AGP

Short for Accelerated Graphics Port, AGP is an advanced port designed for Video cards and 3D accelerators. Designed by Intel and introduced in August of 1997 AGP introduces a dedicated point-to-point channel so that the graphics controller can directly access the system memory. Below is an illustration of what the AGP slot may look like on your motherboard.

The AGP channel is 32-bits wide and runs at 66 MHz. This translates into a total bandwidth of 266 MBps, which is much greater than the PCI bandwidth of up to 133 MBps. AGP also supports two optional faster modes, with throughput of 533 MBps and 1.07 GBps. It also allows 3-D textures to be stored in main memory rather than video memory.

Each computer with AGP support will either have one AGP slot or on-board AGP video. If you needed more than one video card in the computer, you can have one AGP video card and one PCI video card or use a motherboard that supports SLI.

AGP [Accelerated Graphics Port] is a Point-to-Point [Chip-to-Chip] bus using 1.5 Volt or 3.3V signaling. The main use of the AGP bus is as a Local Video bus in IBM compatible Personal Computers [PCs]. The AGP interface bus is based on the PCI [Peripheral Component Interface] spec, using the PCI specification as an operational baseline. The AGP specification adds 20 additional signals not included in the PCI bus. The AGP specification defines the Protocol, Electrical and Mechanical aspects of the bus. Refer to this page for a comparison of Video bus through-put for different expansion buses.

The Mechanical definitions include a connector and AGP Board [Add-in card]. The Card sizes and 1.5v and 3.3v connectors are also defined within the spec. There are five connectors defined: AGP 3.3v, AGP 1.5v, AGP Universal, AGP Pro Universal, AGP Pro 3.3v, and AGP Pro 1.5v. PCI and AGP boards are not mechanically interchangeable.

The AGP 1.0 specification defined 1x and 2x speeds with the 3.3v keyed connector.
The AGP 2.0 specification defined 1x, 2x and 4x speeds with the 3.3v, or 1.5v keyed connector or a 'Universal' connector which supported both card types.
The AGP Pro specification defined 1x, 2x and 4x speeds with the 3.3v, or 1.5v keyed connector or a 'Universal' connector which supported both card types.
The AGP 3.0 specification defined 1x, 2x, 4x and 8x speeds with the 1.5v keyed connector or a 1.5v AGP Universal / Pro connector.
Each up-grade is a supper-set of the 1x mode, so 4x will also support the 1x speed. The base clock rate is 66MHz, but to achieve to 2x, 4x, and 8x speeds the clock is doubled each time. AGP uses both edges of the clock to transfer data.

AGP (1x): 66MHz clock, 8 bytes/clock, Bandwidth: 266MB/s [3.3V or 1.5V signal swing]
AGP 2x: 133MHz clock, 8 bytes/clock, Bandwidth: 533MB/s [3.3V or 1.5V signal swing]
AGP 4x: 266MHz clock, 16 bytes/clock, Bandwidth: 1066MB/s [1.5V signal swing]
AGP 8x: 533MHz clock, 32 bytes/clock, Bandwidth: 2.1GB/s [0.8V signal swing], still uses 1.5 volt motherboard power

The AGP data bus may be 8, 16, 24, 32, or 64 bits. Due to timing requirements the maximum bus length is 9". The trace impedance is specified as 65 ohms +/- 15 ohms (no termination resistor is specified). For the 8x speed the bus requires a parallel termination or 50 ohms. Some lines may require a Pull-Up Resistor to insure the lines come out of reset in the proper state. The AGP Interface is optimized for FR4 PCB designs. Both 4 layer and 6 layer PCBs have been studied.