This is a draft cheat sheet. It is a work in progress and is not finished yet.
Endianess
Big Endian |
01 23 45 67 |
Little Endian |
67 45 23 01 |
32 bit integer is 0x01234567
IPC
Semaphore |
Mutex |
Message Passing |
Shared Memory |
Sockets |
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
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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
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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.
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Detect Endianess
void endian()
{
unsigned int i = 1;
char c = (char)&i;
if (*c)
printf("Little endian");
else
printf("Big endian");
}
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RTOS
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
Paging |
Blocks of memory that are stored in auxiliar memory and replaced in main memory when a program needs it. |
Caching |
Data is temporarily stored in high speed memory for faster access. |
Segmentation |
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");
}
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Define 2 local variables one after other and compare their addresses.
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