Memory by cuiliqing


                    RAM or ROM
 RAM, or random access memory is the temporary memory that the
  computer uses to perform its functions while being used; that is, the
  when the computer is actively running, it is constantly storing and
  erasing bits of information. When you shut off your computer, that
  information is gone. It is like using a whiteboard with erasable ink:
  you can use it over and over and over, but once you erase
  something, it is gone forever; unless you recreate it from scratch.

 ROM, or read only memory. In its pure form, ROM is the data that is
  permanently written to a chip, that there is no way of overwriting. An
  example of ROM is the program that you computer uses to reboot. It
  is essential to the core functioning of the computer. You might be
  familiar with Electrically Erasable Programmable Read Only Memory
  in the form of a flash drive.
RAM Types
   RAM Types and other things to
 100 pin memory
  Used primarily in laser printers, 100 pin SIMM memory is not
  compatible with other formats.

 CAS Latency:
  The main timing of a RAM module is described by a CAS
  (Column Address Strobe) Latency value. This is the length of
  time that a RAM module needs between serving one request
  and when it is "recharged" and able to take the next request.
 Lower CAS Latency values, the faster the RAM can respond.

 Typical DDR CAS values :
   CL3, CL2.5 and CL2 . There are other latency
  measurements but CAS Latency is the most important.
 In older SDRAM (PC66, PC100, PC133) , a CL2
 module is not always faster than a CL3 module - the
 effective speed is determined by the memory
 controller on the motherboard of the computer, and
 in some cases a CL3 module matches an older
 motherboard's timing better and performs faster than
 a CL2 module.

 Non - Synchronous memory isn't measured the
 same way - it has a memory response speed in
 nanoseconds, and the computer waits for the RAM
 chip to be ready. Synchronous RAM, in contrast,
 synchronizes its operations to the computer's
 memory buss clock.
 SDRAM = CAS 2 or CAS 3

 DDR RAM = CAS Latency 2.5 for PC2700 modules & CL3
  for PC3200 modules. Premium = CL2.5 & CL2.0 on higher-
  capacity PC3200 modules. (DDR = 2x op /clock cycle,
  hence "half" a tick)

 DDR-2 RAM = CAS Latency 4 (DDR2-533) or CAS Latency
  5 (DDR2-667). Latencies may drop in the future as
  manufacturing of faster chips improves.

 DDR-3 RAM = CAS latencies in the range of 7 to 9.
  Meaning the latency penalty versus DD-2 RAM largely
  eliminates the advantage of DDR-3 running at higher clock
 DDR - Dual Data Rate RAM
  Packaged in both DIMM (184-pin for Desktops) and SO-DIMM (200-
  pin for laptops) forms.
1. PC2100 (266 MHz)
2. PC2700 (333 MHz)
3. PC3200 (400 MHz).

 DDR-2
  Faster than DDR. DDR and DDR-2 are not compatible with each
  other. Designations are PC2-5300 (DDR2-667 MHz), PC2-4200
  (DDR2-533 MHz, also called by some manufacturers PC4300), and
  PC2-3200 (DDR2-400). Available in both DIMM and SODIMM

 DDR-3
  Introduced in 2007. There are some performance penalties due to
  the higher latency of DDR-3. DDR, DDR-2 and DDR-3 are not
  compatible. Some motherboards have sockets for both DDR-2 and
  DDR-3 memory, however the two formats cannot be combined at the
  same time.
 Density:
  4 x 32Mb chips, 8 x 16Mb chips or 16 x 8Mb chips all come out to the same
  amount of RAM. The memory controller circuit in a computer must be able
  to address the chips on the module - if the memory controller cannot
  address higher density chips, that computer will only function with
  modules built with lower density chips.

 DIMM - Dual Inline Memory Module
  Found in desktop machines and servers, PC66, PC100 and PC133 different
  speeds of DIMMs in a 168 pin package, there are also SDRAM DIMMs that
  predate the PC66 standard.. The 168-pin DIMM format is 5.25 inches long
  and may be from .75 to 2 inches tall.

