Beginners Guides memory.doc
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Beginners Guides: RAM, Memory and Upgrading
What is memory? Well, let me think... - Version 1.0.2
Bookmark this PCstats guide for future reference.
Modern computer processors can perform several billion operations per second,
creating and changing incredible amounts of data in a short period of time. To
perform at this level, they have to be able to juggle the information they process, to
have someplace to store it until it is needed again for modification or reference.
As a metaphor, the more jobs a technician takes on at once, the more bench space
they are going to need to place the components they are assembling, and the more
shelf space they will need to place the finished products. Similarly, computers need
space to store data while they are working on it, and space to store data that is not
being worked on, but will be needed in the future. This is provided by RAM (Random
Access Memory) and hard disk drives respectively.
Computers have a memory structure which can be easily (if somewhat sloppily)
compared to the human brain. The hard drive provides long-term memory storage
similar to our long-term memories, a place where data is put to be permanently
stored. RAM (Random Access Memory) provides a pallet that the computer can work
from in normal operation, similar to our short-term memory. It holds information
that is essential now but may or may not be transferred to long-term memory,
depending on need.
Modern processors also include a memory cache, a comparatively small amount of
high-speed memory which stores the data that is currently being used most often.
This could be compared to our awareness, the memory that connects one moment to
the next and keeps us doing what we were doing a second ago.
Random Access Memory (RAM)
can be thought of as the short-
term memory, in the sense that
once the power is turned off, all
information stored there is not
saved. All modern computers
have hard drives which storedata permanently as magnetic
information, but even with the
improved speed of today's hard
drive technology. Hard drives
are still too slow to keep up with
the needs of the processor since
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it can operate on considerably more information per second than can possibly be
transferred to and from the hard drive.
This is where the need for a fast, short-term memory solution comes in, a memory
space that provides very fast access for the processor so data can be written and
read as needed without slowing down the system appreciably.
RAM fulfills this need, specifically DRAM (Dynamic RAM), the template for all modern
memory types.
DRAM consists ofsemiconductor chips arranged on a small circuit board, each
containing a logical arrangement of cells laid out in rows and columns. These cells
use a combination of a capacitor and a transistor to achieve one of two states, filled
with electrons (1) or empty (0), thus allowing binary (digital) information to be
stored.
The dynamic aspect of this type of memory is that it needs to be constantly
refreshed with an electric charge to keep its information stored. When the computer
is turned off, all data in the DRAM is lost. In all modern desktop computers, DRAM
can be added directly to the motherboard in the form of memory modules, a circuit
board with mounted memory DRAM chips.
Types of memory
There are three main types of memory in common use today. SDRAM (Synchronous
Dynamic RAM), DDR-SDRAM (Double Data Rate SDRAM) and RDRAM (RAMBUS
Dynamic RAM). This article will detail all three, though it should be mentioned at thistime that DDR-SDRAM in its various forms is by far the dominant type in today's PC
market.
Beginners Guides: RAM, Memory and Upgrading
SDRAM
Synchronous Dynamic Random Access Memory
SDRAM started life as an
evolution of the EDO (Extended
Data Output) DRAM memory
type, as seen in 486 and older
Pentium systems. The main
drawback of these older memory
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technologies was that they ran at a different speed than the rest of the system
components (asynchronously).
This resulted in occasional delays, or wait-states while the processor was waiting for
the RAM to be available to receive data, which in turn reduced the overall speed of
the system. SDRAM, initially available at the 66Mhz specification, was synchronizedwith the system clock, eliminating any unnecessary wait times. Ideally, as long as
the SDRAM used was fast enough to keep up with the system clock, it would perform
the requested data storage or retrieval action within one clock cycle, and be ready to
receive or transmit data again on the next cycle.
To give an example, a 66Mhz system clock, as seen in older Intel Pentium II and
Celeron systems, performed 66.6 million cycles per second, each cycle taking 15
nanoseconds to complete. The SDRAM would theoretically perform one read or write
action every cycle. We say theoretically since the actual memory chips used to make
SDRAM are not generally significantly faster than the older DRAM types, and still
take considerably more than one clock cycle (generally 5 cycles for SDRAM) to begin
a read/write action by locating the correct row and column to begin reading or
writing from. After this first cell is located, subsequent read/write actions on the
adjacent memory cells are much faster, happening one per clock cycle. This is known
as burst mode.
Tying the memory to the system clock enabled memory access to keep pace with the
increasing speed of modern computers, while also putting pressure on manufacturers
to increase the quality of the memory to cope with the increasing demands put on it.
