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Understanding PC Buses

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

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.

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.

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.

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.

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
Last updated: November 04, 2005

Related Links

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.

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.

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