# Control Unit - Q13

Consider a CPU where 150 instructions take 8 clock cycles each to complete the execution. A horizontal micro-programmed control unit has to generate 125 control signals. If '$x$' is the minimum number of bits required in control word and '$y$' is the maximum number of bits that can be used in control word using micro-programming, then what is the value of $x + y$?

(A). 1250 bits

(B). 1898 bits

(C). 1480 bits

(D). 1336 bits

reshown Jun 9

+1 vote

For minimum use decoder in addition to select next control word in addition with existing combinational circuit to decide for next micro-instruction:

total number of bits = control signals (due to horizontal micro-progrmming) + additional bits

= 125 + ceil (log(150 * 8)) = 125 + 11 = 136

For maximum dont put any decoder and use control signals along with a selector that tests against various control addresses.

Total bits required = 150 * 8 = 1200

x + y = 1336
answered Jun 9 by (2,940 points)
selected Jun 9
Here 150 * 8 = 1200 is the total clock cycles i.e 1200 micro instructions. we have to store those 1200 micro instruction in control memory. so 10bits are needed to address them uniquely.  Why are you giving 1bit for each clock cycle?
We are supposed to give 1bit for each control signal(horizontal)
We are not giving 1 bit for each CLOCK CYCLE. We are giving 1 bit for each micro-instruction, which in min was done by decoder and for max done without one.

Always remember, don't try to by-heart the architecture. Rather try to see the bigger picture i.e. the problem being solved and be open to make any change in the hardware you put to suit the question.
control word size is minimum when we use vertical microprogramming and control word is maximum when we use horizontal microprogramming,with this fact,if we find out,none of the options match.

@getgatebook

Sir please provide a detailed solution for it
answered Jun 9 by (1,720 points)
+1 vote
X=136 11+25=136 because horizontal is given

total 150 instructions *8 clock cycle 1200 so 11 bit neede

For y total clock cycle is 1200 so 1 bit for each so max 1200 do I think 1336 is correct  not sure about y but sure about X
answered Jun 9 by (19,700 points)