Cache Memory

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Cache Memory

• Nimi Berman
• CS 147
• Fall 2003

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Cache Memory

• Nimi Berman
• CS 147
• Fall 2003

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Today we will talk about

• What is Cache Memory?

• Types of Cache Memory

• Associative Mapping
• Direct Mapping
• Set-Associative Mapping

• Replacing and Writing Data into Cache

• Cache Performance

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1. So, what is cache memory?

• We want to minimize memory accesses by processor.

• Cache memory is just like regular memory, only
  it has faster access time of 10 ns.

• Constructed using static RAM or associative
  memory ? more expensive then RAM.

A lot of cache ? a lot of cash
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Static RAM vs. associative memory

• Receives address from CPU
• Access data at that address in cache

• Receives portion of data
• Search all cache locations in parallel
• Return matches

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How is Data stored in Cache?

• Valid bit 1 if holds valid data, 0 if garbage.

• CPU specifies data value to be matched.

• Example

• Find data with 1010 as four high-order bits.
• CPU loads 1111 0000 0000 0000 to mask register
  and 1010 XXXX XXXX XXXX to data register.
• Find matches.

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2.a. Associative Mapping

• First 16 bits memory address
• Last 8 bits the actual data stored in that
  memory address

• CPU outputs 16-bit address to be accessed and
  8-bit dont cares into data register and the
  value
• 1111 1111 1111 1111 0000 0000
• to mask register.

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Alternate Configuration
wider memory, acts on blocks of data, not just
individual locations.
Locality of Reference Next instruction is likely
to be in adjacent memory location.
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Result

• Entire block is copied to cache.
• Next instruction is likely to already be in
  cache.
• No need for main memory access.

• Faster processing

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2.b. Direct Mapping

• Can be larger than Associative cache but still
  cheaper.

• 10 Low-order bits (index) specific location in
  cache to store data (1-1 relation).

• High-order bits (tag) same as high-order bits
  of original main memory address.

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Problem Any data from location XXXX XX11 1111
1111 in main memory map to same location in cache.
Solution Tag stores high-order bits from
original main memory address, cache address is
same as 10 low-order bits. CPU knows it accesses
correct data in cache.
? No ambivalence
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Alternate Configuration
wider memory, acts on blocks of data, not just
individual locations.
Locality of Reference Next instruction is likely
to be in adjacent memory location.
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Direct Mapping vs. Associative Cache

• Cheaper than Associative

• More flexible than Direct Mapping

0000 0000 0000 0000 (0000H) JUMP 1000H 0001
0000 0000 0000 (1000H) JUMP 0000H

• In direct mapping
• Both map to same location (overwriting)
• Other cache locations not in use

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2.c. Set-Associative Mapping

• Low cost SRAM

• Each address in cache contains several data
  values an n-way set-associative cache contains
  n values.

• Tag field is same as in Direct Mapping but has
  one extra bit (9 bits for location, 7 bits for
  tag).

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• Count/Valid field valid bit as before, count
  bit to track when data was accessed.

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Alternate Configuration
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Which method is used?
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3. Replacing and Writing Data

• When valid bit 0 memory location is free.

Problem What do we do when all memory locations
are occupied?
Solution Must overwrite on existing data. But,
which one?
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Direct Mapping Each data value can only map to
specific location, that location must be used,
old value written to main memory.

• Associative Mapping
• FIFO fill cache from top to bottom and start
  replacing from top. Produces good performance.

• Least Recently Used using a counter keep track
  of which data was last accessed, replace least
  recently used.

• Random produces good performance.

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Set-associative Mapping Often utilizes the Least
Recently Used method.
Access D
Access E
Access A
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4. Cache Performance

• Main purpose of cache memory is to improve
  system performance by minimizing slow accesses to
  main memory.

• Cache hits when CPU accesses cache instead of
  main memory.

• Cache miss when CPU accesses main memory and
  not cache.

• Hit ratio h of cache hits.

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• As h ? 1 we get faster and better system
  performance.

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Associative cache h 0.389 and TM 40.56 ns
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Direct-mapped cache h 0.167 and TM 50.67 ns
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Two-way set-associative cache h 0.389 and TM
40.56 ns
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Conclusion

• Cache Memory is fast (and expensive)

• Types of Cache Memory

• Associative Mapping
• Direct Mapping
• Set-Associative Mapping

• Replacing and Writing Data into Cache

• Cache Performance

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Bottom Line
Cache memory speeds up system performance thus
making the users life a lot easier.