CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [117]
Figure 8-7 Sixteen-bit ISA or AT slots
Even though IBM allowed third parties to copy the PC and AT expansion bus architecture, they never released the complete specifications for these two types of expansion buses. In the early 1980s, a number of clone makers pooled their combined knowledge of the PC/XT and AT buses to create the Industry Standard Architecture (ISA).
The ISA bus enabled manufacturers to jump the first of the three hurdles for successful expansion cards, namely connectivity. If a company wanted to build a new kind of adapter card for the PC, they simply followed the specifications in the ISA standard.
ISA Bus
16 bits wide
7-MHz speed
Manual configuration
Essentials
Modern Expansion Buses
The ISA expansion bus was both excellent and cutting edge for its time, and was the expansion bus in every PC for the first ten years of the PC’s existence. Yet ISA suffered from three tremendous limitations that began to cause serious bottlenecks by the late 1980s. First, ISA was slow, running at only about 7 MHz. Second, ISA was narrow—only 16 bits wide—and, therefore, unable to handle the 32-bit and 64-bit external data buses of more modern processors. Finally, techs had to configure ISA cards manually, making installation a time-consuming nightmare of running proprietary configuration programs and moving tiny jumpers just to get a single card to work.
Manufacturers clearly needed to come up with a better bus that addressed the many problems associated with ISA. They needed a bus that could take advantage of the 33-MHz motherboard speed and 32-bit-wide data bus found in 386 and 486 systems. They also wanted a bus that was self-configuring, freeing techs from the drudgery of manual configuration. Finally, they had to make the new bus backward compatible, so end users wouldn’t have to throw out their oftentimes substantial investment in ISA expansion cards.
False Starts
In the late 1980s, several new expansion buses designed to address these shortcomings appeared on the market. Three in particular—IBM’s Micro Channel Architecture (MCA), the open standard Extended ISA (EISA), and the Video Electronics Standards Association’s VESA Local Bus (VL-Bus)—all had a few years of modest popularity from the late 1980s to the mid 1990s. Although all of these alternative buses worked well, they also had shortcomings that made them less than optimal replacements for ISA: IBM charged a heavy licensing fee for MCA, EISA was expensive to make, and VL-Bus only worked in tandem with the ISA bus. By 1993, the PC world was eager for a big name to come forward with a fast, wide, easy-to-configure, and cheap new expansion bus. Intel saw the need and stepped up to the plate with the now famous PCI bus.
PCI
Intel introduced the peripheral component interconnect (PCI) bus architecture (Figure 8-8) in the early 1990s, and the PC expansion bus was never again the same. Intel made many smart moves with PCI, not the least of which was releasing PCI to the public domain to make PCI very attractive to manufacturers. PCI provided a wider, faster, more flexible alternative than any previous expansion bus. The exceptional technology of the new bus, combined with the lack of a price tag, made manufacturers quickly drop ISA and the other alternatives and adopt PCI.
PCI really shook up the PC world with its capabilities. The original PCI bus was 32 bits wide and ran at 33 MHz, which was superb, but these features were expected and not earth-shattering. The coolness of PCI came from its capability to coexist with other expansion buses. When PCI first came out, you could buy a motherboard with both PCI and ISA slots. This was important because users could keep their old ISA cards and slowly migrate to PCI. Equally impressive was that PCI devices were (and still are) self-configuring, a feature that led to the industry