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System Clocking (aka Front Side Bus speeds):
All of the major components on the motherboard - Processor, Memory, PCI, and AGP devices are connected via signal busses that require some form of external clock signal to set their operating speed and to synchronize them during operation. Unfortunately, they don't all operate at the same speed. Processors and Memory run at higher speeds than AGP (video cards) and PCI (io) devices. Also, processor and memory speeds can vary dramatically from processor make/model to another, while AGP and PCI speeds are typically fixed. Processor external clock speeds are set by the Manufacturer (Intel, AMD) and vary by processor model. Common processor external clock speeds are 100 MHz, 133 MHz, 166 MHz, 183 MHz, and 200 MHz. The AGP bus speed is fixed at 66.66 MHz, and the PCI bus speed is fixed at 33.33 MHz by industry standards.
When building a PC or attempting overclocking, all these speeds need to be kept in mind and controlled and matched appropriately through jumpers on motherboards (older boards) or through software settings in the BIOS (current boards). When building a PC or overclocking one, the external processor speed can be set using the BIOS setting screens. Some motherboards allow only specific settings mentioned above, while others allow settings in 1 MHz increments. These fine-grained settings allow you to overclock by changing the clock speed in small increments until you hit the highest speed possible without experiencing instability. Note that machines from Compaq, Dell, HP, etc seldom provide users with the ability to make these adjustments.
PCI and AGP bus speeds can often be locked at their spec speeds or allowed to rise in proportion to the processor FSB speed depending on the settings in the BIOS. For example if you allow the PCI and AGP speeds to increase proportionately as you raise the FSB speed when trying to overclock the CPU, you may wind up overclocking the video card or PCI devices beyond their capability, thereby causing them to fail even though the processor is still fine. Under this condition, youd probably go into the BIOS and lock the AGP & PCI speeds at 66 MHz and 33 MHz respectively to keep them in spec. If on the other hand, your video card and PCI devices can tolerate the higher clock speed without giving errors or causing instability, then let the speeds adjust and be happy that you have great components!
The bottom line is the main system clock setting, which is usually labeled Processor Speed, FSB Speed, etc. in the BIOS is the primary adjustment and other components take their settings from this one. Clock dividers are used to set these other speeds as a ratio of the FSB speed.
To confuse matters a bit, internal processor speeds are quite different from the external clock speeds you set in the BIOS. They are often advertised or reported in terms of internal bus and clock speeds and in reality, they are a multiple of the external clock speed. For example the Pentium 4 is considered a quad-pumped processor, so internally (inside the processor chip, that is) the processor bus runs at 4 times the external clock rate. A Pentium 4 with an external clock speed of 200 MHz is really operating at an internal FSB speed of 800 MHz. The effective processor speed is model dependent and based on an overall multiplier. For example a model 2.4B Pentium 4 chip (rated at 2.4 GHz or 2400 MHz) uses a 133 MHz external clock, runs its internal FSB at 133x4 = 533 MHz, and uses an internal core multiplier of 18 thereby achieving a rated speed of 2400 MHz (133.3 MHz x 18 = 2399 MHz).
Memory speeds are also related directly to the Processor external (and internal) clock speeds. Current technology DDR (Dual Data Rate) memory modules are rated by their speed and bandwidth. Speed is how fast they run, bandwidth is how many megabytes per second can be read/written to the module. Since DDR moves data on both the rising and falling edge of the clock, speed ratings are expressed in terms of the external clock frequency times 2. So if you are running a 200 MHz system clock (FSB speed), you need memory rated at 400 MHz or DDR400 memory. These same modules are labeled and sold based on their bandwidth. Since memory busses are 8 bytes wide, memory bandwidth = clock speed x 2 x bus width (8). Therefore, a DDR400 memory module is generally labeled a PC3200 memory module (200 MHz x 2 x 8 Bytes = 3200 Megabytes/second).
