1. Bus Systems
ISA
PCI
AGP

2. Introduction
The components inside your computer talk to each other in various different ways
Most of the internal system components, including the processor, cache, memory, expansion cards and storage devices, talk to each other over one or more "buses".
The system I/O (input/output) buses are also also called expansion buses

3. ISA
The most common bus in the PC world, ISA stands for Industry Standard Architecture,
The ISA bus is still a mainstay in even the newest computers
The ISA bus eventually became a bottleneck to performance
ISA persists because of the truly enormous base of existing peripherals using the standard
There are still many devices for which the ISA's speed is more than sufficient (standard modems being an example).

4. The specification of modern ISA is:
The ISA bus provides reasonable throughput for low-bandwidth devices and virtually assures compatibility with almost every PC on the market
The specification of modern ISA is:
16 bit bus width max
8-bit cards use only the first part of the ISA slot
16-bit cards use both parts
8MHz bus speed

5. 8 bit ISA

6. 16 bit ISA

7. PCI Peripheral Component Interconnect
Currently by far the most popular I/O Bus
Developed by Intel and introduced in 1993
PCI is a 32-bit bus that normally runs at a maximum of 33 MHz
The PCI bus is controlled by special circuitry in the chipset that is designed to handle it
PCI is also used outside the PC platform, providing a degree of universality and allowing manufacturers to save on design costs

8. PCI performance
PCI is the highest performance general I/O bus currently used on PCs because:
Burst Mode: The PCI bus can transfer information in a burst mode, where after an initial address is provided multiple sets of data can be transmitted in a row
Bus Mastering: PCI supports full bus mastering which leads to improved performance. It acts like a tiny "local area network" inside the computer, in which multiple devices can each talk to each other, sharing a communication channel that is managed by the chipset
High Bandwidth Options:The PCI bus specification version 2.1 calls for expandability to 64 bits and 66 MHz speed. This would therefore be ready for the conversion of the rest of PC technology to 64 bit operation

9. PCI supports plug and play
The PCI chipset circuitry handles the identification of cards and works with the operating system and BIOS to automatically set resource allocations for compatible peripheral cards
The PCI bus offers more expansion slots than most other bus systems
The PCI bus offers a great variety of expansion cards
However, certain functions cannot be provided on the PCI bus. For example, serial and parallel ports must remain on the ISA bus.

10. AGP
The need for increased bandwidth between the main processor and the video subsystem originally lead to the development of PCI
With the need for video bandwidth now starting to push up against the limits of even the PCI bus.
Traffic on the PCI bus was starting to become heavy on high-end PCs, with video, hard disk and peripheral data all competing for the same I/O bandwidth.
To combat the eventual saturation of the PCI bus with video information, a new interface was pioneered by Intel, designed specifically for the video subsystem. It is called the Accelerated Graphics Port or AGP.

11. Another issue has been the increasing demands for video memory
Another issue has been the increasing demands for video memory. As 3D computing becomes more mainstream
much larger amounts of memory become required, not just for the screen image but also for doing the 3D calculations. This traditionally has meant putting more memory on the video card for doing this work. There are two problems with this:
Cost: Video card memory is very expensive compared to regular system RAM.
Limited Size: The amount of memory on the video card is limited.It's not easy to expand this memory, and you can't use it for anything else if you don't need it for video processing.

12. AGP gets around these problems by allowing the video processor to access the main system memory for doing its calculations.
This is more efficient because this memory can be shared dynamically between the system processor and the video processor, depending on the needs of the system.
AGP is considered a port, and not a bus, because it only involves two devices (the processor and video card) and is not expandable.
One of the great advantages of AGP is that it isolates the video subsystem from the rest of the PC

13. AGP Bus width and bandwidth
The AGP bus is 32 bits wide, just the same as PCI is, but instead of running at half of the system (memory) bus speed the way PCI does, it runs at full bus speed.
This means that on a standard Pentium II motherboard AGP runs at 66 MHz instead of the PCI bus's 33 MHz. This of course immediately doubles the bandwidth of the port; instead of the limit of MB/s as with PCI, AGP in its lowest speed mode has a bandwidth of MB/s. Plus of course the benefits of not having to share bandwidth with other PCI devices.

14. In addition to doubling the speed of the bus, AGP has defined a 2X mode, which uses special signalling to allow twice as much data to be sent over the port at the same clock speed.
One performance enhancing benefit of AGP is its ability to pipeline requests for data. Pipelining was first used by modern processors as a way to improve performance by letting the sequential parts of tasks overlap