Caching I

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Caching I

• Andreas Klappenecker
• CPSC321 Computer Architecture

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Memory

• Current memory is largely implemented in
• CMOS technology. Two alternatives
• SRAM
• fast, but not area efficient
• stored value in a pair of inverting gates
• DRAM
• slower, but more area efficient
• value stored on charge of a capacitor (must be
  refreshed)

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Static RAM
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Static RAM
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Dynamic RAM
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Dynamic RAM
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Memory

• Users want large and fast memories
• SRAM is too expensive for main memory
• DRAM is too slow for many purposes
• Compromise
• Build a memory hierarchy

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Locality

• If an item is referenced, then
• it will be again referenced soon
• (temporal locality)
• nearby data will be referenced soon
• (spatial locality)
• Why does code have locality?

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Memory Hierarchy

• The memory is organized as a hierarchy
• levels closer to the processor is a subset of any
  level further away
• the memory can consist of several multiple
  levels, but data is typically copied between two
  adjacent levels at a time
• initially, we focus on two levels

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Memory Hierarchy
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Two Level Hierarchy

• Upper level (smaller and faster)
• Lower level (slower)
• A unit of information that is present or not
  within a level is called a block
• If data requested by the processor is in the
  upper level, then this is called a hit, otherwise
  it is called a miss
• If a miss occurs, then data will be retrieved
  from the lower level. Typically, an entire block
  is transferred

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Cache
A cache represents some level of memory between
CPU and main memory More general definitions
are often used
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A Toy Example

• Assumptions
• Suppose that processor requests are each one
  word,
• and that each block consists of one word
• Example
• Before request C X1,X2,,Xn-1
• Processor requests Xn not contained in C
• item Xn is brought from the memory to the cache
• After the request C X1,X2,,Xn-1,Xn
• Issues
• What happens if the cache is full?

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Issues

• How do we know whether the data item is in the
  cache?
• If it is, how do we find it?
• Simple strategy direct mapped cache
• exactly one location where data might be in the
  cache

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Direct Mapped Cache

• Mapping address modulo the number of blocks in
  the cache, x -gt x mod B

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Direct Mapped Cache

• Cache with 1024210 words
• tag from cache is compared against upper portion
  of the address
• If tagupper 20 bits and valid bit is set, then
  we have a cache hit otherwise it is a cache
  missWhat kind of locality are we
  taking advantage of?

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Direct Mapped Cache Example
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Direct Mapped Cache Example
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Direct Mapped Cache Example
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Direct Mapped Cache

• Taking advantage of spatial locality

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Hits vs. Misses

• Read hits
• this is what we want!
• Read misses
• stall the CPU, fetch block from memory, deliver
  to cache, restart
• Write hits
• can replace data in cache and memory
  (write-through)
• write the data only into the cache (write-back
  the cache later)
• Write misses
• read the entire block into the cache, then write
  the word

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Hits vs. Miss Example