Expansion Slots
In addition to the processor and the memory, the video processor is crucial to the working order of your computer. As the latter part of the name applies, a video processor is a microprocessor just like the one that is running your computer. However, the operations that this processor does is much more specific and focused on producing graphical output to a monitor from your computer.
Motherboards used to just hold the processor, memory, drive controller and a couple of ports for things such as keyboards, mice, and printers. Other functionality such as video, sound and networking would have to be added by individual expansion cards. Today, even though a number of these things have made their way directly onto the motherboard, they don't always handle the most advanced features such as surround sound and 3d graphics. Expansion slots are still needed to add greater functionality and to allow for future expansion of features.
PCI
The most ubiquitous expansion slot type is PCI (Peripheral Component Interconnect). It was introduced in 1992 and doubled both the width (to 32 bits) and speed (to 33 MHz) of the previous expansion slot type, ISA. PCI is a parallel, shared bus design. This means that it uses many lines to communicate a data chunk at once and that only one device can talk on the bus at a time. Later revisions to PCI allowed for a 64 bit data path and at a speed of 66 MHz. It was common to find three to five (or even more) PCI slots on a motherboard and most current motherboards still have them today.
AGP
One of the first functions that found it self in need of more bandwidth than the PCI bus could accommodate was 3d graphics. In 1996 AGP (Advanced Graphics Port) was created to give a 3d graphics card a faster and direct access to the computer's central processor. It is a 32 bit parallel bus that started out at 66 MHz (AGP-1x) and eventually scaled up to 266 MHz (AGP-8x). Except possibly in some unusual cases, you'll only see one APG port on a motherboard.
PCI-X
PCI-X (PCI eXtended) was designed as a follow up to PCI and is found mostly in servers and workstations. It was introduced in 1999 and was designed to be backwards compatible with PCI. PCI cards will work in a PCI-X slots and PCI-X cards will work in PCI slots (if they're the appropriate size) but at the lower speed of PCI. PCI-X came in both 32 bit and 64 bit versions. They started at 133 MHz with the highest end revisions going all the way up to 533 MHz. The later versions also added the option of being point to point instead of just shared bus like PCI.
PCI Express
The newest expansion slot standard is PCI Express (PCI-E). It was introduced in 2002 and is gaining widespread adoption. PCI Express is very different from the previous expansion slot types. Instead of being a parallel bus (using many wires to transmit a data chunk at once) it is a high speed serial bus (transmits smaller pieces of the data chunk in succession). Additionally, it uses a switched communications pathway as opposed to a shared bus. This means that multiple devices can talk at the same time. A PCI Express channel is capable of transmitting .5 Gbps of data in each direction. This is equivalent to the most basic version of PCI-X and is referred to as PCI Express x1. However, multiple channels can be bound together (up to 32) in one slot to achieve even greater throughput. Many newer motherboards have a PCI Express x16 slot to replace the AGP slot for 3d video cards.
For more information about PCI-E, see:
http://arstechnica.com/articles/paedia/hardware/pcie.ars
For further details on how much data the various expansion slot types can transmit, see:
http://www.techweb.com/encyclopedia/defineterm.jhtml?term=PCI%2DSIG
Fig. 1: Slot Comparison
Fig. 2: This motherboard has a variety of expansion slot types.
Starting from the left, there are three PCI, one PCI Express x4,
two PCI Express x1 and one PCI Express x16
Laptop Expansion Slots
Obviously none of those full size cards will fit inside a laptop. In 1990, a group called PCMCIA established standards for expansion slots for laptops. These expansion cards are about the size of a credit card except thicker.
CardBus
In 1995 PCMCIA established a new standard called CardBus. These had the same form factor as the older cards but used PCI technology.
ExpressCard
The latest PCMCIA standard uses cards that can vary a bit in size. At their widest, they are the same as previous generations but their connector is narrower. Also there are cards that are the width of the connector for their whole length. Instead of using PCI technology like CardBus, ExpressCard can use either a single channel of PCI Express or USB 2.0.
For more detailed information, see:
http://www.techweb.com/encyclopedia/defineterm.jhtml?term=PCCard
