Contiguous Memory Allocation
Each process is contained in a single contiguous section of memory.
There are two methods namely :
v Fixed – Partition Method
v Variable – Partition Method
Fixed – Partition Method :
· Divide memory into fixed size partitions, where each partition has exactly
· The drawback is memory space unused within a partition is
wasted.(eg.when process size < partition size)
ü Divide memory into variable size partitions, depending upon the size of the incoming process.
· When a process terminates, the partition becomes available for another process.
· As processes complete and leave they create holes in the main memory.
· Hole – block of available memory; holes of various size are scattered throughout memory.
Dynamic Storage-Allocation Problem:
How to satisfy a request of size =n‘ from a list of free holes?
v First-fit: Allocate the first hole that is big enough.
v Best-fit: Allocate the smallest hole that is big enough; must search entire list, unless ordered by size. Produces the smallest leftover hole.
v Worst-fit: Allocate the largest hole; must also search entire list. Produces the largest leftover hole.
NOTE: First-fit and best-fit are better than worst-fit in terms of speed and storage utilization
External Fragmentation – This takes place when enough total memory space exists to satisfy a request, but it is not contiguous i.e, storage is fragmented into a large number of small holes scattered throughout the main memory.
Ø Internal Fragmentation – Allocated memory may be slightly larger than requested memory.
hole = 184 bytes
Process size = 182 bytes.
We are left with a hole of 2 bytes.
Coalescing :Merge the adjacent holes together.
Compaction: Move all processes towards one end of memory hole towards other end of memory, producing one large hole of available memory. This scheme is expensive as it can be doneif relocation is dynamic and done at execution time.
Permit the logical address space of a process to be non-contiguous. This is achieved through two memory management schemes namely paging and segmentation.