 DRAM - Dynamic Random Access Memory

 Dual Channel memory - In Dual-Channel Mode, the memory controller
  addresses a pair of matched modules as if they were one big module. This
  doubles the theoretical bandwidth of the memory. Benchmark
  improvement of memory performance can be up to 40%, typically around
  20%, but real-world improvement are only a few percent because your
  machine spends a lot of time on tasks other than waiting for the memory.
 ECC - Error Correcting RAM - also called Parity RAM
     ECC RAM has an extra error-correcting bit (9 bits instead of 8) for
     higher reliability in servers and high-end machines. Most forms of
     RAM (SIMMs, DIMMs, RDRAM and DDR RAM) are available in either
     ECC or Non-ECC.
    In order to use ECC RAM,
1.     The machine's motherboard must support ECC, and
2.     All of the RAM in the machine must be ECC. If you install Non-ECC
       and ECC together, the machine might work, but all of the RAM will
       revert to Non-ECC mode.
3.     Although some machines can accept either ECC or Non-ECC, there is
       little advantage to installing ECC RAM in a general purpose machine
       and possibly a small speed penalty to using ECC.

 Registered vs. unregistered (unbuffered)
     Memory modules can be built with additional support chips to buffer
     the flow of data on and off the module. This provides less load to the
     motherboard, increases reliability, and allows for the use of larger or
     more memory modules. There is sometimes a small speed penalty to
 Non-ECC RAM also called Non-Parity RAM - this is the standard
  memory used in desktops and laptops.
  Unless specified otherwise, all standard RAM is Non-ECC,
  unbuffered and unregistered.

 EDO - Extended Data Out

 FPM - Fast Page Mode
  These two terms refer to older (c. 1998 and earlier) RAM produced
  before the introduction of the PC66 standard. SIMMs, DIMMs and SO-
  DIMMs can be either EDO or FPM, Some desktop machines can take
  EDO or FPM interchangeably (although it is recommended to match
  the existing RAM type), in many cases EDO and FPM cannot be
  substituted for each other. Some EDO modules are rising in price
  rapidly because the components are no longer being manufactured by
  most chipmakers. It is not unusual for an EDO SO-DIMM for an older
  laptop to cost hundreds of dollars. The distinction between EDO and
  FPM is irrelevant with PC66, PC100, PC133, RAM Bus and DDR RAM
 Flash RAM:
  Any of a number of types of RAM that stores data even when the
  power is turned off. The firmware (BIOS) of most machines is
  kept in flash RAM (also called NVRAM - Non-Volitile Random
  Access Memory). Digital Camera memory cards such as
  Compact Flash and Secure Digital use Flash RAM technology.
  The term Flash card is loosely used to cover any form of portable
  non-volitile memory card.

 Memory Speed: Measured in megahertz (MHz). Putting in
  faster RAM will not necessarily make the machine run any faster.
  The memory is accessed at the speed set by the machine's
  memory controller. RAM with a faster speed rating will simply be
  run at the controller's normal speed. The motherboard must
  support adjusting to higher memory bus speeds (either manually
  through a BIOS setting, or automatically with Intel's PAT
  technology) for there to be any advantage to faster rated RAM.
 Micro-DIMM
  Rare, smaller than the SODIMM format, found in a few Sony,
  Panasonic. Fujitsu, Acer and Toshiba laptops. The MicroDIMM
  package can be either PC100. PC133 or DDR PC2100 or PC2700 or
  DDR2-4200, and there may be 144, 172 or 214 pins. The different
  formats are not interchangeable. These are difficult to find and many
  variations are discontinued or not available.

 PC66, PC100, PC133:
  These are standards for SDRAM DIMMs and SO-DIMMs that specify
  memory bus speeds of 66 MHz, 100 MHz and 133 MHz In broad terms,
  you can substitute faster RAM into a machine (putting PC133 into a
  machine that specifies PC100, for example), but there is no
  performance benefit to doing so.

 Profile:
  The physical size of the RAM module, both in height and in thickness.
  Some machines, particularly laptops, have physical space limitations
  that demand low-profile or in some cases super-low profile modules.
 Proprietary:
 Any memory format that does not conform to the
 common standards. Some machines, predominantly
 laptops built earlier than 1998 and some laser printers,
 use a proprietary memory packaging. These modules
 have mainly been discontinued by manufacturers and
 may be difficult or impossible to source.

