How fast is SDRAM?

How fast is SDRAM?

Before SDRAM, we would typically describe the speed of memory in terms of nanoseconds. A 70 nanosecond part would be a "7," a 60 nanosecond part a "6," etc. The lower the number, the faster the memory.
With SDRAM, however, this is somewhat inaccurate. The reason is that the speed of SDRAM is dependent upon the speed of the front side bus (FSB) in the motherboard. Because of this, you will often see SDRAM rated as 66MHz, 100MHz, 133MHz, etc.
100MHz SDRAM typically has a speed rating of 10 nanoseconds (the reciprocal of 100,000,000 cycles/second) when running on a motherboard with a 100MHz front side bus. However most 100MHz SDRAM parts are actually designed to run as fast as 125MHz (we'll explain why later) and are often referred to as "8" (8 nanosecond parts). 133MHz parts are referred to as "7.5" (7.5 nanosecond parts).

Get on the Bus

The front side bus (FSB), sometimes just called the bus, is the main circuitry highway in the PC that enables all data to get from one place to another. Simply put, the faster the bus runs, the faster that data can move.
Note that the speed of the FSB is not the same as processor speed. If you have a 500MHz processor with a 100MHz front side bus, the information flowing within the processor will run at 500MHz; however, whenever the data goes outside the processor, it will flow at 100MHz. The limiting factor in the overall speed of your system is the bus speed.

The Bus Bottleneck
In the late 80s and very early 90s, 386 and 486 processors typically ran at 100MHz and less with a 33MHz or 66MHz bus. For several years after that, processor speeds increased but bus speeds really did not. This held back performance. It wasn't until 1998 when the Intel Pentium II processor and the BX chipset debuted that we saw a 100MHz bus.