Fixed size partition in OS

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              Fixed size partition

  • This is the oldest and simplest technique used to put more than one processes in the main memory. 
  • In this partitioning, number of partitions (non-overlapping) in RAM are fixed but size of each partition may or may not be same.
  •  As it is contiguous allocation, hence no spanning is allowed. Here partition are made before execution or during system configure.
  • This is static size partition . E.g array used fixed size partition where memory of variable define before executing program.

  • As illustrated in above figure, first process is only consuming 1MB out of 4MB in the main memory.
  • Hence, Internal Fragmentation in first block is (4-1) = 3MB.
  • Sum of Internal Fragmentation in every block = (4-1)+(8-7)+(8-7)+(16-14)= 3+1+1+2 = 7MB.
----> we can reduce fregmentation using partition allocation scheme e.g, first fit ,best fit and worst fit.

  • Suppose process P5 of size 7MB comes. But this process cannot be accommodated inspite of available free space because of contiguous allocation (as spanning is not allowed). Hence, 7MB becomes part of External Fragmentation.
There are some advantages and disadvantages of fixed partitioning.
Advantages of Fixed Partitioning –
  1. Easy to implement:
    Algorithms needed to implement Fixed Partitioning are easy to implement. It simply requires putting a process into certain partition without focussing on the emergence of Internal and External Fragmentation.
  2. Little OS overhead:
    Processing of Fixed Partitioning require lesser excess and indirect computational power.
Disadvantages of Fixed Partitioning –
  1. Internal Fragmentation:
    Main memory use is inefficient. Any program, no matter how small, occupies an entire partition. This can cause internal fragmentation.
  2. External Fragmentation:
    The total unused space (as stated above) of various partitions cannot be used to load the processes even though there is space available but not in the contiguous form (as spanning is not allowed).
  3. Limit process size:
    Process of size greater than size of partition in Main Memory cannot be accommodated. Partition size cannot be varied according to the size of incoming process’s size. Hence, process size of 32MB in above stated example is invalid.
  4. Limitation on Degree of Multiprogramming:
    Partition in Main Memory are made before execution or during system configure. Main Memory is divided into fixed number of partition. Suppose if there are n1 partitions in RAM and n2are the number of processes, then n2 <= n1 condition must be fulfilled. Number of processes greater than number of partitions in RAM is invalid in Fixed Partitioning.

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