Hardware Reference
In-Depth Information
The EISA bus was essentially a 32-bit version of ISA. Unlike the MCA bus from IBM, you could still
use older 8-bit or 16-bit ISA cards in 32-bit EISA slots, providing for full backward compatibility.
As with MCA, EISA also allowed for automatic configuration of EISA cards via software.
To learn more about the EISA Bus, including pinouts of the EISA slot, see “The EISA Bus” in
Chapter 4 in
Upgrading and Repairing PCs,
19
th
Edition, available in its entirety on the DVD
packaged with this topic.
Local Buses (VESA, PCI, PCI Express, AGP)
The I/O buses discussed so far (ISA, MCA, and EISA) have one thing in common: relatively slow
speed. The next three bus types that are discussed in the following few sections use the
local bus
concept explained in this section to address the speed issue. The main local buses found in PC
systems are
• VL-Bus (VESA local bus)
• PCI
• 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 processor bus
increased, the I/O bus realized only nominal speed improvements, 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 processor
throughput. It is interesting to note that the first 8-bit and 16-bit ISA buses were a form of local bus
architecture. These systems had the processor bus as the main bus, and everything ran at full
processor speeds. When ISA systems ran faster than 8MHz, the main 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 architecture. 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 widespread 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 PCI or VL-Bus slots along with ISA
slots. In a typical system built in 1999-2000, 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 chipsets 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 other processors—including
the 386 or even the Pentium—it was designed for the 486 and worked best as a 486 solution only.
