Editing Accelerated Graphics Port (section)

=={{Anchor|APERTURE}}Advantages over PCI==
AGP is a superset of the PCI standard, designed to overcome PCI's limitations in serving the requirements of the era's high-performance graphics cards.<ref>{{Cite web |last=By |date=2024-03-03 |title=A Better Use For The AGP Slot, Decades Later |url=https://hackaday.com/2024/03/03/a-better-use-for-the-agp-slot-decades-later/ |access-date=2024-11-11 |website=Hackaday |language=en-US}}</ref>

The primary advantage of AGP is that it doesn't share the PCI [[Bus (computing)|bus]], providing a dedicated, [[Point-to-point (telecommunications)|point-to-point]] pathway between the expansion slot(s) and the motherboard chipset. The direct connection also allows higher clock speeds.

The second major change is the use of split [[Peripheral Component Interconnect#PCI bus transactions|transactions]], wherein the address and data phases are separated. The card may send many address phases, so the host can process them in order, avoiding any long delays caused by the bus being idle during read operations.

Third, PCI bus handshaking is simplified. Unlike PCI bus transactions, whose length is negotiated on a cycle-by-cycle basis using the FRAME# and STOP# signals, AGP transfers are always a multiple of 8&nbsp;bytes long, with the total length included in the request. Further, rather than using the IRDY# and TRDY# signals for each word, data is transferred in blocks of 4&nbsp;clock cycles (32&nbsp;words at AGP 8× speed), and pauses are allowed only between blocks.

Finally, AGP allows (mandatory only in AGP 3.0) ''sideband addressing'', meaning that the [[address bus|address]] and data buses are separated, so the address phase does not use the main address/data (AD) lines at all. This is done by adding an extra 8-bit "SideBand Address" [[computer bus|bus]], over which the graphics controller can issue new AGP requests while other AGP data is flowing over the main 32 address/data (AD) lines. This results in improved overall AGP data throughput.

This great improvement in memory read performance makes it practical for an AGP card to read [[Texture mapping|textures]] directly from system RAM, while a PCI graphics card must copy it from system RAM to the card's [[video memory]]. System memory is made available using the [[graphics address remapping table]] (GART), which apportions main memory as needed for texture storage.<ref>{{cite web |url=http://www.sysopt.com/features/mboard/article.php/3549951 |title=What is AGP? |access-date=15 September 2014 |url-status=dead |archive-url=https://web.archive.org/web/20120509222057/http://www.sysopt.com/features/mboard/article.php/3549951 |archive-date=9 May 2012 }}</ref> The maximum amount of system memory available to AGP is defined as the ''AGP [[Aperture (computer memory)|aperture]]''.