Types of RAM:
Memory Terminology: This is a simplified overview of RAM terms and definitions
100 pin memory
Used primarily in laser printers, 100 pin SIMM memory is not compatible with other formats.
To oversimplify a complex discussion, 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. All else being equal, and if the motherboard can make use of faster latencies, the lower the CAS Latency value the faster the RAM can respond.
There are other latency measurements but CAS Latency is the most important. See our article on choosing memory speed.
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 can be CAS 2 or CAS 3
DDR RAM is normally CAS Latency 2.5 for PC2700 modules and CL3 for PC3200 modules. Premium modules are available with CL2.5 or CL2.0 on higher-capacity PC3200 modules. (DDR make two operations per clock cycle, which is why it can have “half” of a tick)
DDR-2 RAM is normally CAS Latency 4 (DDR2-533), CAS Latency 5 (DDR2-667) and CAS Latency 5 0r 6 (DDR2-800). Lower latency RAM is available at higher prices.
DDR-3 RAM has CAS latencies in the range of 7 to 11, with latency increasing as the clock rate increases. This means that the latency penalty versus DDR-2 RAM partially rolls back the advantage of DDR-3 running at higher clock speeds. Again, the fastest MHz and the lowest Latency command the highest prices.
DDR – Dual Data Rate RAM
Can be packaged in both DIMM (184-pin for Desktops) and SO-DIMM (200-pin for laptops) forms. Speeds are PC2100 (266 MHz) PC2700 (333 MHz) and PC3200 (400 MHz) DDR . The standards for speeds higher than PC3200 are not finalized, although some manufacturers offer “PC3700” “PC4000” and up DDR modules as a marketing designation.
A format that is 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 PC2-6400 (DDR2-800). Available in both DIMM and SODIMM packages. Faster speed RAM is available at higher prices.
DDR-3 is now the primary RAM type for new desktop and laptop machines and motherboards. DDR, DDR-2 and DDR-3 are not compatible with each other. Some early DDR3 motherboards have sockets for both DDR-2 and DDR-3 memory, however the two formats cannot be combined at the same time.
JEDEC has recently (September 2012) ratified the DDR-4 memory standard. It is dual data rate memory, same as DDR1, 2 and 3. The DDR4 memory speeds will start at 2133 MHz (bus speed 1067 MHz).
When a RAM module is built, the manufacturer can design it using high density or low density chips. For example, modules built with 4 x 32Mb chips, 8 x 16Mb chips or 16 x 8Mb chips all come out to the same amount of RAM. However, 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 are all different speeds of DIMMs in a 168 pin package, there are also SDRAM DIMMs that predate the PC66 standard. (DDR memory is also technically a DIMM format but are usually referred to as DDR rather than DIMM). The 168-pin DIMM format is 5.25 inches long and may be from .75 to 2 inches tall.
DRAM – Dynamic Random Access Memory – the general description of almost all computer memory.
Dual Channel memory – In Dual-Channel Mode, (whether in a PowerMac G5, Mac Intel, or modern PC motherboards), 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 to application performance will be between 6% and 8% because your machine spends a lot of time on tasks other than waiting for the memory. wikipedia definition
In order for dual channel to work, the two memory modules have to be identical in size, speed, and composition — that is, the number and capacity of chips and the organization of the rows and columns on the chips. Note that they don’t HAVE to be the same brand and model, as long as all the other things match… but it is much easier to assure that they will match if two of the same modules are bought together at the same time or as a dual-channel kit. It’s important to note that there is no difference to the individual memory module whether it is single or dual channel – Dual Channel refers only to the use of two modules as a matched pair.
If a motherboard does not support dual channel, there is no way to add the function, and installing matched pairs will not result in any performance increase.
ECC – Error Correcting RAM – also called Parity RAM
ECC RAM has an extra error-correcting bit (9 bits instead of 8 per word) 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. Most common computers use Non-ECC (standard) memory. In order to use ECC RAM,
- The machine’s motherboard must support ECC, and
- 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.
- 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.
If you have a machine that can take either ECC or Non-ECC and are unsure what to order, either check the RAM that is already installed, or check your purchase invoice for the machine, which should indicate whether ECC was installed or not initially. Do not mix ECC and Non-ECC if you can avoid it.
