decoder.

group 1= ⌈ log220⌉=5 (Number of input bits for decoder, given output is number of control word in given group)

group 2= ⌈ log270⌉ =7

group 3= ⌈ log22 ⌉ =1

group 4= ⌈ log210⌉ =4

group 5= ⌈og223 ⌉=5

Total bits required in vertical microprogramming= 5+7+1+4+5=22

So number of control words saved= 125-22=103 hence (B) is answer