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Memory 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 Know! 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 speeds. 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 packages. 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 this. 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 types. 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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