Instructions

Question

A computer has 40-bit instruction format. It uses 1 register operand and 1 memory operand. There are 128 general purpose registers and 256 M bytes of RAM.
Q. 15 How many different operations the computer can perform if the indirect bit (mode) is used as the part of the operation code.

• A. 32
  
• B. 16
  
• C. 17
  
• D. 8

Answer 1

Here the case is like MODE bit is one only as it indicate the Indirect mode 

MODE -1bit

OPCODE-?

REGISTER OPERAND require 7 bits (128 registers are there)

MEMORY OPERAND require 28 bits

Now total length of INSTRUCTION is 40 

so total bits for OPCODE= 40-28-7-1=4 bits ..

So total operations possible are 2^4 = 16

Comments

Why we use 1 bit for mode

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in question they told that we have indirect mode ...so we assign 1 bit only ....now say in question if they told that we have direct , indirect and immediate mode then ..?

Then we have to assign 3 bits for MODE and we can have only 2 bits for OPCODE ..so we might be able to do only 4 operations ...

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How we check how bits used for direct indirect immediate i m confused

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like 3 bits are allocated

so 000-> direct mode

    001-> Indirect mode

     010->Immediate mode like that

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This bits are fixed??

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no ..its a defined at the time of creation of INSTRUCTION set

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How wo can define indirect mode bit for this question