CPU bus speed and overall speed

[Wizzerad]

Hi,

the CPU bus speed is at 100MHz, since many years, both with AMD and INTEL. Take Intel for simplicity. Since data is Quad-pumped with Intel (therefore 64*4=256 bit are transferred at each bus cycle), then data is externally pumped into the CPU at 100MHz*4=400MHz.

I know, the CPU is doing several internal cycles, ruled by the multiplier, which allow working faster on the data that is ALREADY at the CPU. Modern CPU overall clock speed therefore ranges between 1 to several GHz. But still, the clock at which information is transferred from external source (s.a. RAM) to the CPU is 400MHz. Or?

If what I said above is true, why the hell do I need a RAM that runs at 1333MHz or higher as you find on the market?
In other words, the same question would be: how can RAM run at 1333MHz and the bus speed is at 100MHz?

This must be an easy question, but I can't find the answer.

[Carryspider]

The question is determined by where the memory controller is located in.
Several decades ago,the memory controller is intergrated in the North Bridge.The North Bridge is a chip which is connected to the memory,CPU and the South Bridge.The bus connected CPU and North Bridge is called Front-Side Bus(FSB).The FSB detemine how data can be pumped into CPU from memory.For example,Core 2 Duo E8400 can work
in a 1333MHz FSB speed.
But since Intel used PCH chipset for the newer CPU,the memory controller was integrated into CPU,and the North Bridge disappeared from chipset.That means,the memory
speed was detemined by CPU in these kinds of CPU.
In short,the speed of memory is detemined by CPU type.Before buying a new memory,you need to know your CPU can support what speed of memory.

[Calbris]