GATE Overflow Test Series | Mock GATE | Test 5 | Question: 15

Question

Consider a CPU where all the instructions require $4$ clock cycles to complete execution. There are $300$ instructions in the instruction set. It is found that $164$ control signals are needed to be generated by the control unit. While designing the horizontal microprogrammed control unit, single address field format is used for branch control logic. If the minimum size in bits of the control word required is $\alpha$ and that of control address register is $\beta,\; \alpha+\beta = $ _________

Answer 1

Each of the $300$ instructions require $4$ clock cycles.
    
This means total number of microinstructions $=300\times 4 = 1200$
    
We will need $\left\lceil \log_2 1200  \right\rceil = 11$ bits to address them.
   
Horizontal microprogramming means we will use a separate bit for each of the control signals.
    
So, size of control word = $164 + 11 = 175$ and control address register size $= 11$ bits.
    
So, $\alpha + \beta = 175+11=186.$

Comments

yes you are correct. Despite solving pyqs, i still get confused. Because i have read from 2 sources and one considers that we store microinstruction in control memory which has flags, control words and next address. And in other it considers control word as whole.

Even if we see this question https://gateoverflow.in/1061/gate2004-67 it gives an impression that first approach sounds logical.

Whereas in this https://gateoverflow.in/3349/gate2008-it-39 it is different. Also if you this diagram, even it points to the logic i mentioned first.

 

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all ok!

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@reboot Great observation! I guess all the details will be given in the questions about the contents of control word. But the general scenario could be
 

Control word or Micro Instruction have 3 parts. 1) Conditions Part 2) Control Signals part and 3) Next Address part.