Computer Organisation & Architecture

Question

Control field of a microinstruction supports 2 groups of control signals in which group 1 indicates none or one of 325 signals and group 2 indicates at most 8 signals from the remaining.What is the size of a control field in the microinstruction?

Answer 1

see

group 1 indicates none or one of 325 signals ie. vertical micro programmed control unit (log n bit format are required to represent the control signal)

= log ₂ (325) bits (ceiling value) =9

group 2 indicates at most 8 signals ie. horizontal micro programmed control unit (n bit format are required to represent the control signal) 

=8 bits

 so size of control field in micro instruction is = 9 + 8 =17 bits

Comments

Pawan,

Why are we considering based on the terminology here(at most for horizontal microprogramming)

See if it is not mentioned exclusively which CU the default should be vertical CU since practically the horizontal CU is not possible.

Can you please explain in this regard as to why we are taking some part of the control field as vertical and another part as the horizontal format.

Thanks in advance.

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>in horizontal micro programmed allow high degree of parallelism ie. none or more than one

(here 1 bit/control signal)

>in  vertical micro programmed allow low degree of parallelism ie. none or one

(here log n bits are required)