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 inputto 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.

PC Components | Processors (CPUs) | PC Data Storage | PC Multimedia | PC Input/Output | Communications | Mobile Computing

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