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

Marks: 5 Marks

Year: May 2016

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.