ISRO-DEC2017-43

Question

Consider an instruction of the type $\text{LW R1, 20(R2)}$ which during execution reads a $32$-$bit$ word from memory and stores it in a $32$-$bit$ register $R1.$ The effective address of the memory location is obtained by adding a constant $20$ and contents of $R2.$ Which one best reflects the source operand?

• A. Immediate addressing 
• B. Register addressing 
• C. Register Indirect addressing 
• D. Indexed addressing

Comments

Why is (C) wrong?

Answer 1

Answer must be D) Indexed addressing

• Address field stores starting address and Index register stores a signed number (displacement )
• Suitable for iterative operations, array implementation .
• Register R2 acts as index register and 20 is  the Base address.

Edit: ISRO has corrected the answer in revised key :)

Answer 2

The given answer key of this question is also wrong. It is given as C) Register Indirect addressing but answer should be D) Indexed addressing.

https://gateoverflow.in/2123/gate2011_21

Answer 3

 

LW R1, 20(R2)

Addressing modes essentially deal with the question — "Where is the operand?"

1. Option A would give a constant value in the instruction, which is supposed to be our operand. Not the case here.
    
2. Option B would say that the operand is in the register. Not the case here.
    
3. Option C would say that the operand is in the address specified by the register. Not the case here.
    
4. Option D would say that the operand is given by an indexed address. Now see LW R1, 20(R2).
   
   LW R1, 20(R2) — 20 is the base.
   
   LW R1, 20(R2) — Contents of R2 act as index. R2 is essentially the index register.

Hence, Option D is the answer.

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Bonus:-

• For relocatable/position-independent code: Base Register mode, PC relative mode.
• Accumulator uses: Implied mode.
• To implement parameter-passing and pointers: Indirect mode.
• Loops: Auto inc/dec mode.