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Understanding System Memory and CPU speeds: A layman's guide tothe Front Side Bus (FSB)

by Lee Penrod 

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1. Introduction2. What's a bus?

o Bus Types3. The System Clock

o Why Isn't My Processor the Right Speed?4. Double Pumping, Quad Pumping, and DDR

o Double Pumping and DDRo Quad Pumping, the P4, and Rambuso Dual Channel Technology

5. The System Clock, the Front Side Bus, and Overclockingo What is Underclocking?o What is Overclocking?

6. Summary and Conclusion7. Additional Info and Updates

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Introduction

Shopping for a new processor and motherboard can be confusing. Some of 

the most important terms and concepts regarding system performance are

also the hardest to understand. Terms like: System Clock, Quad Pumping,

Double Pumping, DDR, FSB, SDRAM, Dual Channel, and QDR make

many new builders cringe. In this article I will walk you through some of 

these important concepts so that you can make a more informed decision

when upgrading your current system or building a new one.

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Part One: What's a bus

To get anything done with a computer you have to get the information youinput to the CPU and then to any attached devices such as cards, displays,

and other output devices. Inside the computer itself, this information travels

in the form of signals over what is known as a bus. You can think of a bus

as a road and the signals as cars. A wide road (bus) can support more cars

(signals), and a smaller road (bus) supports less. The cars (signals) on the

road (bus) have a speed limit (the bus speed). Although a speed limit can

be broken (an overclocked bus) doing so can have adverse effects on

the cars (signals).

Going along with this analogy: A computer is like a small city. You do not

have just one road, but instead you have several different roads with

different names and speeds.

There are three main buses in most computers:

)1 PCI Bus- The PCI bus connects your expansion cards and drives to your 

processor and other sub systems. On most systems the bus speed of the

PCI bus is 33MHz. If you go higher than that, then cards, drives, and other 

devices can have problems. The exception to this is found in servers. In

some servers you have a special 64-bit (extra wide) 66MHz PCI slots that

can accept special high-speed cards. Think of this as a double sized

passing lane on a major road that allows higher cars to go through. For 

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information about PCI Express please see the PCI Express

Guide.

2) AGP Bus- The AGP bus connects your video card directly to

your memory and processor. It is very high speed compared to standardPCI and has a standard speed of 66MHz. Only one device can be hooked

to the AGP bus as it only supports one video card so the speed is better 

compared to the PCI bus, which has many devices on it at once.

3) Front Side Bus (FSB) - The Front Side Bus is the most important bus to

consider when you are talking about the performance of a computer. The

FSB connects the processor (CPU) in your computer to the system

memory. The faster the FSB is, the faster you can get data to your processor. The faster you get data to the processor, the faster your 

processor can do work on it. The speed of the front side bus depends on

the processor and motherboard chipset you are using as well as the

system clock. Read on for more information about the Front Side Bus later 

in this article.

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Part Two: The System Clock

The system clock is the actual speed of your FSB with out any

enhancements (such as double pumping, or quad pumping) on it. The

system clock is also sometimes just called the bus speed. From the system

clock your PCI bus speed is determined via the use of a divider and then

your AGP bus speed is determined by multiplying the PCI bus speed by 2.

The dividers allow you to have a faster speed on your PCI and AGP bus

while still allowing for the faster operation of the main FSB. In most

systems PCI dividers are set automatically and you can not alter them,however, in newer motherboards geared towards computer enthusiasts --

PCI dividers can sometimes be manually set in order to allow you to raise

the System clock higher then its normal rate. The three most common

dividers built in to motherboards are: 1/5 (used on a 166MHz system

clock), 1/4 (used on a 133MHz system clock), and 1/3 used on a 100MHz

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system clock. A 1/6 divider is sometimes available for overclocking and

future support.

Example: If you have a 166MHz system clock and you set a 1/5 divider in

your motherboard's bios then your PCI bus speed would be 166/5 =~33MHz and your AGP bus speed would be ~33*2 = 66MHz.