SDRAM memory is commonly available in 66, 100 and 133Mhz speeds, also calledPC66, PC100 and PC133 respectively. It should be noted though, that these values
do not refer to the speed of the memory itself, but rather the bus and system clock
speed of the systems it is rated to be used with.
The memory itself works the same way, and is generally backwards compatible, in
that higher rated memory (PC133 for example) will work in a lower speed system
(say 100Mhz) running at the lower speed. You gain no performance benefit from
using the higher rated memory in this scenario though, and be aware that older
SDRAM systems are likely to be incompatible with newer SDRAM memory due to the
memory speed settings that the motherboards may default to.
Manufacturers have been known to put ratings on their SDRAM of up to 166Mhz even
though there are no motherboard/processor combinations using SDRAM which run at
that clock speed by default. They do this to advertise their memory's ability to run at
higher clock speeds for users who wish to overclock their computers. Since if you
increase your computer's clock speed, the frequency at which it will attempt to
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access the memory will also increase, leading to stability problems if the memory can
not keep up.
Most SDRAM memory chips are capable of transferring and receiving data slightly
faster than their rated speed to give some margin for error. SDRAM is typically
available in a 168-pin DIMM (Dual Inline Memory Module) in capacities from 16MB upto 1 GB.
Currently, aside from a few motherboards that support both SDRAM and DDR-
SDRAM, you cannot purchase new systems that use SDRAM. The huge volume of
older computers on the market ensure that SDRAM modules will be manufactured for
at least a few more years, however.
DDR-SDRAM
DDR-SDRAM
Double Data Rate SDRAM
DDR-SDRAM is again an
evolution, this time of the
SDRAM specification. As
the speed of computer
processors has increased
in leaps and bounds,
the amount of data they
are able to process in a set amount of time has also increased vastly. The recent
families of processors from Intel and AMD such as the Pentium 4 and Athlon XP are
capable of several billion operations per second. This is wonderful from a
performance standpoint if you are looking at the speed of the chip alone, but it
presents somewhat of a problem for the system as a whole, since it is limited by the
bandwidth of the memory.
The bandwidth of the memory is how much data it can potentially transfer in a set
period of time. Essentially, the faster a processor can go, the faster the memory
system supporting it needs to be able to carry data.
To increase its bandwidth, DDR-SDRAM transfers data twice on each clock cycle,
achieving twice the theoretical maximum bandwidth of SDRAM running at the same
speed. This does not translate to twice the memory or system performance, since
the efficiency of the memory (expressed as a percentage where 100% efficiency
equals one data transfer performed every clock cycle) suffers as the speed it
attempts to perform operations in increases.
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Despite this, it is still capable of feeding and receiving considerably more data than
SDRAM, and is a suitable memory platform for modern processors like the AMD
Athlon XP and the Intel Pentium 4, both of which rely chiefly on various speeds of
DDR-SDRAM to provide memory support.
DDR-SDRAM has been in development since the late 90's, and was first introduced tothe desktop PC market in the Geforce video card by Nvidia, followed by AMD in late
2000 with their 760 chipset for the Athlon processor. It has since completely
supplanted SDRAM as the memory of choice for the home and small business PCs
using both Intel and AMD processors.
In the form of 184-pin DIMM modules, DDR-SDRAM is currently available in a few
speeds: PC1600 (200Mhz) PC2100 (266Mhz), PC2700 (333Mhz), PC3200 (400Mhz),
PC3500 (433MHz), PC3700 (466MHz), PC4000 (500MHz), PC4200 (533MHz) and
PC4400 (566MHz). The first number, for example 'PC2100' represents the maximum
memory bandwidth the module can provide in Megabytes per second. The Mhz value
is the clock speed it is certified to operate at. DDR-SDRAM is commonly available in
64MB-2GB sizes.
Note that some newer chipsets such as Nvidia's nForce and nForce 2 and the Intel
I865 use dual-channel memory, essentially accessing two separate DDR memory
modules at the same time to double the maximum bandwidth. Dual-channel requires
that memory modules be added in identical pairs to the board. Regular DDR-SDRAM
can still be used in this case, just be sure to purchase identical modules.
RDRAM
RAMBUS Dynamic RAM
Currently all but extinct in the
standard desktop PC market,
RDRAM is a proprietary memory
standard, developed by the
RAMBUS company. RDRAM
originally made a big splash in
1998 with its adoption by Intel to provide memory support for their high-end
Pentium III boards and the early Pentium 4 models.
