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Summary of caches:

Summary of caches:. The Principle of Locality: Program likely to access a relatively small portion of the address space at any instant of time. Temporal Locality : Locality in Time Spatial Locality : Locality in Space Three Major Categories of Cache Misses:

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Summary of caches:

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  1. Summary of caches: • The Principle of Locality: • Program likely to access a relatively small portion of the address space at any instant of time. • Temporal Locality: Locality in Time • Spatial Locality: Locality in Space • Three Major Categories of Cache Misses: • Compulsory Misses: sad facts of life. Example: cold start misses. • Conflict Misses: increase cache size and/or associativity. Nightmare Scenario: ping pong effect! • Capacity Misses: increase cache size • Cache Design Space • total size, block size, associativity • replacement policy • write-hit policy (write-through, write-back)

  2. Virtual Memory • Main memory can act as a cache for the secondary storage (disk) • Advantages: • illusion of having more physical memory • program relocation • protection

  3. Pages: virtual memory blocks • Page faults: the data is not in memory, retrieve it from disk • huge miss penalty, thus pages should be fairly large (e.g., 4KB) • reducing page faults is important (LRU is worth the price) • can handle the faults in software instead of hardware • using write-through is too expensive so we use writeback

  4. Page Tables Virtual page

  5. Page Tables • Each memory reference requires 2 memory accesses (why?)

  6. Making Address Translation Fast TLB • A cache for address translations: translation lookaside buffer

  7. Modern Systems • Very complicated memory systems:

  8. For each of these, where could the data be and how would we find it? • TLB hit • TLB miss • Virtual memory hit • Virtual memory miss • Cache hit • Cache miss

  9. Summary: Levels of the Memory Hierarchy Upper Level Capacity Access Time Cost Staging Xfer Unit faster CPU Registers 100s Bytes <10s ns Registers prog./compiler 1-8 bytes Instr. Operands Cache K Bytes 10-100 ns $.01-.001/bit Cache cache cntl 8-128 bytes Blocks Main Memory M Bytes 100ns-1us $.01-.001 Memory OS 512-4K bytes Pages Disk G Bytes ms 10 - 10 cents Disk -4 -3 user/operator Mbytes Files Larger Tape infinite sec-min 10 Tape Lower Level -6

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