PAGING-SHARED PAGES

• Advantage of paging is sharing of common code.
• This is important in a time sharing environment.
• In multi programming system, when two or more user use program or file at same time. 
• The MMU of OS share pages of the process with the users.
• These pages are called shared pages.
• Shared code

    1. One copy of read-only (re-entrant) code shared among processes (i.e., text editors, compilers,
         window systems).
    2. Shared code must appear in same location in the logical  address space of all processes.
    3. Each process keeps a separate copy of the code and data.
    4. The pages for the private code and data can appear  anywhere in the logical address space.

RE-ENTRANT CODE:(OR PURE CODE)

• is non-self-modifying code.
• If the code is re-entrant,then it never changes during execution.
• Thus two or more processes can execute the same code at the same time.
• Each process has its own copy of registers and data storage to hold the data for the process execution.