Why Isn't My Processor the Right Speed?

An often-misunderstood property of the system clock is its effect on

processor speed. You see, a thing called a "CPU Multiplier" determines the

speed of a processor in MHz. If you take the multiplier of the processor and

multiply it by the system clock speed you get the speed of your processor.Your CPU has its multiplier hard wired in to the chip, and this *normally*

cannot be changed. Your system clock is another matter. It can be set on

your motherboard by using BIOS or a set of switches on the board itself.

This is very important. Most motherboards do not automatically set the

system clock for you when you install a new processor. We often get

reports from new system builders saying that they received the wrong

speed processor when in actuality; the new builder forgot to set the system

clock to the right speed for their processor. For a list of standard system

clock speeds please see part IV of this article.

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Part Three: Double Pumping, Quad Pumping, and DDR

Earlier in this article I compared a bus to a road, and the bus speed to a

speed limit. This isn't entirely correct because unlike a standard speed limit

in real life you are not talking about miles per hour or kilometers per hour,

you are talking about MHz or millions of clock cycles a second. A cycle iseasily represented by a sine wave.

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A clock cycle is how long it takes to go from 1 to 0 (from the peek of the

wave to the bottom of it). So how does that affect your system?

Your processor and memory all can be affected by various enhancements

to speed. To understand this you first must look back at the old way of doing things:

Traditional parts with out any enhancements can only send/receive a signal

once a cycle. A good example of this as far as memory is concerned is

standard SDRAM such as PC133. The traditional approach has been

around for a long time and it matched well to the un-enhanced buses like

you find on processors such as the Intel Pentium / II / III or AMD K6 series.

For these types of systems standard SDRAM made a lot of sense because

the memory and the processor both were able to transmit at the same timeand the bus speed could be synchronized.

Enter the Present - The Double Pumped bus w/DDR

As time progressed processor and memory manufacturers found ways of 

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improving the number of access times per cycle. With the release of the

AMD Athlon Processor the world saw the concept of a "Double Pumped

FSB". With a double pumped bus the processor could send and receive a

signal from the memory sub system twice a cycle. This was a great idea;

however this meant that standard SDRAM memory no longer lined up.Standard SDRAM memory could only send/receive once a cycle. What was

created is what is known as a bottleneck -- or an obstacle to maximum

performance. Removing the bottleneck required a new and faster type of 

memory and the memory that filled this gap was DDR memory or Double

Data Rate memory.

DDR memory can transmit twice a cycle just like the double pumped bus

on an Athlon processor, which means that using it with an Athlon processor 

creates an optimized situation just like you had before with the traditional

system.

Quad Pumping, the P4, and Rambus Memory

When the Pentium 4 came out they introduced a new catch phrase to the

market: "Quad Pumped" (also known as QDR). The Pentium 4 FSB can

handle 4 signals a cycle. When the P4 was first released motherboards

only supported traditional SDRAM accessing once a cycle. As you canimagine, such a combination of single access a cycle memory and a four 

access a cycle processor gives you a massive bottleneck and greatly

reduces the potential performance of the processor. Intel was very quick to

adopt the fastest memory technology available: Rambus RIMM memory.

Although Rambus memory only accesses twice a cycle like DDR, Rambus

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memory comes in much higher speeds than DDR. The base speed of the

popular Rambus memory at the time was a double pumped 400MHz

(800MHz). Although the memory does not handle 4 signals a cycle it does

work very well since 400MHz is also the enhanced speed of the standard

P4 FSB (4 accesses a cycle x 100MHz). The fact that the memory doestwo cycles for every cycle that the bus helps makes up for the two signals a

cycle difference. It is not as good as true QDR would be but the technology

is widely available unlike QDR memory.

