Motherboards and Buses - Upgrading and Repairing PCs - page 274

cessor Bus (Front Side Bus)” in Chapter 4 , “ Motherboards and Buses ,” in Upgrading and
Repairing PCs, 19 th edition ,availableinitsentiretyontheDVDpackagedwiththisbook.
Types of I/O Buses
SincetheintroductionofthefirstPC,manyI/Obuseshavesurfaced.Thereasonissimple:
Faster I/O speeds are necessary for better system performance. This need for higher per-
formance involves three main areas:
• Faster CPUs
• Increasing software demands
• Greater multimedia requirements
Each of these areas requires the I/O bus to be as fast as possible.
One of the primary reasons new I/O bus structures have been slow in coming is compat-
ibility—that old catch-22 that anchors much of the PC industry to the past. One of the
hallmarks of the PC's success is its standardization. This standardization spawned thou-
sands of third-party I/O cards, each originally built for the early bus specifications of the
PC. If a new high-performance bus system was introduced, it often had to be compatible
with the older bus systems so the older I/O cards would not be obsolete. Therefore, bus
technologies seem to evolve rather than make quantum leaps forward.
You can identify different types of I/O buses by their architectures. The main types of I/O
buses are detailed earlier in this chapter.
Themaindifferencesamongbusesconsistprimarilyoftheamountsofdatatheycantrans-
fer at one time and the speeds at which they can do it. The following sections describe the
various types of PC buses.
The ISA Bus
Industry Standard Architecture (ISA) is the bus architecture that was introduced as an
8-bit bus with the original IBM PC in 1981; it was later expanded to 16 bits with the IBM
PC/AT in 1984. ISA is the basis of the modern personal computer and was the primary
architecture used in the majority of PC systems until the late 1990s.
To learn more about the ISA Bus, including diagrams of 8-bit and 16-bit ISA slots and
pinouts, see “ The ISA Bus ” in Chapter 4 , “ Motherboards and Buses ,” in Upgrading and
Repairing PCs, 19 th edition ,availableinitsentiretyontheDVDpackagedwiththisbook.