3/22 How a Bus Works.• Roll Call• Lecture:

– general• traces• addresses• sizes• types

– How a PCI bus works.

The Bus: general• A set of circuits

and chips that transfer data between devices.

• Buses are made of traces (tiny electrical circuits), microchips, & expansion slots.

Traces• Traces work like

the wires in cables: the circuits are used for: – power– data transmission– control commands– & also addresses.

Addresses• An address is a

unique number that is assigned to each component on a particular bus.

• A component will ignore commands and data on the bus if another address is signaled.

Microchips in Buses• Microchips are

used in some buses to increase the capacity of the bus in speed, in the number of components, and to relieve the load on the CPU.

Expansion Slots• Expansion slots are

an easy way to expand and alter the computer’s setup.

• Varieties (some):– 8,16, & 32-bit– ISA, EISA– PCI, AGP

Expansion Slots: 8 & 16 bits• 8-bit:

– 31 slots, 8 used for data. (other: power, control, etc.)

• 16-bit:– ISA: Industry

Standard Architecture

– 49 connectors, 16 for data.

Expansion Slots: 32 bits•EISA:

–“Extended Industry Standard Architecture”

–97 slots, 32 for data•PCI:

–“Peripheral Component Interconnect”

–Local Bus design–47 slots, 32 for data

Expansion Slots: 32 bits• AGP

– “Accelerated Graphics Port”

– Local bus design– 44 pins, 32 for data.

How a Bus Works: PCI Bus• Signals from the

CPU are sent to an I/O controller for the PCI local bus operations.

• The controller diverts the non-PCI signals to a different controller.

How a Bus Works: PCI Bus• The controller

sends the PCI signals on to the local bus slots.

• A PCI bus controller works like a pre-sorter at the post office.

How a Bus Works: AGP Bus• The AGP

controller does all that the PCI controller does.

• The AGP also handles data transfer among memory, the processor and the ISA controller.

How a Bus Works: AGP Bus• The AGP sorts the

data from the CPU to go to the– ISA I/O controller– PCI local bus– Accel. Graph. Port– RAM