8/12/2019 59018943 Advantages and Disadvantages of Virtual Memory Management Schemes

1/2

1. Advantages and Disadvantages of Virtual Memory Management Schemes

a. Paged Memory Allocation

Advantages:

Allows jobs to be allocated in non-contiguous memory locations.

Memory used more efficiently more jobs can fit.

Disadvantages:

Address resolution causes increased overhead.

!nternal fragmentation still e"ists# though in last $age.

%e&uires the entire job to be stored in memory location.

Si'e of $age is crucial (not too small# not too large).

b. Demand Paging

Advantages:

*ob no longer constrained by the si'e of $hysical memory (conce$t of virtual

memory). +tili'es memory more efficiently than the $revious schemes.

Disadvantages:

!ncreased overhead caused by the tables and the $age interru$ts.

c. Segmented Memory Allocation

Advantages:

!nternal fragmentation is removed.

Disadvantages:

Difficulty managing variable-length segments in secondary storage. ,"ternal fragmentation.

d. SegmentedDemand Paged Memory Allocation

Advantages:

arge virtual memory.

Segment loaded on demand.

Disadvantages:

/able handling overhead.

Memory needed for $age and segment tables.

!f given an o$tion# ! would im$lement SegmentedDemand Paged

Memory Allocation because it combines all the features of the $revious

schemes. !t uses both the logical benefits of segmentation and the $hysical

benefits of $aging.

8/12/2019 59018943 Advantages and Disadvantages of Virtual Memory Management Schemes

2/2

0. Segmented Memory Allocation solved the $roblem on internal fragmentation

because through segmentation# each job is divided into segments of different

si'es. ith this# no s$ace is wasted thus# removing internal fragmentation.

2. Associative Memory and 3ache Memory

/he cacheis a small amount of high-s$eed memory# usually with a

memory cycle time com$arable to the time re&uired by the 3P+ to fetch one

instruction. /he cache is usually filled from main memory when instructions or

data are fetched into the 3P+. 4ften the main memory will su$$ly a wider data

word to the cache than the 3P+ re&uires# to fill the cache more ra$idly. /he

amount of information which is re$laces at one time in the cache is called

the line sizefor the cache. /his is normally the width of the data bus between

the cache memory and the main memory. A wide line si'e for the cache means

that several instruction or data words are loaded into the cache at one time#

$roviding a 5ind of $refetching for instructions or data.

hen a cache is used# there must be some way in which the memorycontroller determines whether the value currently being addressed in memoryis available from the cache. /here are several ways that this can beaccom$lished. 4ne $ossibility is to store both the address and the value frommain memory in the cache# with the address stored in a ty$e of memorycalled associative memoryor# more descri$tively# content addressable memory.

An associative memory# or content addressable memory# has the$ro$erty that when a value is $resented to the memory# the addressof thevalue is returned if the value is stored in the memory# otherwise an indicationthat the value is not in the associative memory is returned.Allof the

com$arisons are done simultaneously# so the search is $erformed very &uic5ly./his ty$e of memory is very e"$ensive# because each memory location musthave both a com$arator and a storage element. A cache memory can beim$lemented with a bloc5 of associative memory# together with a bloc5 of66ordinary77 memory. /he associative memory would hold the addressof thedata stored in the cache# and the ordinary memory would contain the data atthat address.