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.
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.
AGP Connectors
Recent motherboards are built with a general AGP
connector which can be identified by its brown color. There are three
types of connectors:
- AGP 1.5 volt connector:
- AGP 3.3 volt connector:
- Universal AGP connector:
Summary
Here is a table summarising the technical specifications for each version and mode of AGP:
| AGP | Voltage | Mode |
|---|---|---|
| AGP 1.0 | 3.3 V | 1x, 2x |
| AGP 2.0 | 1.5 V | 1x, 2x, 4x |
| AGP 2.0 universal | 1.5 V, 3.3 V | 1x, 2x, 4x |
| AGP 3.0 | 1.5 V | 4x, 8x |
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