| Think of a bus as
the electronic highway on which
data travels within a
computer,
from one component to another.
Basically, it's the conduit used by your entire system to
communicate with your
CPU. A bus is a collection of wires
and connectors through which the data is transmitted. When used in reference to
personal computers, the term bus usually refers to what is commonly
called the
local bus
(on older systems) or
system bus
(on newer systems).
This bus is considered the first bus on the electronic highway and
it connects the CPU to the
main
memory (RAM) on the
motherboard. All buses consist of
two parts -- an
address bus and a data bus.
The data bus transfers actual data whereas the
address bus transfers
information about the data and where it should go. The address bus is used to identify
particular locations (addresses) in main memory. The width of the
address bus (that is, the number of wires) determines how many
unique memory locations can be addressed. Modern PCs and Macs
have as many as 36 address lines, which enables them theoretically
to access 64 GB
of main memory. However, the actual
amount of memory that can be accessed is usually much less than this
theoretical limit due to chipset and
motherboard limitations.
The size of a bus, known
as its width, is important because it determines how much data can
be transmitted at one time. The bus size actually
indicates the number of wires in the bus. For example,
a
32-bit bus
has 32 wires or connectors that transmit 32 bits
simultaneously (referred to as
in
parallel).
It would be considered "32-bits wide." A
16-bit bus
has 16 wires or connectors that can transmit 16 bits of data in
parallel. You would say it is "16-bits wide." |
Key Terms To
Understanding Buses
Bus
A collection of wires through which data is transmitted from one
part of a computer to another.
System Bus
The bus that connects the CPU to main memory on the motherboard.
I/O
The term I/O is used to describe any program, operation or device
that transfers data to or from a computer and to or from a
peripheral device.
PCI
Short for Peripheral Component Interconnect, a local bus standard
developed by Intel Corp. |
System Bus & I/O Bus
On older computers, the
local bus,
which was the
only bus, was used for the CPU, RAM and
I/O (input/output) components. All components on the local bus used the
same clock
speed. In the late 80s we saw the separation of the system bus from the
I/O bus allowing them to run at different speeds.
The system bus (also called the frontside
bus, memory bus, local bus or host bus) is what connects the CPU to
main memory on the motherboard. I/O buses are those that connect the CPU
and RAM with all other components, and the I/O buses branch off of the
system bus. I/O buses operate on a speed which is lower than the system bus
speed. PCs offer several types of I/O buses which include the ISA
bus, PCI bus, AGP bus and USB bus.
ISA Bus & PCI Bus
Short for Industry Standard Architecture
bus, the ISA
bus architecture was used in the IBM PC/XT and PC/AT. The AT version of
the bus is called the AT bus and became a de facto industry standard.
Starting in the early 90s, ISA began to be replaced by the
PCI (Peripheral
Component Interconnect) local bus architecture. The
PCI standard was developed by Intel Corp. On modern PCs, the PCI
bus is the central (or main) I/O bus. It's used for connecting adapters
such as hard disks, sound cards, network cards and graphics cards
(although now AGP is more common for 3-D graphics). PCI is a 64-bit bus,
though it is usually implemented as a 32-bit bus, and it can run at
clock speeds of 33 or 66
MHz. At
32-bits and 33 MHz, it yields a throughput rate of 133 MBps (at 66 MHz
266 MBps). The vast majority of today's PCs implement a PCI bus that
runs at a maximum speed of 33 MHz.
PCI 2.1
Also called PCI-X 2.0, the PCI bus specification version 2.1 calls
for expandability to
64-bits
and 66 MHz speed, yielding a throughput rate of 532 MBps.
AGP Bus
Short for
Accelerated Graphics Port, an interface
specification developed by Intel Corporation. AGP is based on PCI, but
is designed especially for the throughput demands of
3-D
graphics. Rather than using the PCI bus for graphics data, AGP
introduces a dedicated point-to-point channel so that the graphics
controller can directly access main memory. The AGP channel is 32-bits
wide and runs at 66 MHz. This translates into a total
bandwidth
of 266 MBps,
as opposed to the PCI bandwidth of 133 MBps. AGP also supports optional
faster modes and allows 3-D textures to be stored in
main
memory rather than
video
memory.
USB Bus
Short for
Universal Serial Bus, an external bus standard
that supports data transfer rates of 12 Mbps. A single USB port can be
used to connect up to 127 peripheral devices, such as mice, modems, and
keyboards. USB also supports
Plug-and-Play installation and
hot
plugging.
USB 2.0
Also referred to as Hi-Speed USB,
USB 2.0
is an external bus that supports data rates up to 480Mbps. USB 2.0
is an extension of USB 1.1. USB 2.0 is fully compatible with USB 1.1
and uses the same cables and connectors.
Bus Speed
Every bus has a
clock speed measured in MHz.
This measurement represents the speed in which information and data
can move across the bus on the motherboard. A fast bus allows data
to be transferred faster, which makes applications run faster. Bus
speed is one of the factors which determines the speed of your CPU.
Backside Bus
The
backside bus is the microprocessor bus that connects the CPU to a
Level 2 cache.
Typically, a backside bus runs at a faster clock speed than the
frontside
bus that connects the CPU to main memory. For example, the
Pentium Pro microprocessor actually consists of two chips — one
contains the CPU and the primary cache, and the second contains the
secondary cache. A backside bus connects the two chips at the same clock
rate as the CPU itself (at least 200 MHz). In contrast, the frontside bus
runs at only a fraction of the CPU clock speed.
|
Did You Know...
Embedded systems do not contain an I/O bus. Communication is
controlled by the CPU, which reads and writes data from the
devices as if they are blocks of memory. |
Vangie 'Aurora' Beal
Writer, www.Webopedia.com
Last updated: November 04, 2005
PCI Special Interest Group
The PCI-SIG provides a forum for its 900+ member companies who develop
differentiated, interoperable products based on its specifications.
These are exciting times for the PCI development community. With the release of
three new specifications, PCI Express. 1.1, PCI-X 2.0 and PCI 3.0, the I/O
environment as we know it will be radically changed.
Introducing the PCI bus
The PCI is the high speed bus of the 1990s. PCI stands for Peripheral Component
Interconnect. This bus is made by Intel. It is used today in all PCs and other
computers for connecting adapters, such as network-controllers, graphics cards,
sound cards etc.
Peripheral Component Interconnect (PCI)
Local Bus
Currently by far the most popular local I/O bus, the Peripheral Component
Interconnect (PCI) bus was developed by Intel and introduced in 1993. It is
geared specifically to fifth- and sixth-generation systems, although the latest
generation 486 motherboards use PCI as well.
PCI Local Bus Technical Summary
The PCI Local Bus is a high performance bus for interconnecting chips, expansion
boards, and processor/memory subsystems. It originated at Intel in the early
1990s as a standard method of interconnecting chips on a board. It was later
adopted as an industry standard administered by the PCI Special Interest Group,
or "PCI SIG". Under the PCI SIG the definition of PCI was extended to define a
standard expansion bus interface connector for add-in boards.
SysOpt
System Optimization and PC Performance
Understanding Video and Graphics Adapters
While the names to describe it are many (video adapter, video card, video board,
video display board, graphics card or graphics adapter) its job and function
within a computer system remains the same, regardless of what you call it.
What Is CPU Overclocking?
While the words CPU and microprocessor are used interchangeably, in the world of
personal computers (PC), a microprocessor is actually a silicon chip that
contains a CPU. At the heart of all personal computers sits a microprocessor
that controls the logic of almost all digital devices, from clock radios to
fuel-injection systems for automobiles. |