Dual channel Technology

Lets say you have a car that can hold 4 people but you've got 8 people to

transport across town. What do you do? Well you could take one load of 

people across town, and then go back and get another load of people (a

standard memory system) or if money was no object you could simply buy

another car and have the other half of the people follow you across town in

the other car (a Dual channel memory bus). With dual channel technology

you use two memory modules at once to further enhance performance.

This essentially doubles the number of signals a second you can handle

and doubles your bandwidth (volume of information that can be transferred

at once). Point Blank: Dual channel technology increases memory

performance but it costs more money because you have to buy memorymodules in pairs. Dual channel technology also costs more because the

motherboard has to support it in the chipset and a chipset that supports

dual channel technology costs more due to the higher complexity of the

memory bus. Higher motherboard cost + higher memory cost = higher 

overall system cost.

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Dual channel Rambus has been around for a long time but Dual Channel

DDR technology is just now hitting the scene in mass. Since DDR memory

is cheaper than Rambus memory and more widely available, Dual Channel

DDR should be a good option for the P4 processor but the problem is thatas of this writing no consumer level chipset supporting Dual Channel DDR

exists for the Pentium 4. (Dual channel DDR is widely available for the

AMD Athlon XP series of processors via the nforce2 chipset by nVidia,)

When Dual Channel DDR solutions emerge for the Pentium 4 they will

quickly become the best price vs performance ratio on the P4 side.

Update [Sept. 2007]: At this point, pretty much any motherboard you buy

is going to have either Dual Channel DDR, DDR2, or DDR3. Most but notall Dual Channel supporting motherboards will operate in a slower single

channel mode if using one stick of ram, but work best with ram in pairs. On

these boards its best not to try the odd 3 stick of ram configuration though

as some motherboards will have problems operating in this fasion or plain

won't POST.

It's worth noting that at this point, RAMBUS is no longer widely available

nor is it supported by any curent motherboard chipset. Those with Rambus

based systems are strongly encouraged to upgrade to systems utilizing

DDR2 or DDR3. As far as which to go with, [DDR2 vs DDR3] right now

DDR2 is much much more common than DDR3 and cheaper. DDR3 will

probably start becoming common sometime in 2008 when more

motherboard chipsets come out with full support for it.

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Part IV: The System Clock, the Front Side Bus, and OverclockingNow that you understand the performance enhancements in the FSB of a

processor it is important that you understand how to figure out the

processor multiplier and the proper system clock. When you go to purchase

a processor you are told in the ad / description for the processor what FSB

it has. To determine the proper system clock for the processor simply

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divide the FSB by the performance enhancer (2 for the double pumped bus

on AMD Athlon XP/Thunderbird/Duron processors or 4 for the quad

pumped bus on the Intel Pentium 4).

If your processor has a ... FSB then the system clock speed should be:

66MHz (Various Celeron and older): 66MHz clock

100MHz (Pentium II / Pentium III / K6): 100MHz clock

133MHz (Pentium II / Pentium III / K6): 133MHz clock

200MHz (Athlon, Duron, Thunderbird): 100MHz clock

266MHz (Thunderbird, XP): 133MHz clock

333MHz (XP): 166MHz clock

400MHz (Pentium 4): 100MHz clock400MHz (AMD XP): 200MHz clock

533MHz (Pentium 4): 133MHz clock

800MHz (Pentium 4): 200MHz clock

800MHz (AMD64): 200MHz clock

1066MHz (Pentium 4/LGA775): 266MHz clock

1333MHz (Pentium 4/LGA775): 333MHz clock

Now, remember what I said about the processor multiplier earlier in this

article? (Processor speed = processor multiplier x system clock)

If you do not know the multiplier for your processor simply take the proper 

system clock speed for it and divide that into the rated processor speed

and then round the dividend to the nearest .5. Examples: The Pentium4

3.06GHz processor has a FSB of 533MHz. Its system clock is 533 / 4 =

~133. The multiplier is 3,060 / 133 = ~23.

