Hardware Reference
In-Depth Information
• PCI Express
• AGP
The speed limitation of ISA, MCA, and EISA is a carryover from the days of the original
PC when the I/O bus operated at the same speed as the processor bus. As the speed of the
processorbusincreased,theI/Obusrealizedonlynominalspeedimprovements,primarily
from an increase in the bandwidth of the bus. The I/O bus had to remain at a slower speed
because the huge installed base of adapter cards could operate only at slower speeds.
A
local bus
is any bus designed to interact more closely with the processor, or closer to
processorthroughput.Itisinteresting tonotethatthefirst8-bitand16-bitISAbuseswere
aformoflocalbusarchitecture.Thesesystemshadtheprocessorbusasthemainbus,and
everythingranatfullprocessorspeeds.WhenISAsystemsranfasterthan8MHz,themain
ISA bus had to be decoupled from the processor bus because expansion cards, memory,
and so on could not keep up. In 1992, an extension to the ISA bus called the
VESA local
bus
(VL-Bus) started showing up on PC systems, indicating a return to local bus archi-
tecture. The PCI local bus, developed at about the same time as VL-Bus, supplanted it as
the most common high-speed connection until the PCI Express (PCIe) bus became wide-
spread in the 2009 and beyond time frame. Since then, PCI slots have become fewer in
number while PCIe slots have increased.
Depending upon the requirements of the devices connected via expansion slots, it has
been common to have two or more bus designs represented in expansion slots on typical
desktop PCs. For example, a typical system built in 1993-1994 would feature one or two
PCIand/orVL-BusslotsalongwithISAslots.Inatypicalsystembuiltin1999-2000,you
might find a single ISA/PCI combo slot, an AGP slot for video, with the rest of the slots
using PCI. Typical systems in the 2006 and later timeframe typically have PCIe and PCI
slots. Some of the most recent chipset from Intel no longer support PCI slots, so in a few
years they will no longer be found in systems.
VESA Local Bus
The Video Electronics Standards Association (VESA) 32-bit local bus (VL-Bus) was the
most popular local bus design from its debut in August 1992 through 1994. It was used
for video and PATA hard disk interfacing. Although the VL-Bus could be adapted to oth-
er processors—including the 386 or even the Pentium—it was designed for the 486 and
worked best as a 486 solution only. Despite the low cost, after a new bus called PCI be-
came popular, VL-Bus fell into disfavor quickly. It never did catch on with Pentium sys-
tems, and there was little or no further development of the VL-Bus in the PC industry.
