GATE CSE 1987 | Question: 4a

Question

Find out the width of the control memory of a horizontal microprogrammed control unit, given the following specifications:

• $16$ control lines for the processor consisting of ALU and $7$ registers.
• Conditional branching facility by checking $4$ status bits.
• Provision to hold $128$ words in the control memory.

Comments

As in question "horizontal micro-programmed control unit" is told that's why we can't use decoder to decrease required number of status bits (like we do in vertical miro-programmed control unit). So, we need 4 status bits in "control word" register. Let me know if i am wrong ?

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@Aalok8523

This is saying the same thing https://www.geeksforgeeks.org/difference-between-horizontal-and-vertical-micro-programmed-control-.

Answer 1

Width of the control memory $=$ size of a control word

• $\text{Control word} = [\text{Condition bits} + \text{control signal bits} + \text{next address}]$
• Condition bits $= 4 \implies \log_2 4$-bits
• $128$ words $\implies 7$-bits for address.

In the case of horizontal micro-programming, there is $1$- bit for each control signal. It is given there are $16$ control signals.

$\implies$ Length of control word $= 2+16+7 = 25$- bits.

Comments

Somebody plzz explain..why are we not considering the 7 registers ?

what is their use in this question ??

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@Ajit_Singh

7 registers are nothing to do with the given question , they just describe that processor consists of ALU and 7 registers.

They just want to confuse us with extra information.

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@jaswanth431

see here, for both horizontal and vertical ,there is different criteria for selecting the flag bits,in case of horizontal all the bits are taken , and in vertical log(n) bits are taken(where n are no. of status bits)

https://www.geeksforgeeks.org/difference-between-horizontal-and-vertical-micro-programmed-control-

so according to this , answer for this question must be $27$ bits.

Answer 2

....

Answer 3

16 bits for 16 control lines.(As Horizontal)

For next address field 7 bits (128 control words.. 2^7=128)

4 status bits.

Width of control memory = control word length = 4+16+7 = 27 bits

Comments

No, $25$ bits will be the correct answer, as we need only 2x1 MUX for 4 status bits.

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:https://gateoverflow.in/1061/gate2004-67 watch this 25 is correct

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Correct me if I am wrong.

In the question https://gateoverflow.in/1061/gate2004-67 It is not given that it is a horizontal microprogrammed unit. Then how can we apply that logic to this question.