Sadly for the company, this was closely followed by a protracted series of court
battles with major memory manufacturing companies such as Infineon and Hyundai
over alleged patent violations among other things. The comparatively high price of
RDRAM, its early stranglehold on the Pentium 4 processor market, and the
perception that the RAMBUS company's series of lawsuits might well drive up
Figure 4
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conventional SDRAM and DDR-SDRAM prices if they succeeded combined to make
RDRAM rather unpopular with the home user and PC enthusiast markets.
RDRAM failed to decrease appreciably in price, so Pentium 4 chipsets supporting
SDRAM were introduced by Intel in 2001 to attract the lower-end market. SDRAM did
not deliver the necessary performance for the Pentium 4, so Intel introducedPentium 4 motherboard chipsets supporting DDR-SDRAM in 2002, all but eliminating
RAMBUS memory from the home and small business PC market. There have been
some signs of a resurgence in RAMBUS fortunes recently, most notably the January
overturning of a court ruling against them in favour of Infineon, so who knows what
the future holds.
In design and operation, RDRAM differs considerably from SDRAM and DDR-SDRAM
memory in several ways. First of all, RDRAM transfers only 16 bits, or 2 bytes of data
at a time, as compared to SDRAM/DDR-SDRAM's 64-bit data channel, but it
transmits those 16 bits at a considerably higher frequency, 400Mhz for basic PC800
RDRAM. Also, RDRAM transmits data twice per clock cycle just like DDR-SDRAM, so
the effective data transfer rate starts at 800Mhz. Using the formula
(Memory frequency) * (# of bits in data channel) / 8 800,000,000 * 16 / 8
To determine maximum memory bandwidth gives RDRAM a theoretical maximum
bandwidth of 1.6 GB per second. This gave it a considerable on-paper advantage to
SDRAM at the time, and was one of the main reasons why Intel decided to use
RDRAM to launch its Pentium 4 processor line.
In reality, while RDRAM's high (at that point in time) bandwidth gave it an advantageover SDRAM in Pentium 4 chipsets, the longer time required for RDRAM to initially
locate memory cells to be written or read from, as compared to SDRAM or DDR-
SDRAM resulted in it actually performing worse than SDRAM when used with the
Pentium 3.
RDRAM's advantage is the speed of the burst or sequential memory transfers after
the initial delay due to the higher frequency of the memory. Newer RDRAM modules
can run at 533/1066Mhz and 600/1200Mhz with a corresponding increase in
bandwidth. RDRAM numbers still trail those of the fastest DDR-SDRAM however.
Just like DDR-SDRAM, some RDRAM chipsets may be dual-channel, requiring two
identical chips to function. RDRAM memory chips can generate considerable heat,
and require a metal heat spreader to help dissipate the excess. RDRAM is available in
PC800, PC1066 and PC1200 types (where 'PC800' indicates the speed in Mhz after
taking into account the doubled data transfer rate), at sizes from 64MB to 512MB.
RDRAM modules are 184-pin packages called RIMMs (RAMBUS inline memory
modules).
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What type of memory should you use?
The answer to the above question is governed by a few factors. First of all, if you
already have a computer that you do not intend to significantly upgrade, you are
limited to the type of memory that your current motherboard supports. The chipsetof a motherboard is the collection of chips and circuits that allow the components of
your computer to work together, and is tied to a specific kind of memory, SDRAM,
DDR-SDRAM or RDRAM.
As a general rule, to add x amount of memory to your system, you need a single
memory module of that size, of the same type and speed that your system currently
uses (note that type does not mean the brand of the memory, but rather whether it
is SDRAM, DDR or RDRAM). This information should be easily attained from your
motherboard manual. There are of course exceptions to this rule.
Here are some common ones....
In theory, any system that uses SDRAM can use memory that is rated faster than
the requirements of the motherboard, as well as memory of the correct speed. For
example, an older Intel Celeron system would use a 66Mhz clock speed, and thus
would require PC66 SDRAM, but could also make use of PC100 and PC133 SDRAM,
which it would simply access at 66Mhz. Note that this is not necessarily possible in
the real world, as voltages and other factors have changed since SDRAM was
introduced, and it is a good idea to stick with the recommended memory, but it is
possible.
Some recent chipsets support more than one type of memory, generally SDRAM and
DDR-SDRAM. Consult your motherboard manual for information on this one, but note
that if your board does support this, you cannot mix both types. Some DDR and
RDRAM chipsets use dual-channel memory, meaning that two separate memory
modules on the motherboard are accessed at the same time, doubling the maximum
memory bandwidth. This requires that the memory be installed in identical pairs on
the board, rather than single modules as is generally the case. Again, consult your
manual.
