Datasheet

40 Chapter 1
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Hardware
Accelerated Graphics Port (AGP); they’re for circuit boards. External expansion buses
include serial, parallel, Universal Serial Bus (USB), FireWire, and infrared. The following
sections explain some of the most common buses.
There are many obsolete bus types, including Video Electronics Standards
Association Local Bus (VESA local bus, or VL-Bus), Microchannel Architec-
ture (MCA), and enhanced ISA (EISA). These were not on the last iteration
of the A+ test and should not appear on this one either.
ISA (Industry Standard Architecture)
This is a 16-bit bus (originally 8-bit on the oldest computers) that operates at 8MHz. Its
slots are usually black. New motherboards will not have this type of slot, because the ISA
bus is old technology and has been phased out.
Besides the slow speed and narrow width, another drawback of the ISA bus is that each
ISA device requires separate system resources, including separate interrupt requests (IRQs).
In a heavily loaded system, this can cause an IRQ shortage. (PCI slots, in contrast, can
share some resources.)
PCI (Peripheral Component Interconnect)
The PCI bus is a fast (33MHz), wide (32-bit or 64-bit) expansion bus that is the modern
standard in motherboards today for general-purpose expansion devices. Its slots are typi-
cally white. PCI devices can share IRQs and other system resources with one another in
some cases. All modern motherboards have at least three PCI slots. Figure 1.15 shows some
PCI slots.
FIGURE 1.15 PCI bus connectors
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