Reading memory chip part numbers: You can sometimes read from the part number whether the RAM in your machine is ECC (Parity). ECC RAM often has a number in the part number that is a multiple of 9, where Non-ECC would have a multiple of 8. Example, RAM that has x18 or x36 or x72 in the number would be ECC, compared to Non-ECC with x16 or x32 or x64. Also look for letters in the speed designation. Examples: PC2-6400U would normally mean Unbuffered, normal non-ECC RAM. PC2-6400UR would stand for Unregistered, PC2-6400R would be Registered ECC, and PC2-6400E would mean ECC – probably unbuffered. 1R (SR), 2R (DR)and 4R (QR) would stand for Single Rank, Dual Rank and Quad Rank.
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.
Because of the added expense of the modules and the performance hit, the use of buffered or registered memory is confined mainly to servers and high end workstations (and some early Athlon 64 motherboards). All non-ECC memory is unbuffered.
If a machine requires registered memory (or unregistered memory) do not install RAM of the opposite type. Even if it supports both, never mix registered and unregistered modules in the same machine.
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, RAMBus and DDR RAM types.
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 CompactFlash and SecureDigital use Flash RAM technology. SSD drives (Solid State Drive) also use Flash memory chips internally, but with a much different controller than CF and SD cards. The term Flash card is loosely used to cover any form of portable non-volatile 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. See our article.
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.
However there are a number of earlier PC66 and PC100 machines that are incompatible with modern PC133 modules for reasons of both memory chip density and differences in the supporting chips on the memory module. Always cross-reference compatibility before ordering.
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.
Any memory format that does not conform to the common standards. Some machines, predominantly laptops built earlier than 1998, some music keyboards and samplers, 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.
RAMBus speeds are PC800 (for machines with 400 MHz and 533 MHz Frontside Busses) and PC1066 (533 MHz FSB). The PC600 and PC700 designations are obsolete and PC800 can be substituted. There are 2 variations on PC800 RAMBus – 45 nanosecond and the faster 40 ns. The 45 ns memory can usually be used on machines with 400 MHz FSB. A 533 MHz FSB machine will take the 40 ns modules, and may not accept the 45 ns modules.
Most manufacturers have discontinued RAMBus, PC1066 is largely unavailable, for the time being there are still supplies of PC800 RAM, and the faster PC800-40ns RAM can sometimes be used to substitute for PC1066. Always cross-reference compatibility with your specific machine before ordering.
32-bit RAMBus modules have been introduced which can be installed singly – however the motherboard has to specifically support 32-bit RAMBus to use these. 32-bit RAMBus (sometimes called PC4200) has been discontinued and is completely unavailable.
It is not possible to convert a RAMBus motherboard to use any other type of RAM. Although you could swap in a modern motherboard with some computers, there are problems with mismatches in power supplies and in Pentium processor pinouts – most RAMBus motherboards use an obsolete CPU socket, so the CPU cannot be swapped into a new motherboard. Ultimately, replacing the machine with a new one is the answer.
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 discription 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 (SODIMM)- Small Outline Dual Inline Memory Module:
Most often found in laptops but also in iMac and Mac Mini desktops.
Most SDRAM SODIMMs are 144 pin packages,
DDR and DDR2 are in 200 pin SODIMMs with different slots, and
DDR3 is in a 204 pin SODIMM package.
There are also some rare 72 pin and 100 pin small-outline modules.
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 versus 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 – VideoRAM:
Memory that is dedicated to the use of the grapics 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 amountsof textures and shader 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, GDDR-3 and GDDR-5
Shared VRAM is a misnomer – this really refers to motherboards with built-in graphics chips with no dedicated VRAM. The graphics system “steals” 4 MB to 512 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.
High speed memory, that acts as a buffer between a processor and lower speed devices (memory, disk storage or a bus) Cache acts like the little jug of coffee creamer at your restaurant table. You can serve yourself small quantities immediately, without having to call back to the kitchen and waiting for each serving.
Similarly, 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 1 GB and 12 GB of cache RAM built right onto the CPU’s die, where it can be accessed directly by the CPU core(s). The days of user-accessible cache RAM are long over.
Hard drives use cache RAM as well, between 8MB and 64MB of SDRAM built onto the drive controller board to buffer reads and writes while the drive platters and heads are positioning in the correct locations. A few manufacturers have used up to 4 GB of SSD flash memory as a larger cache on a hard drive, terming it a “Hybrid” drive.
Intel incorporated hardware support in the Z68 and some newere 6- and 7- series chipset motherboards for caching of the system hard drive on a 32 GB SSD, calling it Smart Response Technology (SRT).
Apple’s new Fusion drive technology uses software to cache onto a 128 GB SSD to speed up the rotating hard drive.