GATE CSE 2008 | Question: 37, ISRO2009-38

Question

The use of multiple register windows with overlap causes a reduction in the number of memory accesses for:

1. Function locals and parameters

2. Register saves and restores

3. Instruction fetches

• A. $\text{I}$ only
• B. $\text{II}$ only
• C. $\text{III}$ only
• D. $\text{I}, \text{II}$ and $\text{III}$

Comments

@tuhin

in order to store the function values in registers in has to access the memory

so register saves requires access to memory

am i right in my understanding?

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Thank you @Tuhin Dutta, the explanation was very helpful.

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Thanks for sharing the link, it is really helpful

Answer 1

1. Functions locals and parameters
   this is true because overlapped registers eliminates the need for memory accesses. we here got to use registers instead.
    
2. Register saves and restores
   this is false bc we need to see where memory accesses are reduced here before also we were using register as it says Register saves... later also (i.e. after using multiple register windows) registers will are referred. So NO memory accesses are reduced here.
    
3. Instruction fetches
   it has nothing to do with reduction in memory accesses.

Hence, option (A) is correct.

Comments

what's the answer in the answer key to the problem.

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@Arjun sir @Bikram sir

doesn't option B means register values save & restore ? Because we know , during subroutine CALL & RETURN, we save PSW ,PC & some other resgiter values used by calling program to memory... 

​​​​So both A & B should be true... 

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most of the questions from the 2008 paper of COA were somewhat from untouched topics…

Answer 2

→ I is true because when we make a function call there are some input registers and some output registers. If function F() is calling function G(), we can make the caller function F()'s output registers the same as the called procedure G()'s input registers this is done using overlapping register windows.This will reduce the memory accesses so that F()'s output need not be put into memory for G() to access again from memory.
→ II is false as register saves and restores would still be required for each and every variable.
→ III is also false as instruction fetch is not affected by memory access using multiple register windows.

Comments

Nicely Explained

Answer 3

A. I only

Ref: http://www.cs.unm.edu/~maccabe/classes/341/labman/node11.html

Comments

Can't understand ..plz explain

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Never  heard about this topic earlier...

Thank You so much sir. It helped a lot.

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Option A is correct

The use of multiple register window with overlap cause a reduction in number of memory access for function local and parameters because overlapped register eliminate the need of memory register..