Basic PC Configuration Rules
The details of how the PC turned from a renegade project at IBM to a mega-industry go a long way toward explaining the limitations of the systems we have today. The basic PC configuration rules explain why even some new components consume both old and new resources.
The original IBM PC provided a meager hardware addressing range 400 bytes wide into which all the possible hardware (at the time) would exchange commands and data. In addition, eight interrupt request lines (IRQs) were available for hardware to signal the CPU and operating system that attention was needed. Further, it was anticipated that some devices would benefit from direct memory access (DMA) capabilities, and six channels were provided for that.
Within the PC, there are a few prescribed devices and functions that are absolutes no matter the generation, architecture, manufacturer, CPU, chipset, or peripherals involved. The system is destined to have timers, clocks, and a keyboard but no reservation for a display or I/O devices, which are optional.
The Bad Old Days
Shortly into the life of the PC, IBM and other vendors began to incorporate some new devices that led to the explosion of the PC into the consumer and business market. For the first three to four years of the PC's existence, the resources available for expansion and functionality were adequate, but in the years after that, they proved quite limiting. The resources, devices, and obvious limitations for expansion are listed in
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