Also, you will need to verify that you have space on your motherboard to install more
memory. Since we are going to tell you how to actually install it later in the article,
the easiest way to do this is to open up the computer and physically check how many
open memory slots you have.
Regardless of the type of memory you use, they will look more or less like this.
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Figure 5
If you are planning on purchasing a new system, your choice of memory is going to
be governed by the type of processor you select. PCStats has several excellentarticles on the newest processor and memory combinations to help you research.
Keep in mind that, as with all other computer components, the price is governed by
the relative newness and availability of the memory.
Right now, DDR2100 and PC133 SDRAM are the cheapest and easiest to find, but since
both Intel and AMD are using higher rated DDR-SDRAM for their newest processors,
this will change soon. It does seem that DDR-SDRAM of PC3200 speed and above is
going to be the memory of choice for the near future anyhow, as both Intel and AMD are
using it for their next generation 64bit processors.
The Advantage of more memory
The relation of memory to the actual perceived speed of a computer is always going
to be a bit nebulous, as it is governed by so many other factors. To make it easier to
think about, try this. Not having enough memory will slow your system down.
That's about the easiest way to express it. Think of memory as enabling your system
to reach its performance potential and you will have the right idea. The processor
governs the overall speed of the system, but the memory provides it with aworkspace to store information it is using.
If you have multiple applications running at the same time, demands on the memory
increase drastically, and if all available memory is used, the system will resort to
virtual memory, which entails using a portion of your hard drive (the swap file) as
extra RAM space. As you can imagine, as soon as virtual memory must be accessed,
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system performance slows down considerably, since hard drives are vastly slower
than RAM in transferring data.
Thus, adding more memory is not so much about speeding up your system as it is
about avoiding slowdowns. Memory upgrades work on the law of diminishing returns
though. You will see a much bigger increase in performance going from say 64MB ofRAM to 128MB on a Windows 2000 system than you would going from 512MB to 1GB
of RAM on the same system.
This is also dependent, of course, on the amount and type of applications you
commonly run, as well as the operating system you use. Newer versions of Windows
like 2000 and XP take better advantage of large amounts of system memory than do
older operating systems like Windows 9x/ME.
For example, we ran through a couple tests with Bapco sysmark 2002 on a 2.4GHz
Intel Pentium 4 computer with 256MB and 512MB DDR. In the first round of tests,
with 256MB, the Internet Content Benchmark came in at 425, Office productivity at
219. By increasing the memory to 512MB, the Internet Content benchmark increased
to 452, and Office productivity to 246.
While not showing a massive increase in performance, doubling the memory on our
test system gave an appreciable increase in performance when the system is under
heavy load, especially in the office application portion of the Sysmark test. This is
consistent with the real life performance benefits you will see by upgrading your
system's memory.
Installing Memory modules.
Before proceeding with this section, please ensure that you have purchased the
correct type and speed of memory for your computer as specified in your
motherboard manual.
Power off and open up your PC.
All modern RAM is keyed so it can only fit into the RAM slots a certain way. With
modern motherboards, it should not matter which slot you use, though if they are
numbered in the manual or on the board, it is always a good idea to go with slot #1
first. Hold the RAM chip next to the slot so that the indentation(s) on the chip line up
with the bumps in the slot.
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Once you are certain of the orientation of your RAM, open the levers on either side of
the RAM slot and push the RAM chip straight down into the slot until both levers snap
closed on either side of the chip. This will require some force. If it does not seem to
be going in with a moderate amount of force, remove the chip and re-insert it,
making sure that it is exactly lined up with the slot.
Now power on the computer. Check on the boot up screen and on the properties of'my computer' in your OS to verify the RAM was installed correctly. Everything
should be good to go once the operating system boots up now!
Memory Bandwidth vs. Latency Timings
When Intel released the i865PE/i875P alongside the Intel Pentium 4C processors, the
DDR memory game changed forever. With a DDR memory controller now capable of
running dual channel, the Pentium 4 was no longer to be bandwidth limited as it had
been with the i845 series. Those single channel DDR chipsets, like the i845PE for
instance, could only provide half the bandwidth required by the Pentium 4 processor
due to its single channel memory controller.
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As the new 800 MHz FSB Pentium 4 processors
allowed users to hit never before seen highs in
terms of bus speed, many memory
manufacturers were trying to capitalize on the
situation by releasing every increasing degrees
of"high speed"memory.
Unfortunately, to run the memory frequency at
the same speed as the FSB (or a 1:1 ratio)
almost all the high speed DIMM's (Dual Inline
Memory Module) have to have very lax timings.