Common Memory Modules and Speeds:
Thanks to Bobaroo330 for providing this data regarding common memory modules
PC1600 DDR xxx = (100 MHz Operating Speed) x (2x Rising & Falling) x (64-bit Bus) / (8 bits per byte) = 1600 MB/s available bandwidth.
PC2100 DDR xxx = (133 MHz Operating Speed) x (2x Rising & Falling) x (64-bit Bus) / (8 bits per byte) = 2128 MB/s available bandwidth.
PC2400 DDR xxx = (150 MHz Operating Speed) x (2x Rising & Falling) x (64-bit Bus) / (8 bits per byte) = 2400 MB/s available bandwidth.
PC2700 DDR 333 = (166 MHz Operating Speed) x (2x Rising & Falling) x (64-bit Bus) / (8 bits per byte) = 2656 MB/s available bandwidth.
PC3000 DDR xxx = (183 MHz Operating Speed) x (2x Rising & Falling) x (64-bit Bus) / (8 bits per byte) = 2928 MB/s available bandwidth.
PC3200 DDR 400 = (200 MHz Operating Speed) x (2x Rising & Falling) x (64-bit Bus) / (8 bits per byte) = 3200 MB/s available bandwidth.
PC3500 DDR 433 = (215 MHz Operating Speed) x (2x Rising & Falling) x (64-bit Bus) / (8 bits per byte) = 3440 MB/s available bandwidth.
PC3700 DDR 466 =(233 MHz Operating Speed) x (2x Rising & Falling) x (64-bit Bus) / (8 bits per byte) = 3728 MB/s available bandwidth.
PC4000 DDR 500 = (250 MHz Operating Speed) x (2x Rising & Falling) x (64-bit Bus) / (8 bits per byte) = 4000 MB/s available bandwidth
Real World Example:
So you want to build a new overclocking rig. Perhaps something that uses the new C revision P4 processor. What components do you need to get it off the ground? Chances are you will be looking at the 2.4C processor as its a really great CPU for the money and overclocks very well. So lets start there.
The P4 2.4C is a quad-pumped processor designed to run at an external clock speed of 200 MHz. Internally it runs its memory bus at 4 x 200 or 800 MHz. Internally the core uses a multiplier of 12, so the processor is designed to run at 12 x 200 MHz or 2400 MHz.
First off, you need a motherboard capable of 200 MHz clock or higher. A nice board using the Intel Canterwood or Springdale chipset will do. I wont suggest any particular board ASUS, MSI, ABIT, EPOX, and others all make good ones.
Next you need memory - memory that runs at 200 MHz or faster. DDR memory runs at 2 x the external clock speed of the processor, so you need at least DDR400 memory. It is also called PC3200 memory (remember 200 x 2 x 8 = 3200 from our discussion above). This of course is if you plan to run at the stock speed of 200 MHz. However, if you plan to overclock the system, you will be raising the FSB clock above 200 so you should plan on using memory that is rated as fast or faster than your expected overclock speed. PC3500 memory is rated for clock speeds up to 218 MHz, while PC3700 is rated to run at up to 230 MHz. Of course depending on the quality of the memory you buy, and the memory timing and voltage tweaks you make, you may be able to run them higher than their rated speed too. Memory timing and voltage are subjects for another FAQ.
When overclocking, remember that for every 1 MHz you raise the FSB clock, you are increasing the core processor speed by its multiplier (12 in our P4 2.4C example). So if you start out at 200 MHz, then bump the P4 2.4C up to say 210 MHz, you are raising the CPU from 2.4 GHz up to 2.52 GHz. If you are lucky and can get your clock up to say 255 MHz, your processor will be running at 3.06 GHz! Remember, though, your memory will also be clocked at 255 MHz. So technically you need memory rated slightly above PC4000 to get there (255 x 2 x 8 = 4080). Some very high quality PC3700 memory can actually run that fast with careful treatment (aka burn-in) and timing experimentation. PC4000 rated and faster memory modules are just starting to show up on the market, though most contain lower rated speed chips that have tested to run stable at the higher speed.