Accelerated Graphics Port
The Accelerated Graphics Port (AGP ) is a standard interface for PC -Mainboards for direct connection of the graphics card with the chipset / Northbridge. It is based on the PCI bus technology.
Properties
Once the graphics card memory is no longer sufficient, AGP uses the normal working memory ( RAM ) of the PC, there to be deposited, for example, monitor image, textures, Z-buffer, alpha blending and other graphic data. So-called " bursts " were used in this contiguous memory management a quick reading of the data. This AGP reduces the cost as a high -end graphics system. It also allows more efficient use of the framebuffer memory and speeds up as likewise 2D graphics. Some RAID controllers, the AGP slot is used due to its higher transfer rate than slot instead of PCI, although he has not actually designed for it. AGP is not a bus, but a point-to- point connection. ( Which must be then each one has its own controller on the motherboard assigned ) There are a few motherboards with multiple independent AGP slots, but normally only one AGP slot is available.
Development
AGP was developed by Intel and first appeared in 1997 i440LX chipset for the Pentium II processor used. From 1998 AGP appeared widely used in commercial PCs, which were equipped with chipsets from other manufacturers. On the motherboard the AGP slot and the AGP Bridge is essentially required, which is mostly located in the Northbridge of the chipset. The graphics card is connected in this way, the fastest way with the CPU and the main memory, both for the graphics performance of essential importance.
Versions
The first version of AGP, AGP 1.0, appeared in 1997 with the modes AGP 1x and 2x AGP. AGP 1x transfers per clock 32 bits of data at a clock frequency of doubled over PCI 66 MHz and achieves a theoretical transfer rate of 266 megabytes per second. AGP 2x transmits the data in the DDR method and obtained at the same clock frequency 533 MB / s The signal voltage for AGP 1.0 is 3.3 volts. The fastest compatible cards based on the ATI Radeon 9700 - or nVidia GeForce FX GPUs.
AGP 2.0 ( 1999) also offered the AGP 4x mode with a maximum transfer rate of 1066 MB / s at a reduced signal voltage of 1.5 volts. The increase in speed has been achieved by now four data packets were transmitted per clock. Most video cards of this generation were fully backwards compatible with the older standard and could be operated on main boards with 3.3 V at AGP 2x. These models are named after the AGP specification universal AGP cards and have two notches in their terminal strip.
AGP 3.0 (2002 ) reached in the new mode AGP 8x at only 0.8 volt signal voltage with 2133 MB / s maximum AGP transfer rate, instead of four eight data packets per clock now been transferred. The 1x and 2x modes no longer exist in this standard. Virtually all video cards of this generation, however, support for the operation in older mainboards also AGP 2.0 1.5 V and the slower modes ( "Universal 1.5V AGP 3.0" ), some even AGP 1.0 3.3 V ( "Universal AGP 3.0" ) - that is recognizable by the two notches in the terminal strip. Since the mechanical coding of AGP 3.0 models of 2.0 is identical to tolerate all 3.0 cards installed in 2.0 systems, but non- universal card not start it. Older cards cause in 3.0 motherboards due to the already foreseen in the standard protective circuit also no harm, but only start at Universal models as it were named.
AGP 3.5 (2004 ) introduced no new transfer modes, but devoted himself to the stabilization of the reaction. Uniform standards for GART and Aperture (see AGP terms ) should bring a uniform AGP drivers for all motherboards, which moved in the form of the file uagp35.sys in Windows XP SP2. The chipset driver of their own previously used repeatedly led to problems with AGP Fast Writes and options such as faster transfer modes. Also dual AGP motherboards for interconnection of multiple graphics cards should be possible, but that came through the development of PCI Express and based on it, multi-graphics card technologies (eg Crossfire or SLI ) is hardly to bear.
In addition, there are still cards that are based on the AGP Pro slot so-called. The contact strips are slightly longer than the normal AGP cards. AGP Pro cards supplied with up to 50 W instead of 25 watts normal slot. As an alternative to power-hungry graphics cards can be powered externally via a specially mounted power supply ( 5V/12V, Molex connector), what the solution was more frequently used due to the small number of pro- slots. One AGP Pro slot also accepts normal AGP cards and was usually found in semi-professional workstation area.
AGP cards have a notch at different points in the plug, in which the corresponding web has to fit in the slot on the motherboard. In AGP 1.0 (AGP 1x/2x ), the web sits further towards slot bracket from AGP 2.0, however, in the direction of the housing front wall. This is to prevent cards with 3.3V signal voltage in motherboards with only 1.5 are plugged or 0.8V signal voltage, otherwise damage at start- threatening ( see next paragraph).
Incompatibilities
The assignment of the lands to the signal voltages accidental incorporation of incompatible graphics card should be physically prevented. However, in both a manufacturer of graphics cards and motherboards installed an AGP universal connector, even though their product was necessarily linked to an AGP version, and thus voltage. One thus possible incorporation of incompatible cards leads most likely to damage the graphics card and motherboard components.
