OPERATING SYSTEM
Memory protection is preventing one process from corrupting the memory. There are methods for protecting memory:
· Fence is preventing faulty user program from destroying part of the resident portion of the operating system. There are two types of fence:
o Fixed fence( predefined memory access)
§ A method to confine user to one side of the boundary.
o Register fence (used hardware register)
§ Contain address of the end of the operating system.
- Relocation is a process of changing all address to reflect actual address which the program is located.
- Base / Bound Register is useful in knowing how much space is available and checking overflows
- Tagged Architecture is used to solve problem in contiguous nature and all or nothing situation for sharing.
- Segmentation is divide process into pieces with different rights.
o Benefits of segmentation
§ Each address reference checked for protection
§ User cannot access to unpermitted segment
§ User can share access with different rights.
o Problem in segmentation
§ Segments cause fragmentation of main memory because they are varying sizes.
§ If swapping is used then additional memory management techniques must be employed
§ The operating system’s lookup of the name in the table can be slow
- Paging is divide program into equal size; frame is divide memory into equal size.
o Advantages of paging
§ Each page is the same size thus fragmentation is reduced
§ Addressing beyond a page is not a real problem since a carry just refers to the next page.
o Disadvantages of paging
§ Loss of individual access rights since there is not necessarily a relationship between lines of code in a page (unlike program segmentation)
- Combine paging with segmentation used to break each segment into equal sized pages.
o Advantages of combining segmentation with paging
§ Retained the logical unity of the segment
§ Permitted differentiated protection for the segments
o Disadvantage
§ Added an additional layer of translation for each address
Password is a character of string that used to authenticate identity of user.
Authentication is verification of someone identity who generated some data.
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