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I/O buses- ISA, EISA, Local bus, VESA

Local bus

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There's one element, however, that

often escapes notice - the bus.

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A bus is a road vehicle designed to

carry passengers.

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Essentially, a bus is a channel or path

between the components in a computer.

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Having a high-speed bus is as important as

having a good transmission in a car.

There are many different types of buses.

1. internal

2. external

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The idea of a bus is simple -- it lets you connect

components to the computer's processor.

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Some of the components that you might want toconnect include hard disks, memory, soundsystems, video systems and so on.

Forexample, to see what your computer is doing,you normally use a CRT or LCD screen.

You need special hardware to drive the screen, sothe screen is driven by a graphics card.

A graphics card is a small printed circuit board

designed to plug into the bus. The graphics card talks to the processor using the

computer's bus as a communication path.

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SYSTEM BUSES

The CPU moves data around the computer on pathways thatinterconnect it to all the other components on the motherboard. Thesepathways are called 'buses'.

The internal bus carries data within the motherboard.

External buses carry data to peripherals and other devices attached tothe motherboard.

The lines or pins of a bus are of three types:

Address - the components pass memory addresses to one another over

the address bus. Control- used to send out signals to coordinate and manage the

activities of the motherboard components.(handshaking)

Data - transferred between peripherals, memory and the CPU.Obviously, the data bus can be a very busy pathway.

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The PC has a hierarchy of different buses.

They are hierarchical because each slower bus

is connected to the faster one above it.

WHY??

Each device in the system is connected to one

of the buses.

SINCE YOU ARE AWAKE YOU SHOULD ASK

WHAT IS A CHIPSET?????

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A chipset, PCchipset or chip set refers to a

group of integrated circuits, or chips, that are

designed to work together.

We know that communication b/w devices

takes place with the help of buses and this

communication is controlled by CHIPSET. They are usually marketed as a single product.

A chipset is usually designed to work with a

specific family of microprocessors. It plays a crucial role in determining system

performance.

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The system chipset is the conductor

that controls communication, and

makes sure that every device in thesystem is talking properly to every

other one.

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A = CPU slot, B = north bridge chip (covered by a

cooling fan), C = south bridge chip

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north bridge

The north bridge controls the communication

between AGP video, the RAM and the CPU

south bridge The south bridge is often referred to as the I/O

controller. It deals with i/o devices.

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SOME COMMON POINTS WITH SYSTEM BUSES

Bus Width: A bus is a channel over which

information flows.

The wider the bus, the more information can

flow over the channel, much as a wider

highway can carry more cars than a narrow

one.

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Bus Bandwidth:Bandwidth, also called

throughput, refers to the total amount of data

that can theoretically be transferred on thebus in a given unit of time.

Using the highway analogy, if the bus width is

the number of lanes, and the bus speed ishow fast the cars are driving, then the

bandwidth is the product of these two and

reflects the amount of traffic that the channel

can convey per second.

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why we need high speed buses

This need for higher performance involves

three main areas: 1.Faster I/O speeds are necessary for better

system performance

2. Faster CPUs3.Greater multimedia requirements

Each of these areas requires the I/O bus to beas fast as possible.

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The main differences among buses consist

primarily of the amounts of data they can

transfer at one time and the speeds at whichthey can do it.

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Processor bus:Also called the front-side bus(FSB), this is the highest-speed bus in the system

and is at the core of the chipset andmotherboard.

the front-side bus (FSB) is the bus that carriesdata between the CPU and the Northbridge.

This bus is used primarily by the processor topass information to and from cache or mainmemory and the North Bridge of the chipset.

The processor bus in a modern system runs at

66MHz, 100MHz, 133MHz, 200MHz, 266MHz,400MHz, 533MHz, or 800MHz and is normally 64bits (8 bytes) wide.

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a back side bus (BSB), or backside bus, is a

computer bus used to connect the CPU to CPU

cache memory.

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older buses:-

ISA bus:Industry Standard Architecture (ISA)

is the bus architecture that was introduced as

an 5MHz 8-bit bus with the original IBM PC in

1981; it was later expanded to 8MHz 16 bits

with the IBM PC/AT in 1984. It is a very slow-speed bus, but it was ideal for

certain slow-speed or older peripherals.

It has been used in the past for plug-inmodems, sound cards, and various other low-

speed peripherals.

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Two versions of the ISA bus exist, based on the

number of data bits that can be transferred on

the bus at a time.

The older version is an 8-bit bus; the newer

version is a 16-bit bus.

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8 BIT ISA BUSES

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16 BIT ISA BUSES

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32 BIT EXPANSION(failed badly)

After 32-bit CPUs became available, it was some

time before 32-bit bus standards becameavailable.

Before MCA (Micro Channel Architecture) andEISA specs were released, some vendors begancreating their own proprietary 32-bit buses,

which were extensions of the ISA bus. The expanded portions of the bus typically are

used for proprietary memory expansion or videocards.

Because the systems are proprietary (meaningthat they are nonstandard), pinouts andspecifications are not available.

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EISA BUS

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EISA stands for Extended Industry Standard

Architecture.

Unlike ISA, here the name is notindicative of

reality, for the EISA bus never became widely

used and cannot by any stretch of the

imagination be considered an industrystandard.

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Some of the key features of the EISA bus:

ISA Compatibility: ISA cards will work in EISA

slots. 32BitBus Width: Like MCA, the bus was

expanded to 32 bits.

Bus M

astering:

The EISA bus supports busmastering adapters for greater efficiency,

including proper bus arbitration.

Plug and Play:EISA automatically configures

adapter cards, similar to the Plug and Playstandards of modern systems.

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The speed problembecame acute whengraphical user interfaces (such as Windows)

became prevalent. These systems require the processing of so much

video data that the I/O bus became a literalbottleneck for the entire computer system.

However, some I/O peripherals need a very lowbandwidth while other need higher bandwidths.

In other words, it did little good to have aprocessor that was capable of 66MHz450MHz or

faster if you could put data through the I/O bus ata rate of only 8MHz.

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The I/O buses discussed so far (ISA, MCA, andEISA) have one thing in common: relatively slowspeed.

LOCAL BUSES(HIGH SPEED BUSES)

The next three bus types that are discussed in thefollowing few sections all use the local busconcept explained in this section to address the

speed issue. The three main local buses found in PC systems

are

VL-Bus (VESA local bus)

PCI AGP

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VESA LocalBUS:

VESA (Video Electronics Standards Association)Local bus was an extension of the standard 16-bitISA bus, and gave a 32-bit expansion slot.

These were seen in late 486 and early Pentiummotherboards, which would often have two orthree VESA Local Bus slots in addition to 16-bitISA slots.

They were short lived, quickly replaced by PCIslots.

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ADDITIONAL READING

ISSTRONGLYRECOMMENDED