AGP graphic cards in 2007 (page 1: Introduction, the norms, the bridges)

Although the majority of graphic cards today are in PCI Express format, there are still AGP models offered by AMD/ATI and Nvidia or a partner that has taken the initiative to release an AGP version of a recent card. We decided to bring together all recent AGP solutions and compare them to several reference cards in order to help those who might want to make a final update to their AGP system.

Updating an AGP system

Is it worth it and wouldn’t be better to update the entire platform to PCI Express? In the majority of cases this is probably preferable, however, if you own one of the last AGP platforms equipped with a more recent CPU, an update could be an option.

As you probably know, before updating your graphic card it’s important to verify if the rest of the system is up to the challenge for the games that interest you. To do this, most of the time it’s possible to just reduce the resolution to the minimum available and verify that the game is fluid. In general, this will allow you to know if the CPU has high enough performances, so that you don’t needlessly buy a graphic card that won’t improve the situation.

Please note that this method won’t work with some games that are strongly limited by geometric power such as Supreme Commander or even Oblivion. In this case, in lower resolutions the limitation could always comes from the graphic card.

AGP and PCI Express: a recap

Introduced in 1997, AGP underwent several evolutions going from the 1.0 norm to the 3.0/3.5. This interface’s debit rate increased with each evolution going from 2x to 8x, which is supported by the graphic cards that are the subject of today’s article. This means a debit rate of 2.1 GB/s that is shared between the sending and receiving of data. The 32 bit bus is based on an ageing PCI bus structure, and thus is the reason for its replacement. The transfer from the graphic card to the rest of the system has a lower performance as opposed to the flow in opposite direction, which is the more useful.
AGP en bas, PCI Express x16 en haut

The PCI Express connection above, the AGP, below.
PCI Express allows an overall simplification of the process with the transition to a more modern point to point bus. Here, we say, “PCI Express lines” to speak of the bus size. Each of these lines has a rate of 250 Mo/s in each direction. In its 16x version, the PCI Express thus provides 4 Go/s in each direction compared to a total of 2.1 Go/s for AGP 8x. Furthermore, addressing the central memory is simplified and managed by the GPU instead of a GART (Graphic Addressing Remapping Table), whose driver was often a source of problems.

Recent AGP graphic cards fall under the 3.0/3.5 norm and are no longer compatible with the former AGP 3.3v slots. They need a 1.5v slot, supported in theory by all motherboards, which will be the subject of potential updates. Just to be safe, verify that your motherboard follows the 1.5v norm or is universal. The illustration on your right shows the different versions. Just to remind you, the AGP Pro has an extended connection featuring an additional power source, but it has never been used by general public cards. They do, however, still fit in such slots.

AGP/PCIe bridge

GPUs cannot manage PCI Express and AGP at the same time. Both Nvidia and ATI/AMD consequently developed a chip that serves as a bridge between the two buses. These bridges, called « HSI » for Nvidia and “Rialto” for ATI, transform PCI Express signals into AGP signals and vice versa. Therefore, any GPU can support the two buses, at least in theory. In practice, there is still a possibility of incompatibility and driver support may be needed. This may prevent initiatives by partners to release AGP versions of chips that aren’t officially designed by Nvidia or AMD.

Nvidia’s HSI bridge and ATI’s Rialto.