A video card sends data to the CPU for the processing of data and graphics. A set of addresses is needed so that it is sent to the right place and these addresses are contained in each packet of data going from the video card to the CPU. Having to look at each data packet to find the addresses can slow down a computer, decreasing its speed and performance.
Sideband addressing is a method of getting around this slowdown by putting those addresses in eight extra lines of data in the data packet. This action allows the addresses to be found and read much faster, improving the computer's performance and allowing 3D graphics to show more quickly. Sideband addressing was first used with AGP video cards and AGP video cards were first supported in Windows 95 OEM Service Release 2. AGP was first supported in Windows NT-based operating systems beginning with Windows NT 4.0 service pack 3. It was also supported in Linux in 1999 with the introduction of the AGPgart kernel module.