CONTIGUOUS ALLOCATION:
1.Introduction:

• Main memory usually have two partitions-memory resident programs,user programs
• Resident operating system, usually held in low memory with interrupt vector.
• User processes then held in high memory.  Single-partition allocation
• Relocation-register scheme used to protect user processes from each other, and from changing operating-system code and data.
• Relocation register contains value of smallest physical address;
•  limit register contains range of logical addresses – each logical address must be less than the limit register.  

2.Memory protection:

    
3.Memory allocation:       

• Each process is contained in a single contiguous section of memory.

• One of the simplest methods for memory allocation is to divide memory into several fixed-sized partitions. Each partition may contain exactly one process.
• In multiple-partition method, when a partition is free, a process is selected from the input queue and is loaded into the free partition.
• When the process terminates, the partition becomes available for another process.
• The operating system keeps a table indicating which parts of memory are available and which are occupied.
•   Depending on the method used, this approach can suffer from external as well as internal memory fragmentation.

•      Hole – block of available memory; holes of various size are scattered throughout memory.
• When a process arrives, it is allocated memory from a hole large enough to accommodate it.
• Operating system maintains information about:
         a) allocated partitions b) free partitions (hole)

Three ways to fit a process in a hole:

• First-fit:   Allocate the first hole that is big enough.
• Best-fit:   Allocate the smallest hole that is big enough; must search entire list, unless ordered by size.Produces the smallest leftover hole.
• Worst-fit: Allocate the largest hole; must also search entire list. Produces the largest leftover hole.
• How to satisfy a request of size n from a list of free holes.
  First-fit and best-fit better than worst-fit in terms of speed and storage utilization.

4.fragmentation
    4.1 internal fragmentation:allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used.
    4.2 external fragmentation:total memory space exists to satisfy a request, but it is not contiguous.

• Reduce external fragmentation by compaction
• Shuffle memory contents to place all free memory together in one large block.
• Compaction is possible only if relocation is dynamic, and is done at execution time.
• I/O problem
• Latch job in memory while it is involved in I/O.
• Do I/O only into OS buffers.