 RAMBus, RIMM (RAMBus Inline Memory Module),
 also called RDRAM:
 RAMBus memory module. Most often, 16-bit RAMBus
 memory must be installed in pairs. In some machines,
 a dummy terminator module or Continuity RIMM
 (CRIMM) is required to fill the empty sockets.
 SIMM - Single Inline Memory Module:
  Found in older desktops and many laser printers, in 72-pin
  packages, and less commonly 30-pin. Commonly must be
  installed in pairs. Whether a SIMM is FPM or EDO memory (see
  below) is significant.

 SDRAM - Synchronous Dynamic Random Access Memory - the
  general description of most modern RAM. Although DDR and
  DDR-2 RAM is technically Synchronous memory, the term
  SDRAM is sometimes used for PC66, PC100 and PC133 RAM to
  distinguish them from DDR memory. Synchronous refers to the
  fact that starting with PC66 memory, the memory module no
  longer ran 'free' according to the refresh rate of its chips, but
  instead took its timing from a clock pulse from the motherboard.
  Once memory modules were designed to synchronize to an
  external clock, manufacturers were able to produce ever faster
  RAM that could transfer large amounts of data reliably.
 SO-DIMM - Small Outline Dual Inline Memory Module:
  Most often found in laptops. Most SO-DIMMs are 144 pin
  packages - there are also DDR 200 pin SO-DIMMs and some rare
  72 pin small-outline modules.

 Overclocking:
  Modifying a motherboard to run at speeds higher than the speed
  it was designed for. RAM is certified to run reliably at the speed
  it is sold as, and may or may not work reliably at increased
  speeds. There is no warranty on RAM run at faster than design
  speeds. There are specific RAM modules sold (notably Kingston
  HyperX and Corsair XMS) with higher speed ratings and lower
  latencies, specifically for the overclocking market. There is
  always a trade-off between pushing the performance envelope of
  hardware, and stability and longer life. The common method of
  overclocking is to increase speeds to the point where the
  machine becomes unstable, and then reducing the speed to the
  point where it runs acceptably reliably.
 VRAM - Video RAM:
  Memory that is dedicated to the use of the graphics processing unit
  (video card). The VRAM is usually faster than normal RAM and more
  expensive. Top of the line video cards have between 128 Mb and 256 Mb
  of fast VRAM. Large amounts of VRAM are used for 3-D gaming and
  where the program is manipulating large amounts of textures and
  shade data. Normal 2-D display requires only 2 to 16 Mb of VRAM
  depending on resolution and colour depth of the display. VRAM is
  usually soldered onto the video card, and is not upgradeable. On a very
  few machines, VRAM is socketed on the motherboard and can be
  upgraded. Other terms that are specific to Video RAM are GDDR,
  GDDR-2 and GDDR-3

 Shared VRAM is a misnomer - this really refers to motherboards with
  built-in graphics chips with no dedicated VRAM. The graphics system
  "steals" 4 to 128 Mb of memory from main system memory. This results
  in lower performance for video, and a loss of RAM available to the
  operating system. An easy speed up for inexpensive machines with
  shared, or integrated, video systems is to install a separate AGP or PCI-
  e video card (if the machine has a suitable slot) and disable the built-in
  graphics system. Of course, the least expensive machines won't have an
  AGP or PCI-e socket either, so you're stuck on those ones.
 Cache RAM:
 High speed memory, that acts as a buffer between a
 processor and lower speed devices (memory, disk storage or
 a buss). Cache RAM allows a processor to get access to the
 most recently used items in memory very quickly, without
 having to process a request for each bit to come from the
 main memory. This makes operations faster, because much
 of a computer's time is spent accessing the same
 instructions or the same data repeatedly. In the mid 80s,
 some computers had upgradeable cache RAM, which was
 expensive, but gave a performance boost. Today, all major
 processors have between 256 Mb and 2 GB of cache RAM
 built right onto the CPU's die, where it can be accessed
 directly by the CPU core. The days of user-accessible cache
 RAM are over. Hard drives use cache RAM as well, between
 2 and 16 Mb of SDRAM built onto the drive controller
 board the buffer reads and writes while the drive platters
 and headers are positioning in the correct locations.
                           Mini Test
 What are the principle characteristics of DRAM?

 Why is Synchronous DRAM so-called?

 What is the primary use for SRAM?

 What is the clock-speed of PC2100 DDR SDRAM?

 How many pins are there on a DIMM stick of DDR2 SDRAM?

 How can you distinguish memory slots on a motherboard?

 When installing a double banked stick of ram, what might you need to check?

 Why would non-ECC memory be registered?

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