Think about it this way, a car built for drag
racing can go dead straight super fast, but
cannot maneuver as well as an F1 race car.
Likewise, the F1 racer is good in the corners but
will be left in the dust on the drag strip. In otherwords, today's high speedmemory modules are
built for one thing only, and that's top speed,
where timings really aren't considered all that
much.
Confused about memory timings?
When one talks about memory timings they're basically talking about how long the
system has to wait for the memory to be in a ready state before data is fetched or
delivered.
You could think about memory timings as people working at a drive through
restaurant; you place your order then wait for the food to be ready. The lower the
timings are, the faster the computer (and quicker your order comes) is able to get
data from the memory, and the faster the rest of the PC will ultimately be.
This rule of thumb applies whether you're on an Intel or AMD based system. As for
why there aren't lower timings then 2-2-2-5, JEDEC (the memory governing body)
does not think it's possible for current dynamic memory technology to run at 0 or 1.
When we refer to timings it is common to quote a four digit number separated by
dashes (ie. 2-2-2-5) The first number always represents CAS (Column Address
Strobe) Latency as it's usually the most important.
Next in line is RAS-to-CAS Delay (Row Address Strobe), RAS Precharge and Act-to-
Precharge Delay (which is always the final, and largest number).
Figure 6
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In the picture to the left here, we can see the the timing
diagram from some Crucial DDR333 memory. If we take this as an example
for all subsequent memory speeds, I think we should be able to illustrate
just what all these 'timing' numbers really represent.
The diagram shows timings of CAS2, CAS2.5 and CAS3 timings (marked asCL=2 for example). Note the vertical dashed lines which indicate a rise or
fall of the clock signal, since this is double data RAM, there are two such
points per 'Time unit'.
CAS latency is the delay between the registration of a read command and
the availability of the first piece of output data. CAS latency is measured in
clock cycles. In the last of the three examples, a read command which is registered
at T0 (Time=0) is not valid until T3 (Time=3).
With all things equal, a stick of DDR memory capable of running 2-2-2-5 will make
the computer operating experience seem faster than a DIMM which may only run at
3-4-4-8. This is because the delay from when the memory receives an instruction,
retrieves the data, and sends it back out is less.
Where it starts to get confusing is when you has the choice of buying high speed
memorywith slow timings. Just about ever PC3700+ rated memory module we've
seen uses conservative timings after all. If your answer would be to buy fast
memorywith tight timings, I'm afraid you're going to be disappointed as there are
no such modules available yet. So, why are we still interested in fast memory with
slow timings then? Well, the answer goes something like this....
DDR memory with slow timings
In highly competitive markets, once a major manufacturer releases a new and innovative
product, the rest will surely follow close behind. If one manufacturer doesn't follow suit, their
products are considered 'old tech'.
As always, everything always boils down to money and that's why we have this dilemma; to
run faster memory with slower access times, or run slower memory with faster access times.
There are two trains of thought on this, the first is that high speed DIMM's (like PC4000 DDR)
can make up for running slower timings by the amount of bandwidth provide the processor.
Specifically, bandwidth is the amount of data that can be moved from one given device to
another.
Figure 7
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Most DIMM's that run tight timings, such as certain
PC3200 & PC3500 modules, have to run the memory at
lower MHz than the FSB. However, when overclocking
to extreme speeds these DIMM's are bandwidth limiting
the processor. What I mean by this, is that when the processor requires a great deal of
bandwidth, the CPU will have to wait for another clock cycle before being filled, as the memory
is just not fast enough to keep up at the same pace. Having a large pool of bandwidth is great
when you're working with applications that process a lot of raw data, such as Photoshop or
databases for example.
The other point of view is that CAS2-rated PC3200 & 3500 memory can make up for the lack
of bandwidth because the memory has a lower latency that in effect moves data between the
CPU and memory faster. Programs that do not require a large amount of bandwidth tend to
benefit more from quicker data transfers between the memory and the rest of the computer;
such as games or 3D applications.
While bandwidth is still very important to the Intel Pentium 4, it's not as important as it once
was in the i845PE days of single channel memory controllers. Thanks to the i865PE/i875P's
dual channel memory controller things are much brighter. On average, a system with the
memory running at 400 MHz (5:4 memory divider enabled) with aggressive memory timings
will perform 2-3% faster than the system using high speed memory with loose timings.
While that may not seem like a lot to most people, it can make a world of a difference to the
enthusiast, especially if you're gunning for that high score in a clan match where every FPS
counts. Many enthusiasts I know, tend to favour slower memory which allows them to run
aggressive timings for just this reason.