Скачать или смотреть OVERLAY | MEMORY MANAGEMENT | OPERATING SYSTEMS | GATE CSE LECTURES

In this video, I have discussed about overlay concept in detail
00:00 - Introduction
00:20 - overview of contiguous memory allocation
03:17 - Overlay 3:17
03:53- Example
07:02 - Advantages
07:52 - Disadvantages
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The main problem in Fixed partitioning is the size of a process has to be limited by the maximum size of the partition, which means a process can never be span over another.In order to solve this problem, earlier people have used some solution which is called as Overlays.

The concept of overlays is that whenever a process is running it will not use the complete program at the same time, it will use only some part of it. Then overlays concept says that whatever part you required, you load it and once the part is done, then you just unload it, means just pull it back and get the new part you required and run it.
Formally,
“The process of transferring a block of program code or other data into internal memory, replacing what is already stored”.
Sometimes it happens that compare to the size of the biggest partition, the size of the program will be even more, then, in that case, you should go with overlays.
So overlay is a technique to run a program that is bigger than the size of the physical memory by keeping only those instructions and data that are needed at any given time.Divide the program into modules in such a way that not all modules need to be in the memory at the same time.

Advantage –

Reduce memory requirement
Reduce time requirement

Disadvantage –

Overlap map must be specified by programmer
Programmer must know memory requirement
Overlapped module must be completely disjoint
Programming design of overlays structure is complex and not possible in all cases