20042602207.pptx

Unit -5
Department of Computer Science - Kamban
College of Arts And Science For Women

SYLLABUS
Auxiliary memory – Main memory – Associative memory – Cache memory
– Virtual memory

AUXILIARY MEMORY
 It is used to overcome the limitations of primary storage
 Unlimited capacity because the cost per bit of storage is very low
 Larger capacity than main memory
 Used to store large volumes of data on a permanent basis
 It is non-volatile in nature
 Also known as secondary memory

MAGNETIC MEMORY

MAGNETIC DISKS

MAGNETIC TAPES

MAIN MEMORY
 Memory unit that communicates directly with CPU
 Programs and data currently needed by the processor reside here
 Also known as primary memory
 RAM and ROM

ASSOCIATIVE MEMORY

ASSOCIATIVE MEMORY ORGANIZATION

ASSOCIATIVE MEMORY ORGANIZATION
 Associative memory is organized in such a way
 Argument register(A): It contains the word to be searched. It has n bit (one
for each bit of the word)
 key register(K): This specifies which part of the argument word needs to
be compared with words in memory. If all bits in register are q. the entire
word should be compared. Otherwise only the bits having k bit set to 1 will
be compared
 Associative memory array: It contains the words which any to be
compared with the argument word
 Match register(M): It has m bits. One bit corresponding to each word in
the memory array. After the matching process, the bits corresponding to
matching words in match register are set to 1

DISADVANTAGES OF ASSOCIATIVE MEMORY

ADVANTAGES OF ASSOCIATIVE MEMORY

CACHE MAPPING
 There are most common methods available in cache mapping. They are,
• Direct Mapped Cache
• Associative Mapped Cache
• Set- Associated Mapped Cache

DIRECT MAPPING
 Each block of main memory maps to only one cache line
i.e., if a block is in cache, it must be is one specific place
 Address is in two parts;
• Least significant w bits identify unique word
• Most significant s bits specify one memory block

DIRECT MAPPING ADDRESS STRUCTURE
 24 bit address
 2 bit word identifier(4 byte block)
 22 bit block identifier
 8 bit tag(=22-14)
 14 bit slot or line

DIRECT MAPPING FROM CACHE TO MAIN MEMORY

DIRECT MAPPING CACHE ORGANIZATION

ADVANTAGES
 The tag memory is much smaller than in associative mapped cache
 No need for an associative search, since the slot field is used to direct the
comparison to a single field.
DISADVANTAGES
 Consider what happens when a program references locations that are 219
words apart, which is the size of the cache. Every memory reference will
result in a miss, which will cause an entire block to be read into the cache
even though only a single word is used

ASSOCIATIVE MAPPED CACHE
 A main memory block can load into any line of cache
 Memory address is interpreted as tag and word
 Tag uniquely identifies block of memory
 Every lines tag is examined for a match
 Cache searching gets expensive

ASSOCIATIVE MAPPED CACHE

ADVANTAGES
 Any main memory block can be placed into any cache slot
 Regardless of how irregular the data and program references are, if a slot is
available for the block, it can be stored in the cache
DISADVANTAGES
 Considerable hardware overhead needed for cache bookkeeping
 There must be a mechanism for searching the tag memory in parallel

SET ASSOCIATIVE MAPPING
 Cache is divided into a number of sets
 Each set contains a number of lines
 A given block maps to any line in a given set
 E.g. Block B can be in any line of set I
 2 way associative mapping

SET ASSOCIATIVE MAPPING

ADVANATAGES
 In our example the tag memory increases only slightly from the direct
mapping and only two tags need to be searched for each memory reference
 The set-associative cache is widely used in today’s micro processors

VIRTUAL MEMORY

NEED OF VIRTUAL MEMORY

ADDRESS SPACE AND MEMORY SPACE

PAGE REPLACEMENT ALGORITHMS

FIFO

LRU

OPT

ADVANTAGES OF VIRTUAL MEMORY

DISADVANTAGES OF VIRTUAL MEMORY

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