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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.
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Best answer
21 votes
21 votes

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.

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4 votes
4 votes
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

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