Overlays are made when there is a need of running a program of larger size than the memory allocated to it. A program is broken into overlays say overlay1 and overlay2, now overlay1 is loaded into main memory and overlay2 is still in secondary memory. When overlay1 finishes its execution, it gets overwritten by overlay2 and it gets executed without loading whole program at a time. Swapping in a scheme in which pages from main memory to secondary memory are being swapped in(rolled in) and swapped out(rolled out) as desired during execution of a process.

OSI is a seven layered architecture used to establish communication between 2 systems which have nothing in common like they are in different networks, having different hardware and software, having different internal architectures. Those seven layers are: physical , datalink, network, transport, session, presentation and application. Each lower layer provides its functionality to the layer above it.

Operating system is an intermediary between computer user and computer hardware, it acts as an interface between user and hardware. It resides in upper portion of main memory and works in privileged mode. Functions of OS are: file management, memory management, device management and process management. Goal of OS are: convenience (eg. Windows OS) and efficiency (eg. Unix). Generations of OS are: 1.sequential,
2.Batch,
3.Multiproggramming,
4.Real time OS.

Contiguous memory believes in storing data in continuous locations. It is the largest chuck of available memory that is not fragmented. When a process needs to execute, memory is requested by the process. The size of the process is compared with the amount of contiguous main memory available to execute the process. If sufficient contiguous memory is found, the process is allocated memory to start its execution. Otherwise, it is added to a queue of waiting processes until sufficient free contiguous memory is available.

Compiler is a program which coverts High Level Language (source code) into assembly level language (executable code). Compiler works fast as compared to interpreter. Compiler comprises of following:

Process is a program in execution. Process is dynamic and active and it resides in Main memory. Process is a data structure which has attributes as well as process states. Attributes are like process id, state, GPR, program counter, list of open files and devices etc. Process states are: ready, running, block, suspended etc. There are 3 types of schedulers namely: long term (new to ready ), short term(ready to run) and medium term(for swapping).