PC Hardware in a Nutshell, 3rd Edition

Interface

Early video adapters, including the first generation of graphics accelerators, used the ISA bus, which soon proved inadequate to carry the high volume of video data required by graphics-based operating systems and applications. VESA Local Bus (VLB) adapters, shipped with many 486 systems and a few early Pentium systems, greatly improved throughput. ISA and VLB video adapters are obsolete but remain in limited distribution, although they are now difficult to find and use video chipsets that are several generations out of date. Any system so old that it accepts only an ISA or VLB video adapter is too old to be worth upgrading. Current video adapters use one of the following interfaces:

PCI

The PCI bus is 32 bits wide and runs at 33.3 MHz, providing peak throughput of 133 MB/s. The fact that a PCI video card shares the bus with other PCI devices means that heavy transfers of video data reduce the bandwidth available to other PCI peripherals, such as hard disk controllers and network cards. This concern is still largely theoretical because PCI bus saturation is very rare with current systems and software.

AGP (Accelerated Graphics Port)

Intel’s concern about PCI throughput limitations and the increasing amount of data that new generations of 3D video adapters would demand led it to introduce AGP. Although AGP supports 2D operations, it provides no real benefit over PCI in 2D. As the name implies, AGP is a port rather than a bus, which means that it connects only two devices. ...