DOS Days

Front Side Bus Explained

The term "front side bus", often abbreviated to FSB, was dubbed around the time of the Pentium II in 1997. It refers to the communication bus between the CPU and the rest of the computer system. The "back side bus", by contrast, refers to the communications that take place inside the CPU die itself, such as between the CPU and its level 1 cache.

The FSB usually connects the CPU to the rest of the PC hardware via a "chipset" (literally, a set of chips!). In the case of Intel-based motherboards, these were called the "Northbridge" and the "Southbridge". These two chips (the "chipset") would be used to handle communications to the PCI bus, AGP bus, and main memory. All these secondary buses typically run at speeds that are derived from the FSB clock.

Here's an example of a typical Northbridge and Southbridge on a motherboard:


The frequency at which the CPU operates is determined by applying a 'clock multiplier' to the FSB speed. For example, if the front side bus is running at 66 MHz, and the system is configured to use a clock multiplier of 6, the CPU will run at 66 x 6 = 396 MHz (which we tend to just round up to 400 MHz). All devices that connect to the front side bus communicate at that slower speed of 66 MHz (in the example above).

On systems based around the original Pentium, the maximum FSB speed was 66 MHz. With the introduction of the "Super 7" socket with AMD's launch of the K6-2, and Intel's 440BX chipset for the Pentium II, this maximum FSB speed was able to be increased to 100 MHz. Later on with the Pentium III and Athlon processors, we saw this rise again to 133 MHz.

Since the FSB is controlled by the chipset, this is a critical component of any motherboard from the 486-era and up - quality and stability really matter here! In my experience, ALi and Intel produced some of the best chipsets during the DOS and early WIndows era, whilst VIA and SiS were often not so good. I've put together a pretty thorough guide to chipsets here.