The AMD Athlon XP2700+ has a main clock speed of 2.17GHz and a FSB

of 333MHz. Its system clock is 333 / 2 = ~166MHz. The multiplier is 2,170 /

166 = ~13

Underclocking and Overclocking

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Underclocking or the act of running a processor or device at under its rated

speed is accomplished by simply running the device at a lower bus speed

(or if possible a lower multiplier). Most underclocking is done by accident by

new system builders. Most motherboards come defaulted to the lowestsystem clock speed that the motherboard supports. Since the system clock

speed is usually not automatically set by the processor you put into the

board, this means that if you put a processor with a higher bus speed than

the lowest one the board supports, you are underclocking the processor.

Example: Lets say I buy an AMD Athlon XP2400+ processor with a FSB of 

266MHz. (XP2400 has a clock speed of ~2000MHz). If I do not set the

system clock to 133MHz then I get the processors multiplier (15) times thedefault bus speed (100). This gives me the wrong processor speed

(1500MHz) and the motherboard will either tell me I have a 1,500MHz

thunderbird processor, or a XP1700+ processor. Changing the system

clock in bios to 133 will make the motherboard detect the processor 

properly and give me the right processor speed.

Overclocking or the act of running a processor or device higher then its

rated speed is accomplished by increasing the system clock (or if possible

the multiplier). The biggest issue with overclocking is keeping your PCI bus

close to its speed limit (33MHz). Since a divider of your system clock

determines your PCI bus, you not only affect your processor when you

increase it, but also other parts of the system. Devices attached to the PCI

bus are much less over clocking friendly then either memory or a CPU.

When you overclock a processor using the system clock your processor 

speed is determined in the same way as one would for finding normal clock

speed: processor multiplier x system clock = processor speed.

Example: An Athlon XP1800+ (1.53GHz) processor with a FSB of 266 and

its system clock overclocked to 145MHz would give you a speed of 

~1.67GHz and cause the board to detect the processor as a XP2000+.

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Part V: Summary and Conclusion

When you are choosing and installing components in a system you should

now know how to properly set the system clock in order to achieve the full

potential of the system. You should also now understand more about

matching memory with a processor. Go with a motherboard/system that

complements your CPU and provides it with memory support that well

matches the FSB potential. Slower memory technologies such as PC133

SDRAM do not work well with current processors such as the Pentium 4

and Athlon XP. Although synchronizing the memory speed and the FSB

speed is best it is OK to use memory that is faster then the FSB of your 

processor provided that the motherboard supports it.

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Updates

Since this guide was written a few new technologies have emerged such as

DDR2 memory and 64 bit processors for Desktop. Here are a few

additional pieces of information about these technologies:

DDR2 Short and Sweet

There are a few major things you need to know about DDR2 when building

a system:

Basic Functionality: DDR2 memory has a different approach to design at

the chip level then DDR. The simplest way to understand how it works

would be to think that at the low level it had two chips of half the stated

memory speed working in tandem together to achieve the full speed stated.

So for DDR2 400 it would be something like 2 chips of DDR200 working

together to achieve the full 400 speed. Notice that I say "chips" not sticks of 

memory. All this happens on 1 stick of memory.

The overall effect of this trickery is that manufacturers can scale up the

speed of the memory beyond the limits of DDR, with only taking a small hit

to the timing of the memory ( how long it takes for the memory to respond

back to a request ).

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This means that it's possible, and expected to see memory speeds of 

533MHz or higher for DDR2. In fact, the current concensus is that if you

want to build a system, and have a choice between a motherboard with

DDR1 and DDR2 then to see a benefit to DDR2 you need to at least getone speed grade higher than that of the max normal speed of DDR1. (

400MHz ). This is because the timings ( latency ) of DDR2 are worse than

DDR. Essentially in most situation DDR400 ( especially low latency

DDR400) is faster than DDR2 400. However, when you get to DDR2 533

the speed boost makes up for the slower timings.

