INTERFACES

The PC's adaptability - its ability to evolve many different interfaces allowing the connection of many different classes of add-on component and peripheral device - has been one of the key reasons for its success. In essence, a modern PC system of today is little different to IBM's original design - a collection of components, both internal and external, interconnected by a series of electrical data highways over which data travels as it is completes the processing cycle that transforms it from an item of input to an item of output. These "buses", as they are called, connect all the PC's internal components and external devices and peripherals to its CPU and main memory (RAM).

The fastest bus of all is the connection between the processor and its primary cache, and this is kept within the CPU chip. The next level down is the system bus, which links the processor with memory, both the small amount of Static RAM (SRAM) secondary cache and the far larger main banks of Dynamic RAM (DRAM). The system bus is 64 bits wide and, for Intel-based designs, was capped at 66MHz until early 1998, when a new Pentium II chipset raised this to 100MHz. The CPU doesn't communicate directly with the memory, but through the intermediary of the System Controller chip, which manages the host bus and bridges between it and, in modern PCs, the PCI bus.

Processors using a Dual Independent Bus (DIB) architecture - present on Intel designs from the Pentium II onwards - have replaced the single system bus with two independent buses, one for accessing main memory and the other for accessing the Level 2 cache. These are referred to as the frontside bus and the backside bus respectively.

The key concept was of an open architecture based on a simple expansion bus that facilitated the easy connection of additional components and devices. Nearly two decades after its introduction, it was still possible to fit original add-on cards into a modern PC - a tribute to the staying power of the design. Whilst there have been a number of dead ends along the way, the evolution of standard expansion bus designs has been remarkably robust over the years.

Bus terminology

A modern-day system can be viewed as comprising just two classes of bus: a System Bus, connecting the CPU to main memory and Level 2 cache, and a number of I/O Busses, connecting various peripheral devices to the CPU - the latter being connected to the system bus via a "bridge", implemented in the processor's chipset.

In Dual Independent Bus (DIB) architecture systems the single system bus is replaced by a "frontside bus" for shuttling data between the CPU and main memory, and between the CPU and peripheral buses and a "backside bus" for accessing Level 2 cache. The use of dual independent buses boosts performance, enabling the CPU to access data from either of its buses simultaneously and in parallel. Over time, the terms "FSB" and "system bus" came to be used interchangeably.

The evolution of PC bus systems over a period of more than a decade has given rise to a profusion of terminology, much of it confusing, redundant or obsolete. The system bus is often referred to as the "main bus", "processor bus" or "local bus". Alternative generic terminology for an I/O bus includes "expansion bus", "external bus", "host bus" as well as, rather confusingly, "local bus".

A given system can use a number of different I/O bus systems and a typical arrangement is for the following to be implemented concurrently: