co control word

Question

Q:- micro programmed CU supports 400  control signals micro instruction is designed in such a way that  4 signals are active . What is the size of a control field in the instruction ?

Comments

14

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we have to select 0 or 1 signal from g1 => 8 bits.

now there are 255 ignals remaining for g2

we have to select atmost 6 signals and each signal is of 8 bits so at max we need 8*6 bits =48 bits.

so total = 8+48 =56 bits.

where am i doing it wrong ?

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i don't know man for this I m also seeking explaination ?

Answer 1

at a time 4 signals are active...

so  400C4 combinations are there...

so no. of bits = ceil( log(400C4) ) = 30(ANSWER)

Answer 2

there are total of 400 signals but we don't know which 4 signals are active, so we will make 4 groups by taking 400 in each, out of those groups only one signal will be active at a time.to represent that signal we need 9 bits so for 4 groups we need a total of 36 bits