As far as matching FSB to DDR2 speed my recommendations are to skip

DDR2 400 and opt for going with the following:

800MHz FSB = DDR2 533MHz ( Ideal ) or DDR2 400MHz ( Matched but

Slow. ) 1066MHz FSB = DDR2 667 ( Good ) or DDR2 533MHz ( Matched )

Generally you want to keep the system clock of your memory matching with

the root clock of your memory or one step above. So the system clock on a

800MHz FSB P4 is 200 (quad pumped) so that matches DDR2 400

(essentially 200 unimproved) or is good with 1 step up DDR2 533MHz

(essentially 266 unimproved). Note however that if you only had a 800MHz

FSB processor then DDR2 667 really probably isn't going to help much.

Once you pass the 1 step above mark on the memory you have diminishing

returns unless you can get to double (DDR2 800MHz).

Compatability: Generally a motherboard is only going to accept DDR1 or 

DDR2 not both. The slots are physically different and have a different

number of pins, however people have been known to force memory into the

wrong slots ( And that ends in horrible results! ). Be careful when installing

it and make sure the motherboard takes that kind of memory before

attempting.

At the time of this writing only motherboards for Pentium4 or Xeon

accepted DDR2. AMD Socket 754 and Socket 939 motherboards can't

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accept DDR2 due to the integrated memory controller in the CPU. AMD is

making a line of CPUs that can work with DDR2. They will use new

motherboards and have a new socket called M2.

Additional Notes on DDR2: 1) DDR2 is not QDR like I mentioned earlier,the technology is different. 2) DDR2 does give you definate benefits and it

is recommended. 3) At the time of this writing ALL motherboards that used

DDR2 were Dual Channel Ready. 4) It is not uncommon to hear of 

problems from people trying to use 3 sticks of DDR2. This stems somewhat

from what I mentioned in #3. I recommend either using 1,2 or 4 sticks of 

DDR2. ( more is ok if you are doing a server but add them in pairs, don't

use a odd number of sticks if you can avoid it ).

A few notes on 64 bit

1) You can use 32 bit operating systems with a 64 bit AMD CPU or EMT64Enabled Pentium 4/Xeon.

2) If you plan to run a 64 bit OS with your 64 bit processor, and are actuallygoing to use 64 bit applications ( not just 32 bit applications in 64 bit OS )then it is recommended that you double the amount of memory you thinkyou need. So for example if you think you would be comfortable with

512mb of memory, use 1GB. If you wanted 1 GB use 2 GB. [Generally 2GBis fine for most anything].

3) The most common issue with 64 bit CPUs and 64 bit operating systemsis that you need all new drivers for your hardware. Often the driver CDsthat come with hardware lack the 64 bit driver and you have to downloadnew ones from the web.

Specification standards

[edit] Chips and modules

For use in computers, DDR2 SDRAM is supplied in DIMMs with 240 pins and a single locatingnotch. Laptop DDR2 SO-DIMMs have 200 pins and often come identified by an additional S in

their designation. DIMMs are identified by their peak transfer capacity (often called bandwidth).

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Standard

name

Memory

clock 

(MHz)

Cycle

time

(ns)

I/O bus

clock 

(MHz)

Data

rate

(MT/s)

Module

name

Peak 

transfer rate

(MB/s)

Timings[2][3]

(CL-tRCD-tRP)

DDR2-400B

DDR2-400C100 10 200 400 PC2-3200 3200

3-3-3

4-4-4DDR2-533BDDR2-533C

133 71/2 266 533 PC2-4200* 4266

3-3-34-4-4

DDR2-667C

DDR2-667D166 6 333 667 PC2-5300* 5333

4-4-4

5-5-5

DDR2-800CDDR2-800D

DDR2-800E

200 5 400 800 PC2-6400 64004-4-45-5-5

6-6-6

DDR2-1066E

DDR2-1066F

266 33/4 533 1066 PC2-8500* 8533

6-6-6

7-7-7

* Some manufacturers label their DDR2 modules as PC2-4300, PC2-5400 or PC2-8600 instead 

of the respective names suggested by JEDEC. At least one manufacturer has reported thisreflects successful testing at a higher-than standard data rate

[4]whilst others simply round up for 

the name.

