Protection:
The word protection means security from unauthorized
references from main memory. To provide protection, the OS follows different
types of mechanisms. In paging memory management scheme, the operating system
provides protection bits that are
associated with each frame. Generally, these bits are kept in a page table. Sometimes,
these bits are said to be valid / not
valid bit. Consider the following fig:
Fig: Protection in paging
The page map table consists of a specified field that is valid/ invalid bit. If the bit shows valid, it means the particular page is loaded in main memory.
If it shows invalid, it means the page is not in the process’s logical
address space. Illegal addresses are trapped by using the valid /
invalid bit. The operating system sets this bit for each page to allow or
disallow access to that page.
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