What are various ways for memory and address protection

Mumbai University > Computer Engineering > Sem 7 > Cryptography and System Security

Marks: 5 Marks

Year: May 2016

1 Answer

One major problem in multiprogramming is preventing one program from affecting the data and program in the memory space of other users. Some methods to prevent it is:

1) Fence:

  • We confine users to inside of the memory and system memory to another side. Each group cannot come to their opposite memory area.

2) Relocation

  • Programs are written as if starting from location 0 in the memory.
  • But in reality its stored at location n determined by OS.
  • Before a user instruction is executed, a relocation factor (i.e. n)is added to it.

3) Base/bounds register

  • A program is forced to above a base register & is always checked to be below bounds register.
  • Base register A variable fence that determines the lower limit for program address
  • Bounds registerdefines the upper limit for program instruction.

4) Segmentation

  • A very convenient method where program is divided into logical pieces called segments.
  • Consecutive program segments can be stored in non-consecutive locations.
  • A data item is addressed as name_of_segment + offset_of_data_inside_segment.
  • The segment name and address is stored in Segment translation table(STT)
  • Also, Two processes can share a same segment

5) Paging

  • Programs are here divided into equal sized pages & memory is also divided into same size page-frames.
  • Paging address works similar to segmentation. It has also has a page translation table.(PTT)
  • While segments are who block of code (complete function or module), pages are of fixed size and contain part of executing code/data
  • They are usually small in size.

6) Paging +segmentation combined

  • The principle here used is that “paging offer efficiency” and “segmentation provides logical protection”
  • So, here the programmer is given the freedom to define segments on his own and later the OS breaks down these segments into pages.
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