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RAM speed (FSB & Memory bus)


The diagram below shows the CPU connected to the Northbridge Chipset via the front-side-bus (FSB), and the memory connected to the Northbridge Chipset via the memory bus.

The speed of memory that you require depends on the front-side-bus (FSB) speed. If for example your motherboard is designed to work with DDR (Double-data-rate) memory and the FSB is 100MHz then you would require DDR-200 memory as DDR memory transfers data twice per clock cycle.

RAM - Diagram of CPU and Northbridge chipset connected to Memory

RAM - Diagram of CPU and Northbridge chipset connected to Memory.


Be aware that manufacturers usually quote the FSB as much higher (usually called Rated FSB) because an Intel FSB is quad-pumped and an AMD FSB is double-pumped meaning that data is transferred four times every clock cycle (Intel), or twice every clock cycle (AMD). In the diagram above an Intel rated FSB would be 400MHz (100MHz x4) and an AMD rated FSB would be 200MHz (100MHz x2).

The front-side-bus (FSB) and memory bus usually have to operate at the same frequency (1:1) so if the FSB is 100MHz then the memory bus also has to operate at 100MHz as in the diagram above. This is not always the case and you can have an unbalanced FSB and memory bus with a ratio such as 2:3 (FSB:RAM).

A DDR memory module transfers data at 64-bits at a time which equates to 8-bytes at a time. Therefore we can work out the speed of the memory. DDR-200 runs at 200MHz which works out at 200 x (64/8) = 1600MB/s (megabytes per second). That is why a DDR-200 memory module is called PC1600.

Below are three tables showing the different speeds of DDR, DDR2, and DDR3 memory:

DDR:
NameI/O Bus clockData rateTransfer rateModule name
DDR-200100 MHz200 MT/s1600 MB/sPC-1600
DDR-266133 MHz266 MT/s2100 MB/sPC-2100
DDR-333166 MHz333 MT/s2700 MB/sPC-2700
DDR-400200 MHz400 MT/s3200 MB/sPC-3200

DDR2:
NameI/O Bus clockData rateTransfer rateModule name
DDR2-400200 MHz400 MT/s3200 MB/sPC2-3200
DDR2-533266 MHz533 MT/s4264 MB/sPC2-4200
DDR2-667333 MHz667 MT/s5336 MB/sPC2-5300
DDR2-800400 MHz800 MT/s6400 MB/sPC2-6400
DDR2-1066533 MHz1066 MT/s8528 MB/sPC2-8500

An example for a computer which supports DDR2 memory with an unbalanced 2:3 ratio (FSB:RAM):
If the FSB is 266 MHz then the memory bus will be 400 MHZ ((266MHz / 2) x 3)) so you would require DDR2-800 (400MHz x 2) which is PC2-6400 (800MHz x 8) which runs at a speed of 6400 MB/s.

DDR3:
NameI/O Bus clockData rateTransfer rateModule name
DDR3-800400 MHz800 MT/s6400 MB/sPC3-6400
DDR3-1066533 MHz1066 MT/s8500 MB/sPC3-8500
DDR3-1333666 MHz1333 MT/s10600 MB/sPC3-10600
DDR3-1600800 MHz1600 MT/s12800 MB/sPC3-12800
DDR3-1866933 MHz1866 MT/s14900 MB/sPC3-14900
DDR3-21331066 MHz2133 MT/s17000 MB/sPC3-17000
The transfer rate in MB/s is approximate.

DDR4:
NameI/O Bus clockData rateTransfer rateModule name
DDR4-1600J/K/L800 MHz1600 MT/s12800 MB/sPC4-1600
DDR4-1866L/M/N933 MHz1866 MT/s14933 MB/sPC4-1866
DDR4-2133N/P/R1066 MHz2133 MT/s17066 MB/sPC4-2133
DDR4-2400P/R/U1200 MHz2400 MT/s19200 MB/sPC4-2400
The transfer rate in MB/s is approximate.

Sometimes it is possible to add faster memory depending on what the motherboard supports (see your motherboard manual) but there is little point in adding more expensive memory for little or no advantage.

If you have different memory modules which are rated at different speeds then the memory will run at the speed of the slowest memory module.

Some motherboards support Dual-channel memory and you will see that the memory slots on the motherboard are colour coded. It is recommended that you match each coloured memory slot with the same type and speed of memory module. In this case the motherboard can combine two 64-bit memory modules on each channel so that it can transfer data 128-bits at a time.

It is possible to increase the FSB speed on some motherboards to allow you to overclock the CPU so that you can speed the computer up but the memory bus will need to operate at the same speed as the FSB to keep the system balanced and you may need faster memory modules. There are more details on this on the Overclocking page.

More recent computers now have the memory controller integrated into the CPU instead of the Northbridge chipset. You will need to look at your motherboard manual to find the type of memory required. See the Chipset page.