Operating System (OS)
An operating system is a program or a group of programs that

Helps in checking the spelling of Word
Manages the resources of the Computer
Maintain the relationship in Database
Performs the calculations of cells in Excel

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Operating System (OS)
Addressing modes

defines the fundamental method of determining effective operand addresses
None of these
are variations in the use of fundamental addressing structures, or some associated actions which are related to addressing.
All of these
performs indicated operations on two fast registers of the machine and leave the result in one of the registers.

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Operating System (OS)
A critical region is

a program segment that has not been proved bug-free
none is the above
one which is enclosed by a pair of P and V operations on semaphores
a program segment where shared resources are accessed
a program segment that often causes unexpected system crashes

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