GATE CSE 1998 | Question: 1.19

Question

Which of the following addressing modes permits relocation without any change whatsoever in the code?

• A. Indirect addressing

• B. Indexed addressing

• C. Base register addressing

• D. PC relative addressing

Comments

The question makes more sense if they remove the "in the code" part.

If a swap happens, the base register needs to updated to reflect the new memory location at which the process is loaded.

However, no such management is required in PC relative addressing.

Hence, PC relative addressing requires no change whatsoever during both - the intial load and also while swapping the process into new memory location.

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Answer: $Option (D)$

Think of it like this - 

• If the code has already been written by using PC relative indexing addressing mode. In this case, if this code is reloacted to any position, nothing has to be changed. It will simply work as it is.

 

• But if the code was written using base register addressing mode, then if the code is relocated to some other location, then the contents of the base register will have to be changed.

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pc relative mode addressing

option D is correct

"relocatable code aims to be independent of absolute memory without requiring changes of the code itself. to achieve relocatibility , assembly often provide the feactres such as symbolic register relative addressing mode , base addressing mode or the use if labels".

Answer 1

(D) PC relative addressing is the best option. For Base register addressing, we have to change the address in the base register while in PC relative there is absolutely no change in code needed.

Comments

it is the best answer thanks buddy

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but base register is not part of code , instead it is updated by loader(os).
 

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@suraj_soni

yes you are correct both C and D should be the answers ;  i'm just adding few more lines 

@Abhrajyoti00 

No Base Register AM is also correct because we may need to change the address value in base register that's fine but we don't represent that address value in the instruction we only use base register in instructions

In PC Relative AM : 

LOAD R1, 100(PC)

Initially PC = 1000 

Now, EA =  PC + offset 

               =  1100

if the program relocated to address 5000 then PC automatically updated to  5000 

EA = 5000 + 100 = 5100

As you can see the instruction LOAD R1, 100(PC) (which is part of program didn't changed a bit) so it supports program relocation without making any changes to program code.

In Base Register AM : 

LOAD R1, 100(RB)

Initially RB (Base register) = 1000

EA = 1000 + 100 = 1100

If program relocated to address 5000 then RB updated to 5000 by OS Loader Program 

EA = 5000 + 100 = 5100

As you can see this instruction  LOAD R1, 100(RB) (which is part of program code) didn't changed a little bit 

so,Base Register AM also supports program relocation without making any changes to program code.

Hence,both C and D should be the answers.  

Answer 2

Ans: D

Code relocation can be done whenever addresses are not directly specified in the program (absolute), but are relative to some variable or other modifiable source.

For 3), base addressing, a register points to the base of the structure and the effective address is obtained by computing an offset from the base. The base address register can be loaded at run time to point to the base of the structure so the code doesn't have to be modified.

For 4), PC relative, the effective memory address is obtained by computing an offset from the current PC. No matter where the code is loaded in memory, the offsets are always the same so the code needs no modification.

ref: https://cs.stackexchange.com/questions/48730/which-addressing-modes-permits-relocation-without-any-change-whatsoever-in-the-c

Comments

So ,you are supporting both the options,then why the answer is d?

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Bcz Option D is more appropriate

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@rahul sharma sir ,PC-relative AM is applicable only on a particular segment inside the memory at a time,but Base AM is applicable on the whole memory at a time,so it have wide area so (C) should be most appropriate choice

Answer 3

Option D, PC relative addressing

w.r.t Morris Mano,

The base register addressing mode is used in computers to facilitate the relocation of programs in memory. When programs and data are moved from one segment of memory to another, as required in multiprogramming systems, the address values of instructions must reflect this change of position. With a base register, the displacement values of instructions do not have to change. Only the value of the base register requires updating to reflect the beginning of a new memory segment.

In PC Relative Addressing Mode, nothing need to  be changed. 

Answer 4

C- Base register addressing

Answer 5

c) Base Register Addressing

d) PC- relative addressing

Above is the correct option.

Answer 6

PC (Program Counter) Relative Addressing Mode permits relocation without any change whatsoever in the code.

Therefore, Option D is correct.