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OS Interview Questions Cheat Sheet (DRAFT) by

OS Interview Questions

This is a draft cheat sheet. It is a work in progress and is not finished yet.


Big Endian
01 23 45 67
Little Endian
67 45 23 01
32 bit integer is 0x01234567


Message Passing
Shared Memory


Priority Inversion

In a scenario where a low priority task holds a shared resource (example semaphore) that is required by a high priority task. This causes the execution of the high priority task to be blocked until the low priority task has released the resource. This scenario is averted by the OS by increasing the priority of the low­ priority process until it completes the task and releases the resources

Stack and Heap

Stack Allocation
The allocation happens on contiguous blocks of memory. It happens in the function call stack. The size of memory to be allocated is known to compiler and whenever a function is called, its variables get memory allocated on the stack. And whenever the function call is over, the memory for the variables is deallo­­cated. Handled by the compiler
Heap Allocation
The memory is allocated during execution of instru­­ctions written by progra­­mmers. It is called heap because it is a pile of memory space available to progra­­mmers to allocated and de-all­­ocate. If a programmer does not handle this memory well, memory leak can happen in the program.

Detect Endianess

void endian() 
  unsigned int i = 1;
  char c = (char)&i;
  if (*c)    
     printf("Little endian");
     printf("Big endian");


An RTOS will have a determ­­in­istic scheduler.
For any given set of tasks your process will always execute every number of micros­­econds or millis­­econds exactly, and the same number from schedule to schedule.
OS services consume only known and expected amounts of time.
In UNIX or Windows the scheduler are usually soft­ RT (as opposed to some hard ­real­ time RTOS). Soft ­real­time means that the scheduler tries to assure your process runs every X number of millis­­ec­onds, but may fail to do so on some occasions.
Modern RTOSs simply make sure that a) no interrupt is ever lost, and b) no interrupt can be blocked by a lower priority process.

Memory Management

Blocks of memory that are stored in auxiliar memory and replaced in main memory when a program needs it.
Data is tempor­­arily stored in high speed memory for faster access.
Segmen­­tation is a memory management scheme. This is the technique used for memory protec­­tion. Any accesses outside premitted area would result in segmen­tation fault.
Virtual Memory
This technique enables non­co­­nt­i­guous memory to be accessed as if it were contig­uous. Same as paging

Stack Growth Direction

void checkStack()
   int i=2;
   int j=3;

   if(&i > &j) printf("downwards");
   else printf("upwards");
Define 2 local variables one after other and compare their addresses.