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The memory unit of a computer has $256$ K words of $32$ bits each. The computer has an instruction format with four fields: an operation code field, a mode field (to specify one of seven addressing modes), a register address field (to specify one of $60$ processor registers), and a memory address.

What are the number of bits, in each of the below given fields, when the instruction is $one$ memory word long?

  1. Op-code
  2. Mode
  3. Register address
  4. Memory address 

 

  1. $6,5,3,18$
  2. $5,6,3,18$
  3. $5,18,6,3$
  4. $5,3,6,18$
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  1.  Op-code => 32 - ( 3 + 6 + 18 ) = 5  bits
  2.  Mode => we need to identify 1 of 7 addressing modes so 23 = 8 hence 3 bits are required .
  3. Register Address => 60 registers implies 26 = 64 so 6 bits
  4. Memory Address => 256 K = 28 * 210  = 218  means 18 bits 

Hence option D , 5,3,6,18 .

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