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Unformatted text preview: of the 286’s 16-bit external data bus, yet also supported 8-bit cards. IBM
achieved this by simply adding a set of connections to the end of the PC bus, creating a
new 16-bit bus (Figure 8-7). Many techs called this bus the AT bus after the first system
to use these slots, the 286-based IBM Advanced Technology (AT) computer. The AT bus
ran at the same speed (approximately 7 MHz) as the earlier PC bus.
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
Manual configuration ch08.indd 294 12/14/09 2:49:19 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 8
All-In-One Chapter 8: Expansion Bus 295 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 narrowonly
16 bits wideand 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
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This note was uploaded on 04/27/2010 for the course COMPTIA 1201 taught by Professor N/a during the Spring '10 term at Galveston College.
- Spring '10