Introduction to the AGP bus

The AGP bus (short for Accelerated Graphics Port) was released in May 1997 for Slot One chipsets, then was later released for Super 7 chips in order to manage graphical data flow, which had grown to large to be handled by a PCI bus. The AGP bus is directly linked to the processor's FSB (Front Side Bus) and uses the same frequency, for increased bandwidth.

The AGP interface was developed specifically to connect with the video card, by opening a direct memory access (DMA) channel to the graphics board, bypassing the input-output controller. Cards which employ this graphics bus theoretically require less on-board memory; because they can directly access graphical data (such as textures) stored in central memory, their cost is hypothetically lower.

Version 1.0 of the AGP bus, which used 3.3 V of power, had a 1X mode that could send 8 bytes every two cycles, and a 2x mode for transferring 8 bytes per cycle.

In 1998, AGP version 2.0 added AGP 4X, which could send 16 bytes per cycle. Version 2.0 of AGP was powered by 1.5 V, and AGP 2.0 "universal" connectors which could support either voltage were released.

AGP version 3.0, released in 2002, doubled the speed of AGP 2.0 with a new AGP 8x mode.

Characteristics of AGP

The AGP 1X port operates at 66 MHz, as opposed to 33 MHz for a PCI bus, giving it a top speed of 264 MB/s (vs. 132 MB/s, shared between all the cards, for PCI). This gives AGP better performance, especially when displaying complicated 3D scenes.

When AGP 4X was released, its speed went up to 1 GB/s. This generation of AGP used 25 W of power. The next generation was named AGP Pro and used 50W.

AGP Pro 8x offers speeds of 2 GB/s.

The transfer speeds for the various AGP standards are:

AGP 1X: 66.66 MHz x 1(coef.) x 32 bits /8 = 266.67 MB/s
AGP 2X: 66.66 MHz x 2(coef.) x 32 bits /8 = 533.33 MB/s
AGP 4X: 66.66 MHz x 4(coef.) x 32 bits /8 = 1.06 GB/s
AGP 8X: 66.66 MHz x 8(coef.) x 32 bits /8 = 2.11 GB/s

It should be noted that each of these AGP standards is backwards-compatible, meaning that AGP 4X or AGP 2X cards can be inserted into an AGP 8X slot.