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Contigeous Memory Allocation


  The main memory must accommodate or store both the operating system and the various user processes. So we need to allocate main memory in the most efficient way possible. As we know the memory is usually divided into two partitions: one for the operating system and other for the user processes. We can place the operating system in either low memory or high memory. Since the interrupt vector is often in low memory, programmers usually like to place the operating system in low memory.

  We usually want several user processes to reside in memory at the same time. We therefore need to consider how to allocate available memory to the processes that are in the input queue waiting to enter into memory. In contiguous memory allocation, each process is contained in a single section of memory that is contiguous to the section containing the next process. Thus we can say if the blocks are allocated to the file in such a way that all the logical blocks of the file get the contiguous physical block in the hard disk then such allocation scheme is known as contiguous allocation.The contiguous memory allocation can be implemented with the help of two registers:- Base and limit registers.

 When a process want to execute, memory is requested by the process. Then the size of process is compared with the amount of contiguous main memory available. If sufficient contiguous memory is found, the process is allocated memory to start its execution. Otherwise, it is added to a queue of waiting processes until sufficient free contiguous memory is available.


ADVANTAGES:-  
  i] This scheme is simple to implement. 
 ii] This scheme provides best read performance.
iii] This also supports random accessing.



 DISADVANTAGES:-
   i] The disk becomes fragmented.
  ii] The degree of multiprogramming is reduced.
iii] It may be difficult to have a file grow.


 CONTIGEOUS  V/S  NON-CONT. MEMORY ALLOCATION :-



click here for Tyes of memory allocation


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