In a micro programmed control unit, a control field of one...

Name: Problem 16, Chapter 6: Computer Science and Information Technology
Uploaded: 2023-02-02T22:56:19-08:00
Duration: 1 min
Channel: James Kiss
Description: In a micro programmed control unit, a control field of one address control instruction has to support two groups of control signals. In groupl it is required to generate either one or none of the 32 control signals. In group 2 at most 5 from the remaining, what will be the number of bits needed for the control field? (A) 8 (B) 10 (C) 35 (D) 37

In a micro programmed control unit, a control field of one address control instruction has to support two groups of control signals. In groupl it is required to generate either one or none of the 32 control signals. In group 2 at most 5 from the remaining, what will be the number of bits needed for the control field? (A) 8 (B) 10 (C) 35 (D) 37

Key Concepts

One?of?n Encoding

This concept refers to the encoding method used when exactly one (or none) out of a group of signals is allowed to be active. Instead of having one bit per signal (as in a one?hot scheme), the designer can encode the 32 possibilities (plus the option of no signal) using a binary code. The minimal number of bits needed is the ceiling of the base?2 logarithm of the total number of possibilities.

Combination Encoding for Non?Mutually Exclusive Signals

When several control signals can be active at the same time (up to a fixed maximum), one may not simply assign a bit to each control line if one wishes to compress the microinstruction. Instead, the designer can count the number of possible combinations (given by the sum of certain binomial coefficients) and then encode that set of possibilities in the minimum number of bits (again by taking the log base?2, rounded up).

Calculation of Bit?Field Length

This is the process of determining how many binary bits are required to represent a given number of distinct states or combinations. For a group that is mutually exclusive (or one?of?n), the number of codes needed is one more than the number of signals if the option of ‘none’ is allowed. For groups where multiple signals may be activated, counting the number of valid combinations and then computing the ceiling of the base?2 logarithm of that total gives the required number of bits.

Microprogrammed Control Unit

In a microprogrammed control unit, the control word (or microinstruction) specifies which control signals are to be activated. By partitioning these signals into groups – one group where at most one signal is active and another where several signals may be simultaneously active – designers can use different encoding schemes for each group and thereby reduce the overall size of the microinstruction word.

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