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Consider a CPU where all the instructions require $7$ clock cycles to complete execution. There are $140$ instructions in the instruction set. It is found that $125$ 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. What is the minimum size of the control word and control address register?

  1. $125, 7$
  2. $125, 10$
  3. $135, 9$
  4. $135, 10$
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45 votes
Its answer should be $(D)$ because $140$ instructions, each requiring $7$ cycles means $980$ cycles which will take $10$ bits.

Since it is horizontal for control word, $125 \text{ control signals} + 10$ bits $=135$ bits will be required.
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Ans  is  D Part.

Explanation -->

140 (No. of instructions)

7 (No. of cycles required per instruction) 

We need 980 (140*7) memory locations to store control signal corresponding to different instructions. 980 location could be addressed by 10 bits ($\lceil{\log_2(980)}\rceil$) (= Minimum Size Control Address Register).

Horizontal Micro-programmed Control Unit (means no encoding)

Single Address Field (means in each address of control memory contains control signals + address).

125 (No. of different control signal) 

Minimum size of the control word = 10 + 125 = 135 (Width of Control Memory)

21 votes
21 votes

control address register will contain address of control word..control word will is a micro operation (cond flags+control signals+ next control word address)..since 980 cycles are needed..in each cycle 1 micro operation can be performed so 980 micro operations are there in control memory so 10 bits are required for addressing purpose
and control word size=125(control signal bits) +10=135

13 votes
13 votes
each microintruction in an instruction require one cycle, so thats why 140 is multiplied with 7 to get total no of microinstructions which are  980

these 980 micro instruction will be stored in control memory (rom or ram) so to uniquely identify each of them we require 10 bits,

microinstruction is divided into 3 parts - >   flag      microoperation     nextAddress

since nothing mentioned about flags take it 0 - these flags tells about branch and non branch microinstruction

microoperation - > 125 contol signal mentioned are nothing but microoperation and since we are using horizontal
                                   microinstruction these 125 bits will not be encoded so take it 125

next address -> 10 bits from those 140 instructions we calculated are next address bits

total size of control word = 0+125+10 = 135

now control address register size is equal to next address field bits = 10 bits
so answer us -- >   135, 10
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