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CS1104: Computer Organisation http://www.comp.nus.edu.sg/~cs1104 http://www.comp.nus.edu.sg/~cs1104 School of Computing National University of Singapore
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CS1104-P2-8Memory Hierarchy2 PII Lecture 8: Memory Hierarchy Memory Technologies Memory Hierarchy Analogy Illusion of Large, Fast, Cheap Memory Principle of Locality Terminology
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CS1104-P2-8Memory Hierarchy3 PII Lecture 8: Memory Hierarchy Reading: Chapter 8 of textbook, which is Chapter 5 in “Computer Organization” by Hamacher, Vranesic and Zaky.
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CS1104-P2-8Memory Hierarchy4 Memory Technologies RAM (Random Access Memory): access time is the same for all locations (in nanoseconds). DRAM: Dynamic RAM High density, low power, cheap, slow (access time: 60-120 ns). Dynamic: needs to be “refreshed” regularly. SRAM: Static RAM Low density, high power, expensive, fast (access time: 5-25 ns). Static: non-volatile – content lasts “forever” (until power is removed).
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CS1104-P2-8Memory Hierarchy5 Memory Technologies (2) ROM (Read-Only Memory): non-volatile, allows for read only. PROM: Programmable ROM EPROM: Erasable PROM EEPROM: Electrically EPROM Flash Memory Similar to EEPROM Difference: read a single cell, but write a block
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CS1104-P2-8Memory Hierarchy6 Recap: Memory Organisation Another example: Organization of a 2M 32 memory module using 512K 8 static memory chips. Chip select 512K x 8 memory chip 19-bit address 8-bit data input/output
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CS1104-P2-8Memory Hierarchy7 Technology Trends CapacitySpeed (latency) Processor:--4x in 3 yrs DRAM:4x in 3 yrs2x in 10 yrs Disk:4x in 3 yrs 2x in 10 yrs DRAM YearSizeCycle Time 198064 Kb250 ns 1983256 Kb220 ns 19861 Mb190 ns 19894 Mb165 ns 199316 Mb145 ns 199764 Mb120 ns 1000:1!2:1!
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CS1104-P2-8Memory Hierarchy8 Technology Trends (2) DRAM 9%/yr. (2X/10 yrs) µProc 60%/yr. (2X/1.5yr) Processor-Memory Performance Gap: (grows 50% / year) 1 10 100 1000 DRAM CPU 198019811983198419851986198719881989199019911992199319941995199619971998199920001982 Time
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CS1104-P2-8Memory Hierarchy9 Analogy: Term Paper in Library Working on a paper at a desk in library. Option 1: Every time a book is needed… Leave desk to go to shelves (or stacks) Find the book Bring one book back to desk Read section interested in When done, leave desk and go to shelves carrying book Put book back on shelf Return to desk to work
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CS1104-P2-8Memory Hierarchy10 Analogy: Term Paper in Library (2) Option 2: Every time a book is needed… Leave some books on desk after fetching them Only go to shelves when a book not on desk is needed At the shelves, fetch related books in case you need them; sometimes you will need to return books not used recently to make space for new books on desk (replacement algorithm) Return to desk to work Illusion: whole library on your desk
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CS1104-P2-8Memory Hierarchy11 Illusion of Large, Fast, Cheap Memory Registers in processor: Very fast, in order of nanoseconds to picoseconds. Disks: Very slow, in order of milliseconds. Large memories (disks) are slow; fast memories (registers) are small. How do we create a memory that is large, cheap and fast (most of the time)? Memory hierarchy.
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CS1104-P2-8Memory Hierarchy12 Memory Hierarchy Pyramid Central Processor Unit (CPU) “Upper” “Lower” Levels in memory hierarchy Increasing distance from CPU, decreasing cost/bit Size of memory at each level Level 1 Level 2 Level 3... Level n (Data cannot be in level i unless it is also in i+1.)
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CS1104-P2-8Memory Hierarchy13 Principle of Locality Principle of locality (or locality of reference): Program accesses a relatively small portion of the address space at any instant of time. Temporal locality and spatial locality. Address Space 02 n - 1 Probability of reference
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CS1104-P2-8Memory Hierarchy14 Principle of Locality (2) Temporal locality (locality in time): Keep most recently accessed data items closer to the processor. Library analogy: Recently read books are kept on desk. Spatial locality (locality in space): Move blocks consisting of contiguous words to ‘upper’ levels. Block is a unit of transfer. Library analogy: Bring back nearby books on shelves when fetching a book, hoping that you might need them soon.
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CS1104-P2-8Memory Hierarchy15 Principle of Locality (3) What programming constructs lead to principle of locality? SpatialTemporal Dataarraysloop counters Codeno branch/jumploop
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CS1104-P2-8Memory Hierarchy16 Memory Hierarchy: Idea Temporal locality: keep recently accessed data items closer to processor. Spatial locality: move contiguous words in memory to upper levels of hierarchy. Use smaller and faster memory technologies closer to the processor If hit rate is high enough, hierarchy has access time close to the highest (fastest) level and size equal to the lowest (largest) level.
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CS1104-P2-8Memory Hierarchy17 Memory Hierarchy: Terminology Hit: data appears in some block in the upper level. Hit rate: the fraction of memory access found in the upper level. Analogy: fraction of time you find the book on desk. Miss: data is not at upper level and needs to be retrieved from a block in the lower level. Miss rate: 1 – Hit rate Analogy: fraction of time you need to go to shelves for the book.
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CS1104-P2-8Memory Hierarchy18 Memory Hierarchy: Terminology (2) (Average) Hit time: time to access the upper level which consists of Time to determine hit/miss + memory access time. Analogy: time to find and pick up book from desk. (Average) Miss penalty: time to replace a block in the upper level + time to deliver the block to the processor. Analogy: time to go to shelves, find needed book, and return to your desk. Hit time << Miss penalty.
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CS1104-P2-8Memory Hierarchy19 Current Memory Hierarchy Increasing size Processor Primary cache Secondary cache Main memory Magnetic disk secondary memory Register L1 L2 Increasing speed Increasing cost per bit
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CS1104-P2-8Memory Hierarchy20 Current Memory Hierarchy (2) Control Data path Processor Regs Second- ary Memory L2 Cache L1 $ Main Memory Speed(ns):0.5ns2ns6ns100ns10,000,000ns Size (MB):0.00050.051-4100-1000100,000 Cost ($/MB):--$100$30$1 $0.05 Technology:RegsSRAMSRAMDRAMDisk
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CS1104-P2-8Memory Hierarchy21 Management of the Hierarchy Registers Memory By compiler (or assembly programmer). Cache Main memory By the hardware. Main memory Disks By the hardware and operating system. By the programmer (through files).
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CS1104-P2-8Memory Hierarchy22 Virtual Memory Main memory smaller than address space Example: 32-bit address allows an address space of 4G bytes, but main memory may only be a few hundred megabytes. Parts of program not in main memory are stored on secondary storage devices, such as disks. Operating system moves programs and data automatically between the physical main memory and secondary storage (virtual memory).
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CS1104-P2-8Memory Hierarchy23 Things To Remember Principle of locality and hierarchy of memories of different speed and cost; exploit locality to improve cost- performance. Terms: Hit, miss, hit rate, miss rate, hit time, miss penalty, block, upper-level memory, lower-level memory.
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