RAM CHIP MODULES AND
THE SYSTEM DESCRIPTIONS
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RAMCHIP MODULES AND
THE SYSTEM DESCRIPTIONS
RAM chip - Example
A view example of a single RAM chip which are placed on a module board in series
to make up a module.
The 240-pin DIMMs are used to provide DDR2 SDRAM memory for desktop computers.
The 200-pin DIMMs are used to provide DDR SDRAM memory for MAC.
The 184-pin DIMMs are used to provide DDR SDRAM memory for desktop computers.
The 168-pin DIMMs are commonly used in Pentium® and Athlon® systems.
The 144-pin Micro DIMMs are commonly found in sub-notebook computers.
The 100-pin DIMMs are commonly used in Printers.
The 72-pin SIMMs are commonly found in older desktop computers,
such as the
486 and early Pentium® models.
The 30-pin SIMMs are commonly found in much older desktop computers,
the 486 PC and MAC.
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
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
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
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
DIMM – Dual Inline Memory
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
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,
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.
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
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
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.
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
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:
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 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
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.
– 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
SO-DIMM (SODIMM)- Small Outline Dual Inline Memory Module:
often found in laptops but also in iMac and Mac Mini desktops.
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
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