Operating System Question and Answers:

1. What are the basic functions of an operating system? - Operating system controls and 
coordinates the use of the hardware among the various applications programs for various uses. 
Operating system acts as resource allocator and manager. Since there are many possibly 
conflicting requests for resources the operating system must decide which requests are allocated 
resources to operating the computer system efficiently and fairly. Also operating system is 
control program which controls the user programs to prevent errors and improper use of the 
computer. It is especially concerned with the operation and control of I/O devices. 

2. Why paging is used? - Paging is solution to external fragmentation problem which is to permit 
the logical address space of a process to be noncontiguous, thus allowing a process to be 
allocating physical memory wherever the latter is available. 

3. While running DOS on a PC, which command would be used to duplicate the entire 
diskette? diskcopy 

4. What resources are used when a thread created? How do they differ from those when a 
process is created? - When a thread is created the threads does not require any new resources to 
execute the thread shares the resources like memory of the process to which they belong to. The 
benefit of code sharing is that it allows an application to have several different threads of activity 
all within the same address space. Whereas if a new process creation is very heavyweight 
because it always requires new address space to be created and even if they share the memory 
then the inter process communication is expensive when compared to the communication 
between the threads. 

5. What is virtual memory? - Virtual memory is hardware technique where the system appears to 
have more memory that it actually does. This is done by time-sharing, the physical memory and 
storage parts of the memory one disk when they are not actively being used. 

6. What is Throughput, Turnaround time, waiting time and Response time? – 
1. Throughput : number of processes that complete their execution per time unit. 
2. Turnaround time: amount of time to execute a particular process. 
3. Waiting time; amount of time a process has been waiting in the ready queue. 
4. Response time: amount of time it takes from when a request was submitted until the first 
response is produced, not output (for time-sharing environment). 

7. What is the state of the processor, when a process is waiting for some event to occur? - 
Waiting state 

8. What is Dispatcher? - Dispatcher module gives control of the CPU to the process selected by 

the short-term scheduler; this involves: Switching context, Switching to user mode, Jumping to 

the proper location in the user program to restart that program, dispatch latency ‚ time 

it takes for the dispatcher to stop one process and start another running. 

9. What is CPU Scheduler? - Selects from among the processes in memory that are ready to 

execute, and allocates the CPU to one of them. CPU scheduling decisions may take place when a 

process: 1.Switches from running to waiting state. 2.Switches from running to ready state. 

3.Switches from waiting to ready. 4.Terminates. Scheduling under 1 and 4 is non-preemptive. 

All other scheduling is preemptive. 

10. What is Context Switch? - Switching the CPU to another process requires saving the state of 

the old process and loading the saved state for the new process. This task is known as a context 

switch. Context-switch time is pure overhead, because the system does no useful work while 

switching. Its speed varies from machine to machine, depending on the memory speed, the 

number of registers which must be copied, the existed of special instructions(such as a single 

instruction to load or store all registers). 

11. What is cache memory? - Cache memory is random access memory (RAM) that a computer 

microprocessor can access more quickly than it can access regular RAM. As the microprocessor 

processes data, it looks first in the cache memory and if it finds the data there (from a previous 

reading of data), it does not have to do the more time-consuming reading of data from larger 

memory. 

12. What is a Safe State and what is its use in deadlock avoidance? - When a process requests an 

available resource, system must decide if immediate allocation leaves the system in a safe state. 

System is in safe state if there exists a safe sequence of all processes. Deadlock Avoidance: 

ensure that a system will never enter an unsafe state. 

13. What is a Real-Time System? - A real time process is a process that must respond to the events 

within a certain time period. A real time operating system is an operating system that can run real 

time processes successfully 

14. What is hard disk and what is its purpose? - Hard disk is the secondary storage device, which 

holds the data in bulk, and it holds the data on the magnetic medium of the disk.Hard disks have 

a hard platter that holds the magnetic medium, the magnetic medium can be easily erased and 

rewritten, and a typical desktop machine will have a hard disk with a capacity of between 10 and 

40 gigabytes. Data is stored onto the disk in the form of files. 

15. What is fragmentation? Different types of fragmentation? - Fragmentation occurs in a 

dynamic memory allocation system when many of the free blocks are too small to satisfy any 

request. 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. Internal Fragmentation: Internal fragmentation is the space wasted inside of 

allocated memory blocks because of restriction on the allowed sizes of allocated blocks. 

Allocated memory may be slightly larger than requested memory; this size difference is memory 

internal to a partition, but not being used