What is AGP and how does it work?

October 2016

AGP is the short term for Accelerated Graphics Port and is concretely a point-to-point bus. The bus, compatible with IBM computers, is hence used as a Local Bus and operates as a Peripheral Component Interface that includes 20 supplementary signals not currently found on a PCI bus.

How does it work?

The AGP bus has a width of 32 bits and functions at a full bus speed compared to the normal PCI. AGP specifies mainly for the Protocol, Electrical and Mechanical part of the bus and uses 1.5 or 3.3 Volt signals. If installed on a Pentium II motherboard, the AGP can run up to 66 MHz with a minimum bandwidth of 254.3 MB/s.

AGP makes use of a particular signaling that allows the user to send a double amount of data over the port at an equal clock speed. The bus hence sends the information on the rising edge, defined as the "0" to "1" transition signal and falling edge of the clock, defined as the "0" transition signal. It consequently makes transitions by using both signals compared to the normal PCI that transfers data on individual transitions at each cycle.

Different AGP buses

  • AGP 1x uses a 3.3/1.5 V signal swing over a bandwidth of 266MB/s
  • AGP 2x uses a 3.3/1.5 V signal swing over a bandwidth of 533MB/s
  • AGP 4x uses a 1.5 V signal swing over a bandwidth of 1066 MB/s
  • APG 8x uses a 0.8 V signal swing over a bandwidth of 2.1 GB/s

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