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Computer Hardware:
     • Tools, Static & Cleaning
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     • RAM
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     • Overclocking

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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

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:

Name	I/O Bus clock	Data rate	Transfer rate	Module name
DDR-200	100 MHz	200 MT/s	1600 MB/s	PC-1600
DDR-266	133 MHz	266 MT/s	2100 MB/s	PC-2100
DDR-333	166 MHz	333 MT/s	2700 MB/s	PC-2700
DDR-400	200 MHz	400 MT/s	3200 MB/s	PC-3200

DDR2:

Name	I/O Bus clock	Data rate	Transfer rate	Module name
DDR2-400	200 MHz	400 MT/s	3200 MB/s	PC2-3200
DDR2-533	266 MHz	533 MT/s	4264 MB/s	PC2-4200
DDR2-667	333 MHz	667 MT/s	5336 MB/s	PC2-5300
DDR2-800	400 MHz	800 MT/s	6400 MB/s	PC2-6400
DDR2-1066	533 MHz	1066 MT/s	8528 MB/s	PC2-8500

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:

Name	I/O Bus clock	Data rate	Transfer rate	Module name
DDR3-800	400 MHz	800 MT/s	6400 MB/s	PC3-6400
DDR3-1066	533 MHz	1066 MT/s	8500 MB/s	PC3-8500
DDR3-1333	666 MHz	1333 MT/s	10600 MB/s	PC3-10600
DDR3-1600	800 MHz	1600 MT/s	12800 MB/s	PC3-12800
DDR3-1866	933 MHz	1866 MT/s	14900 MB/s	PC3-14900
DDR3-2133	1066 MHz	2133 MT/s	17000 MB/s	PC3-17000

The transfer rate in MB/s is approximate.

DDR4:

Name	I/O Bus clock	Data rate	Transfer rate	Module name
DDR4-1600J/K/L	800 MHz	1600 MT/s	12800 MB/s	PC4-1600
DDR4-1866L/M/N	933 MHz	1866 MT/s	14933 MB/s	PC4-1866
DDR4-2133N/P/R	1066 MHz	2133 MT/s	17066 MB/s	PC4-2133
DDR4-2400P/R/U	1200 MHz	2400 MT/s	19200 MB/s	PC4-2400

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.

MENU (RAM):

1. RAM	2. Types of RAM	3. Memory Upgrade
4. Memory Speeds	5. Memory Timings