Chapter Twelve Memory Organization

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Chapter TwelveMemory Organization
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Memory Hierarchy
Magnetic tapes
Main memory
I/O processor
Magnetic discs
Auxiliary memory
Cache
CPU
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Auxiliary vs. Main Memory

• Provides backup storage
• Low-cost
• Large capacity
• Holds data file, system programs, etc not
  currently needed
• No direct access by CPU

• Holds programs and data currently needed
• Smaller capacity
• Direct access by CPU
• Higher cost
• Faster access, but still not fast enough

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Cache Memory

• Very-high speed memory
• Smaller capacity, higher cost
• Access time is close to processor logic clock
  cycle time
• Stores program segments currently being executed
  and data frequently accessed
• Increases performance of computer

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Memory Hierarchy

• Get the performance of fast expensive memory
  for the price of slow cheap memory!
• GP Registers (2-5 ns)
• Cache
• Level 1 Cache (2-10 ns)
• Level 2 Cache (5-20 ns)
• Main Memory (4080 ns)
• Disk (10 ms seek, 5-100 Mb/s throughput)

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Foundations

• Convention Registers are lowest, Disk is
  highest
• Inclusion Data found at one level of the
  hierarchy is also found at all higher levels
• A miss at level i implies data is not at any
  lower level
• Coherence Copies of data at multiple levels of
  the hierarchy must (eventually) be consistent
• Write through
• Write back
• Locality
• Temporal Recently accessed items are likely
  to be accessed
• Spatial Items that are close in address are
  likely to be accessed

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Basic Operation of Cache

• When CPU needs to access memory, first checks
  cache
• If found, then it is a hit -- data is read
• If not found, then it is a miss -- access main
  memory and transfer block to cache

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Metrics

• Hit ratio hi is the probability that a datum is
  present at at level i
• Access frequency
• fi (1-h1)(1-h2) . (1-hi-1)hi
• Effective access time
• Teff sum fiti

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Goals

• Performance level 1
• Cost per bit level n
• Overall performance ? effective hierarchical
  access time per memory reference is small
• Foundation is based on locality

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Mapping Procedures

• Associative mapping fastest and most flexible
• Direct mapping most rigid
• Set-associative mapping compromise between the
  two

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Replacement Policies

• Choose the victim
• Least Recently Used (LRU)
• Least Frequently Used (LFU)
• First In First Out (FIFO)
• Random
• Optimal

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Analysis of Write-Back

• Simple Write-Back all replaced pages are
  written back to disk
• Average reference time is
• Tm 2(1-h)Tp
• Where
• Tm is the memory access time for one word
• Tp is the page transfer time
• 2 ? one for loading and a for writing back

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Flagged Write Back

• Use dirty bit, set when a page is modified
• Write back only if it is dirty
• Average reference time Tm (1-h) (Tp Wp Tp)
• Where Wp is the probability that a page is
  modified

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Write-through

• Update main memory and disk same time
• Main memory always has the same data as disk
• Page is loaded on a write-miss and read miss
• Ave. time Tm (1-h)Tp Wt (Td-Tm)
• Where Wt is the fraction of writes and Td is the
  disk access time