XiLinx Interview Questions and Answers for 4 years Experience

• In order to align the data in memory,  one or more empty bytes (addresses) are inserted (or left empty) between memory addresses which are allocated for other structure members while memory allocation. This concept is called structure padding.

• Architecture of a computer processor is such a way that it can read 1 word (4 byte in 32 bit processor) from memory at a time.


• To make use of this advantage of processor, data are always aligned as 4 bytes package which leads to insert empty addresses between other member’s address.

• Because of this structure padding concept in C, size of the structure is always not same as what we think.

       For example, please consider below structure that has 5 members.

 

struct student

{

       int id1;

       int id2;

       char a;

       char b;

       float percentage;

};..

• As per C concepts, int and float datatypes occupy 4 bytes each and char datatype occupies 1 byte for 32 bit processor. So, only 14 bytes (4 4 1 1 4) should be allocated for above structure.


• But, this is wrong.  Do you know why?

• Architecture of a computer processor is such a way that it can read 1 word from memory at a time.


• 1 word is equal to 4 bytes for 32 bit processor and 8 bytes for 64 bit processor. So, 32 bit processor always reads 4 bytes at a time and 64 bit processor always reads 8 bytes at a time.


• This concept is very useful to increase the processor speed.


• To make use of this advantage, memory is arranged as a group of 4 bytes in 32 bit processor and 8 bytes in 64 bit processor.