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Re entrant code is code which does not rely on being executed without interruption before completion. Reentrant code can be used by multiple, simultaneous tasks. Reentrant code generally does not access global data. Variables within a reentrant function are allocated on the stack, so each instance of the function has its own private data. Nonreentrant code, to be used safely by multiple processes, should have access controlled via some synchronization method such as a semaphore.
A RTOS offers services that allow tasks to be performed within predictable timing constraints.
unix is a multitasking operating system, multiprocessing means it can run on multiple processors, the multiproceesing os coordinates with multiple processors running in parallel.
A core dump is the recorded state of the working memory of a computer program at a specific time, generally when the program has terminated abnormally includes the program counter and stack pointer, memory management information, and other processor and operating system flags and information a fatal error usually triggers the core dump, often buffer overflows, where a programmer allocates too little memory for incoming or computed data, or access to null pointers, a common coding error when an unassigned memory reference variable is accessed.
RTOS has predictable timing constraints.
We can use thr fork system call to create a process in UNIX and in OSE the system call create_process is used.
Paging: Paging is a technique where in the OS makes available the data required as quickly as possible. It stores some pages from the aux device to main memory and when a prog needs a page that is not on the main memory it fetches it from aux memory and replaces it in main memory. It uses specialised algorithms to choose which page to replace from in main memory.
Caching: It deals with a concept where the data is temperorarily stored in a high speed memory for faster access. This data is duplicated in cache and the original data is stored in some aux memory. This concepts brings the average access time lower.
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.
if(&i > &j) printf("stack grown downwards");
else printf("stack grows upwards");
define 2 local variables one after other and try to print the address
For process synchronization, it is a mechanism to invoke the sleeping process to become ready for execution. Its mechanism where a process can wait for resources to be available.typical example is producer consumer process. The producer process creates resources and signals the semaphore saying resource is available. Consumer process waiting on the semaphore gets the signal that resource is available.
Binary semaphore and counting semaphore. Binary semaphore is same as mutex. Binary semaphore tries to protect only one resource.
Counting semaphore is used in case of multiple resource.
For ex: we have 4 printers then the counting semaphore value will be init to 4. When it reaches 0, the task waiting on the semaphore is suspended.
semaphore, mutex, message passing, shared memory, socket connections.
The bss section contains uninitialized data, and is allocated at runtime. Until it is written to, it remains zeroed.
Memory Segments: Code segment
This phrase used to refer to a portion of memory or of an object file that contains executable computer instructions. It is generally readonly segment.
Data segment: This is one of the sections of a program in an object file or in memory, which contains the global variables that are initialized by the programmer. It has a fixed size, since all of the data in this section is set by the programmer before the program is loaded. However, it is not readonly, since the values of the variables can be altered at runtime.
.bss: This segment of memory is part of data segment that contains uninitialized data (static variables). These are initialized to 0 and later assigned values during runtime.
Stack segment: This segment of memory is a special stack which stores information about the active subroutines of a task. It contains the return address to be branched to after a subroutine has finished execution. It contains local variables of the subroutines and the parameters that are passed to those sub routines.
Heap segment: The segment of memory which is used for dynamic memory allocation is known as heap. It is the responsibility of the programmer to deallocate it after its use. Or alternatively it will be garbage collected by the OS.
Executable and Linking Format is a common standard file format for executables, object code, shared libraries, and core dumps.
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 lowproi process until it completes the task and releases the resources.
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