Note: DDR2-xxx denotes data transfer rate, and describes raw DDR chips, whereas PC2-xxxxdenotes theoretical bandwidth (with the last two digits truncated), and is used to describe

assembled DIMMs. Bandwidth is calculated by taking transfers per second and multiplying by

eight. This is because DDR2 memory modules transfer data on a bus that is 64 data bits wide,and since a byte comprises 8 bits, this equates to 8 bytes of data per transfer.

In addition to bandwidth and capacity variants, modules can

1. Optionally implement ECC, which is an extra data byte lane used for correcting minor 

errors and detecting major errors for better reliability. Modules with ECC are identified by an additional ECC in their designation. PC2-4200 ECC is a PC2-4200 module with

ECC.2. Be "registered", which improves signal integrity (and hence potentially clock rates and

 physical slot capacity) by electrically buffering the signals at a cost of an extra clock of 

increased latency. Those modules are identified by an additional R in their designation,whereas non-registered (a.k.a. "unbuffered") RAM may be identified by an additional U

in the designation. PC2-4200R is a registered PC2-4200 module, PC2-4200R ECC is the

same module but with additional ECC.3. Be fully buffered modules, which are designated by F or FB and do not have the same

notch position as other classes. Fully buffered modules cannot be used with motherboardsthat are made for registered modules, and the different notch position physically prevents

their insertion.

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 Note: registered and un-buffered SDRAM generally cannot be mixed on the same channel.

 Note that the highest-rated DDR2 modules in 2009 operate at 533 MHz (1066 MT/s), comparedto the highest-rated DDR modules operating at 200 MHz (400 MT/s). At the same time, the CAS

latency of 11.2 ns = 6 / (Bus clock rate) for the best PC2-8500 modules is comparable to that of 

10 ns = 4 / (Bus clock rate) for the best PC-3200 modules.

EXAMPLE:

On a P4 system, 533MHz FSB doesn't actually run at 533MHz, it runs at

133MHz. The bus is "quad-pumped" so 533MHz is more theoretical speed

than actual.

When choosing RAM, you have to think in terms of bandwidth & bus speeds,

not just bus speed alone. Remember that DDR & DDR2 RAM is "double-

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pumped", so when you see the RAM listed as DDR266 (for example), the

actual bus speed is 1/2 of that, or 133MHz. And to calculate bandwidth, you

simply multiply the theoretical  bus speed by 8.

For CPUs:

400FSB (@100MHz) x 8 = 3200MB/sec bandwidth533FSB (@133MHz) x 8 = 4267MB/sec

800FSB (@200MHz) x 8 = 6400MB/sec

1066FSB (@266MHz) x 8 = 8533MB/sec

For RAM:

DDR266 (@133MHz) x 8 = 2133MB/sec (PC2100)

DDR333 (@166MHz) x 8 = 2667MB/sec (PC2700)

DDR400 (@200MHz) x 8 = 3200MB/sec (PC3200)

DDR533 (@266MHz) x 8 = 4267MB/sec (PC4200 or PC4300)DDR667 (@333MHz) x 8 = 5333MB/sec (PC5300)

DDR800 (@400MHz) x 8 = 6400MB/sec (PC6400)

So in the case of this system with a 533FSB CPU with 4267MB/secbandwidth, you'd need 2 x PC2100 running @ 133MHz in dual channel mode

to have a 1:1 bandwidth ratio. The P4 tends to perform better if the RAMhas a slightly higher bandwidth than the CPU, so you'd be better off getting

2 x PC2700 running at 166MHz in dual channel mode.