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Understanding Different Memory Types and Cache Role

Learn about various memory types like DRAM and SRAM, cache functionality, and the evolution of memory technology. Understand the differences between asynchronous DRAM, synchronous DRAM, DDR-SDRAM, Rambus DRAM, and memory packaging. Explore the importance of cache, how memory is accessed, and the role of Serial Presence Detect. Discover the impact of memory types on system performance and efficiency.

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Understanding Different Memory Types and Cache Role

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  1. Memory

  2. Types of DRAM • Asynchronous • The processor timing and the memory timing (refreshing schedule) were independent. Thus the processor might have to wait until the memory “window” was open for access. • Synchronous (SDRAM) • The processor and memory timing are linked. This allows for more efficient processor-memory interaction. • Note SDRAM is NOT static dynamic RAM – that doesn’t make sense

  3. The Role of Cache • It’s important to remember the role of cache (SRAM) when trying to understand the distinction between various types of DRAM. • Over 90% of the time the processor finds what it needs in cache. • But when one needs to access memory, one caches not only that particular value held in that particular location but also many of the values in the nearby locations (because they are likely to be needed as well – locality of reference). • So the differences in memory types are often not in an individual access but in the accessing of a larger amount of data for the purposes of caching it.

  4. Asynchronous DRAM • Asynchronous DRAM was common until the mid to late 1990’s but now is out-dated. • Fast Page Mode • What made FPM fast was that the same row but different columns of data could be accessed without forcing one to reselect the row strobe. • Extended Data Out (EDO) • What was “extended” about EDO was that it could go longer between refreshes. • Burst Extended Data Out (BEDO) • Consecutive data was fetched in “bursts” saving on the addressing part of access time.

  5. Synchronous DRAM • Since the mid to late 1990’s SDRAM has taken over as the standard for use in main memory. • JEDEC (Joint Electron Device Engineering Council) or PC66 or Ordinary SDRAM operates with bus speeds up to 66MHz, now is outdated. • PC100 SDRAM that works at higher bus speed of 100 MHz

  6. Synchronous DRAM (Cont.) • PC133 SDRAM operates at bus speed of 133 MHz and slower. This is a standard memory these days. • There are versions of PC133 SDRAM that differ in “latency.” • Latency is the time you spend waiting until conditions are right to proceed with some action. • This is the CAS Latency mentioned previously.

  7. DDR-SDRAM • Double Data Rate Synchronous DRAM • One allows data to be accessed on both the positive and negative edge of the clock (double pumping). This effectively doubles the throughput. • The associated chips go by PC200 (double PC100) or PC266 (double PC 133) • But the memory modules are designated by throughput. With a 64-bit bus (8 bytes) operating at PC200 (double pumped 100MHz bus), the DDR module goes by PC1600 • 1600 = 200  8

  8. DDR2 from Wikipediahttp://en.wikipedia.org/wiki/DDR2_SDRAM • “The key difference between DDR and DDR2 is that in DDR2 the bus is clocked at twice the speed of the memory cells, allowing transfers from two different cells to occur in the same memory cell cycle. Thus, without speeding up the memory cells themselves, DDR2 can effectively operate at twice the bus speed of DDR.”

  9. Rambus DRAM • Rambus DRAM or RDRAM • A proprietary memory technology produced by Rambus. • It is a competitor of the DDR-SDRAM but it seems to be losing out. • Which is better depends to the situation. • For very high memory usage in P4 systems, RDRAM might be better.

  10. Serial Presence Detect • With all of the different types of memory, the computer needed a way to determine the specs of the memory installed so that it could be used efficiently. • This is the job of the Serial Presence Detect (SPD). • A set of data usually stored on the memory stick itself that informs the BIOS of the module's size, data width, speed, and voltage. • Previously done by the parallel presence detect (PPD).

  11. Memory Packaging • How memory is packaged • DIP: Dual inline pin Package (OLD) • Rectangular, has pins on both sides • 386 • SIPP: single inline chip packages (OLD) • Little circuit board with memory chip • Has pins, hard to install

  12. Memory modules (Cont.) • SIMM single inline memory module • Like SIPP but no pins, easier to install • a 32-bit path to the memory 30-pin SIMM 72-pin SIMM

  13. Memory modules (Cont.) • DIMM dual in-line memory module, • a 64-bit path • RIMM the Rambus version of DIMM (is not really an acronym) • SO-DIMMs (small outline) are memory modules for laptops • Initially laptop memory was proprietary and not easily changed or updated. Furthermore there was no easy access to its location.

  14. Row versus Bank • Row: Physical unit of memory • Bank: logical unit • The bank is based on bus width. Pentiums use a 64-bit width. • Two SIMMs to make a bank • One DIMM to make a bank • One RIMM to make a bank

  15. Installing Memory

  16. References • http://www.pcguide.com/ref/ram/types.htm • http://www.webopedia.com • http://www.whatis.com • PC Hardware in a Nutshell, Thompson and Thompson • All-in-One A+ Certification, Meyers and Jernigan

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