Contiguous allocation is an early method for allocating main memory between the operating system and user processes. Each process is contained within a single contiguous section of memory. Relocation and limit registers are used to protect processes from each other and allow transient changes to the operating system. Multiple-partition allocation allocates variable sized partitions to processes out of memory holes. Storage allocation approaches like first-fit and best-fit are used to allocate partitions from the holes. Fragmentation can occur both externally and internally, taking up space that cannot be used.

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Contiguous Memory Allocation.ppt

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  8.1 Silberschatz, Galvinand Gagne ©2013Operating System Concepts – 9th EditionContiguous Allocation Main memory must support both OS and user processes Limited resource, must allocate efficiently Contiguous allocation is one early method Main memory usually into two partitions: Resident operating system, usually held in low memory withinterrupt vector User processes then held in high memory Each process contained in single contiguous section ofmemory
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  8.2 Silberschatz, Galvinand Gagne ©2013Operating System Concepts – 9th EditionContiguous Allocation (Cont.) Relocation registers used to protect user processes from eachother, and from changing operating-system code and data Base register contains value of smallest physical address Limit register contains range of logical addresses – eachlogical address must be less than the limit register MMU maps logical address dynamically Can then allow actions such as kernel code being transientand kernel changing size
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  8.3 Silberschatz, Galvinand Gagne ©2013Operating System Concepts – 9th EditionHardware Support for Relocation and Limit Registers
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  8.4 Silberschatz, Galvinand Gagne ©2013Operating System Concepts – 9th EditionMultiple-partition allocation Multiple-partition allocation Degree of multiprogramming limited by number of partitions Variable-partition sizes for efficiency (sized to a given process’ needs) Hole – block of available memory; holes of various size are scatteredthroughout memory When a process arrives, it is allocated memory from a hole large enough toaccommodate it Process exiting frees its partition, adjacent free partitions combined Operating system maintains information about:a) allocated partitions b) free partitions (hole)
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  8.5 Silberschatz, Galvinand Gagne ©2013Operating System Concepts – 9th EditionDynamic Storage-Allocation Problem First-fit: Allocate the first hole that is big enough Best-fit: Allocate the smallest hole that is big enough; mustsearch 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 holeHow 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 storageutilization
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  8.6 Silberschatz, Galvinand Gagne ©2013Operating System Concepts – 9th EditionFragmentation External Fragmentation – total memory space exists tosatisfy a request, but it is not contiguous Internal Fragmentation – allocated memory may be slightlylarger than requested memory; this size difference is memoryinternal to a partition, but not being used First fit analysis reveals that given N blocks allocated, 0.5 Nblocks lost to fragmentation 1/3 may be unusable -> 50-percent rule
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  8.7 Silberschatz, Galvinand Gagne ©2013Operating System Concepts – 9th EditionFragmentation (Cont.) Reduce external fragmentation by compaction Shuffle memory contents to place all free memory togetherin one large block Compaction is possible only if relocation is dynamic, and isdone at execution time I/O problem Latch job in memory while it is involved in I/O Do I/O only into OS buffers Now consider that backing store has same fragmentationproblems