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Unix OS Interview Questions

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A data representation at the machine level, that has initial values when a program starts and tells about how much space the kernel allocates for the un-initialized data. Kernel initializes it to zero at run-time.

This is the Kernel process that makes rooms for the incoming pages, by swapping the memory pages that are not the part of the working set of a process. Page-Stealer is created by the Kernel at the system initialization and invokes it throughout the lifetime of the system. Kernel locks a region when a process faults on a page in the region, so that page stealer cannot steal the page, which is being faulted in.

The two paging states are:
1. The page is aging and is not yet eligible for swapping,
2. The page is eligible for swapping but not yet eligible for reassignment to other virtual address space. 

1. Page stealer finds the page eligible for swapping and places the page number in the list of pages to be swapped.
2. Kernel copies the page to a swap device when necessary and clears the valid bit in the page table entry, decrements the pfdata reference count, and places the pfdata table entry at the end of the free list if its reference count is 0.

Page fault refers to the situation of not having a page in the main memory when any process references it. There are two types of page fault :
1. Validity fault,
2. Protection fault. 

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