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.

Figure 3
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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.

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