GATE CSE 2008 | Question: 33, ISRO2009-80

Question

Which of the following is/are true of the auto-increment addressing mode?

1. It is useful in creating self-relocating code

2. If it is included in an Instruction Set Architecture, then an additional ALU is required for effective address calculation

3. The amount of increment depends on the size of the data item accessed

• A. I only
• B. II only
• C. III only
• D. II and III only

Comments

Video Solution: https://youtu.be/CS26a7fXN58?si=1JN0fGoyOFpzsPoT

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For option B:

Just like PC increments, all auto-increments can be implemented without the ALU (using a small adder in the address generation unit). But in architectures that keep hardware simpler (RISC), increments are often forced through explicit ALU instructions instead.

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Auto Increment mode 

credit : Go claases(resources)

Answer 1

In auto increment addressing mode, the base address is incremented after operand fetch. This is useful in fetching elements from an array. But this has no effect in self-relocating code (where code can be loaded to any address) as this works on the basis of an initial base address. 

An additional ALU is desirable for better execution especially with pipelining, but never a necessity. 

Amount of increment depends on the size of the data item accessed as there is no need to fetch a part of a data. 

So, answer must be C only. 

Comments

1. It is useful in creating self-relocating code

Can anyone tell what is basically means to say self-relocating code and is there any addressing mode that is self relocating code.?? 

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Self Relocating

means where the operand given is accessed using offset and not by its absolute address(as in direct/absolute addressing). addressing modes which use base register and operand offset to calculate the address of operand. as these block of code can be reloacted and will still work perfectly fine because they rely only on the offset from the base address of the frame. 

Addressing modes which are for self relocating : Relative Addressing , Base Addressing

Addressing modes which are not for self relocating code : Direct Addressing(Absolute addresssing) etc

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auto indexing = register indirect + auto incr/decr

auto indexing : Depends on absolute register values

Auto-indexing uses register contents directly as memory addresses.
So if the program is moved to another memory region,
→ the absolute address stored in the register becomes invalid.

PC relative addressing mode is indeed very useful in creating self-relocating code

Answer 2

From Hamcher,

so answer is (C).

Answer 3

Correct answer (C)

Auto increment mode is used for incrementing and depends on size of data.

Answer 4

no alu required and also no relocation is involved 

 

Answer 5

After determining the effective address, the value in the base register is incremented by the size of the data item that is to be accessed.
For example, (A7)+ would access the content of the address register A7, then increase the address pointer of A7 by 1 (usually 1 word). Within a loop, this addressing mode can be used to step through all the elements of an array or vector.

So, option (C) is right

Answer 6

Ans:(C) 

No relocation required. And no additional ALU is required for Effective Address Calculation