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Main Memory

Main Memory. CS448. Main Memory. Bottom of the memory hierarchy Measured in Access Time Time between a read is requested and data delivered Cycle Time Minimum time between requests to memory Greater than access time to ensure address lines are stable Three Important Issues Capacity

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Main Memory

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  1. Main Memory CS448

  2. Main Memory • Bottom of the memory hierarchy • Measured in • Access Time • Time between a read is requested and data delivered • Cycle Time • Minimum time between requests to memory • Greater than access time to ensure address lines are stable • Three Important Issues • Capacity • Bell’s law - 1 MB per MIP needed for balance, avoid page faults • Latency • Time to access the data • Bandwidth • Amount of data that can be transferred

  3. Early DRAMs • Dynamic RAM • Number of address lines was a large cost issue • Solution: multiplex the address lines • e.g., :Address: 10101010101010101 CAS CAS = Column Address Select RAS = Row Address Select DRAM Multiplexed RAS

  4. DRAMS • Dynamic RAM • Transistor stores each bit • Loss over time • Must periodically “refresh” the bits • All bits in a row can be refreshed by reading that row • Memory controllers periodically refresh, e.g. every 8 ms • If the CPU tries to access memory during the refresh, we must wait (hopefully won’t occur often) • Typical cycle times 60-90ns

  5. SRAMs • Static RAM • Does not need a refresh • Faster than DRAM, generally not multiplexed • But more expensive • Typical memories • DRAM 4-8 times the capacity of SRAM • Used for main memory • SRAM 8-16 times faster than DRAM • Typical cycle times 4-7ns • Also 8-16 times as expensive • Used to build cache • Exceptions; Cray built main memory out of SRAM

  6. Memory Example • Consider the following scenario • 1 cycle to send the address • 4 cycles per word of access • 1 cycle to transmit the data • If main memory is organized by word • 6 cycles for every word • Given a cache line of 4 words • 24 cycles is the miss penalty Yikes! • What can we do about this penalty?

  7. #1 : More Bandwidth to Memory • Make a word of main memory look like a cache line • Easy to do conceptually • Say we want 4 words, so send all four words back on the bus at one time instead of one after the other • In the previous scenario, we could send 1 address, access the four words (4 cycles if in parallel or 16 if sequential), and then send all data at once in 1 more cycle for a total of 6 or 18 cycles • Still better than 24 even if we don’t access each word in parallel • Problem • Need a wider bus, which is expensive • Especially true if this contributes to the pin count on the CPU • Usually the bus width to memory will match the width of the L2 cache

  8. #2 : Interleaved Memory Banks • Take advantage of potential parallelism by interleaving memory • 4-way interleaved memory Allow simultaneous access to data in different memory banks Good for sequential data

  9. #3: Independent Memory Banks • Can take the idea of interleaved banks a bit farther… make independent banks altogether • Multiple independent accesses • Multiple memory controllers • Each bank needs separate address lines and probably a separate data bus • Will see this idea appear again with multiprocessors that share a common memory

  10. Summary of Techniques

  11. #4 : Avoiding Memory Bank Conflicts • Have the compiler schedule code to operate on all data in a bank first before moving on to a separate bank • Might result in cache conflicts • Similar to previous optimizations we saw using compiler-based scheduling

  12. Other Types of RAM • SDRAM • Synchronous Dynamic RAM • Like a DRAM, but uses pipelining across two sets of memory addresses • 8-10ns read cycle time • RDRAM • Rambus Direct RAM • Also uses pipelining • Replace RAS/CAS with a packet-switched bus, new interface to act like a memory system not memory component • Can deliver one byte every 2ns • Somewhat expensive, most architectures need a new bus systems to really take advantage of the increased RAM speed

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