Question

As processes are loaded and removed from memory , the free memory space is broken into little pieces ,causing fragmentation … but how does this happen ?
And what is the best solution to external fragmentation ?

Answer 1

External Fragmentation

External fragmentation happens when a dynamic memory allocation algorithm allocates some memory and a small piece is left over that cannot be effectively used. If too much external fragmentation occurs, the amount of usable memory is drastically reduced. Total memory space exists to satisfy a request, but it is not contiguous.
see following example

 0x0000 0x1000  0x2000  
   A    B     C               //Allocated three blocks A, B, and C, of size 0x1000.
   A          C         //Freed block B

Now Notice that the memory that B used cannot be included for an allocation larger than B’s size