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     The Non-contiguous memory allocation allows a process to acquire the several memory blocks at the different location in the memory according to its requirement. The noncontiguous memory allocation also reduces the memory wastage caused due to internal and external fragmentation. As it utilizes the memory holes, created during internal and external fragmentation.

Paging and segmentation are the two ways which allow a process’s physical address space to be non-contiguous. In non-contiguous memory allocation, the process is divided into blocks (pages or segments) which are placed into the different area of memory space according to the availability of the memory.

The noncontiguous memory allocation has an advantage of reducing memory wastage but, but it increases the overheads of address translation. As the parts of the process are placed in a different location in memory, it slows the execution of the memory because time is consumed in address translation.

Here, the operating system needs to maintain the table for each process which contains the base address of the each block which is acquired by the process in memory space.

Working:
Here a process can be spanned across different spaces in main memory in non-consecutive manner. Suppose process P of size 4KB. Consider main memory have two empty slots each of size 2KB. Hence total free space is, 2*2= 4 KB. In contiguous memory allocation, process P cannot be accommodated as spanning is not allowed.

In contiguous allocation, space in memory should be allocated to whole process. If not, then that space remains unallocated. But in Non-Contiguous allocation, process can be divided into different parts and hence filling the space in main memory. In this example, process P can be divided into two parts of equal size – 2KB. Hence one part of process P can be allocated to first 2KB space of main memory and other part of processP can be allocated to second 2KB space of main memory. Below diagram will explain in better way:

But, in what manner we divide a process to allocate them into main memory is very important to understand. Process is divided after analysing the number of empty spaces and their size in main memory. Then only we divide our process. It is very time consuming process. Their number as well as their sizes changing every time due to execution of already present processes in main memory.

In order to avoid this time consuming process, we divide our process in secondary memory in advance before reaching the memory memory for its execution. Every process is divided into various parts of equal size called Pages. We also divide our main memory into different parts of equal size called Frames. It is important to understand that:

Size of page in process 
= Size of frame in memory

Although their numbers can be different. Below diagram will make you understand in better way: consider empty main memory having size of each frame is 2 KB, and two processes P1 and P2 are 2 KB each.

Resolvent main memory,

In conclusion we can say that, Paging allows memory address space of a process to be non-contiguous. Paging is more flexible as only pages of a process are moved. It allows more processes to reside in main memory than Contiguous memory allocation.

contiguous Memory allocation