So some TNT2 and SiS305 - based graphics cards were produced that work with AGP 1.0, due to their misleading universal connection with two scores but also in motherboards with pure AGP 2.0 connector fit. However, this pure AGP 2.0 motherboards are incompatible with the intended version 1.0 for voltage. Therefore, some of this motherboard models equipped with a protection circuit to prevent in such 3.3 V cards have a start with concomitant damage. This case, they show mostly acoustically or via LED.
On the motherboard side, there were also models with the Intel 845 and 850 - and nVidia nForce chipset, which accidentally had an AGP universal connector, although they do not support AGP 1.0. Again, the so possible insertion of a 3.3 V card resulted mostly a voltage caused damage to both components. The use of these universal connections could be due to the fact universally compatible previous models of motherboard chipsets. Has supported the Intel 815 chipset AGP 1.0 and 2.0 cards. He appeared two years before the i845, which severed the support for version 1.0, which all 3.3V cards.
AGP 3.0 cards have standard according to the same port as 2.0 cards, but therefore have gaps on the intended protection circuit to not take damage when operating in 2.0 motherboards with those higher voltage. For compatibility see the above paragraph to AGP 3.0.
AGP terms
Aperture size is an amount of memory that can be used for storing textures of the graphics chip on the AGP port. He is also referred to as AGP memory and should allow cheap graphic card with lesser amounts of local, installed on the graphics card memory. The Aperture area is not swapped out by the operating system to ensure high speed, compared to the local graphics memory but slower ( in a concrete example, the rendering performance declined thus around 30% off ). There are two modes of use, its name is often mixed. In DiME mode (Direct Memory Execute), the graphics chip only use textures from the Aperture section for completion of an image, its local store is used exclusively for rendering and frame buffer. The Intel i740 chip used this mode to demonstrate the Aperture, although he was also the Dimel mode (Direct Memory Execute and Local Memory) capable. This mode allows the sensible combination of local and AGP memory. For this purpose, the most commonly used textures are kept locally, which corresponds to a texture cache of the graphics chip. This approach dominated previously also some PCI graphics cards; However, AGP caused Dimel with the advantage of the faster AGP bandwidth to memory and the security that textures are not swapping to disk.
Memory system called normal working outside the aperture size, which is like their predecessors also available AGP graphics cards. Probably the first chip that used this memory, was the exotic in many ways nVidia NV1.
GART - The to offload textures provided Aperture Size is reserved in memory only when needed. She previously available to the system for other purposes. If then they used, which located in the motherboard chipset memory controller can hardly find a contiguous memory section of this size. Therefore, the data is distributed ( fragmented) stored. The graphics chip but a contiguous memory is mediated. The translation takes a GART (Graphics Address Remapping Table). This is similar to the memory management unit of a main processor.
Side Band Addressing is the delayed delivery of addresses on the AGP. On these eight " Sideband lines" of the graphics chip can request new data while the current is still transferred to the 32 address and data lines. Thus, the new data directly after sending the current available.
Fast writes was only introduced with AGP 2.0 and first implemented by the Nvidia GeForce 256 graphics cards. This graphics card and CPU can exchange data directly, instead of writing them only in memory and can be read by the receiver again from there. This transfer duplication without fast writes thus effectively halved the RAM bandwidth. Comes to fruition that once the graphics card requests for the corresponding number of data. Fast writes fixes this and includes all data exchange, 2D as 3D. Often supports the graphics card but AGP modes that they can not utilize your computing power. Especially in the high -level segment AGP modes were implemented more because of their marketing impact. Missing here is the fast writes feature which therefore causes as little detriment as with all AGP motherboards, where the RAM bandwidth corresponds to about twice the AGP data rate ( as in models with dual-channel DDR). In addition, the game manufacturer stayed mostly on a bandwidth-saving programming as possible, so that the maximum capacity of the AGP, and thus the need for fast writes came less frequently. Challenge in the fast writes - implementation is a clean data signal. Therefore, Intel 815 chipset disabled in this function as well as the ATI Catalyst drivers in diagnosed as unstable systems. A corresponding test he takes after his installation. Fast writes is down by setting the AGP mode often possible where it was unstable before, and so can still help to a speed increase. However, this requires individual measurements using benchmarks. Fast writes is up available for the AGP 2x mode.
AGP as a discontinued model
With the latest version of AGP 8x, this technique is designed to end, because the speed can not be further increased due to timing issues at high clock rates, which result from the parallel data transmission, and thus increasingly sophisticated board design. Also, the special feature of AGP to offer a quick point-to- point connection to memory, has now lost its importance because most graphics cards are so well equipped by the sharp decline in prices for memory chips. In addition, AGP is designed as a special solution for graphics cards, but also more and more other components, which were until then still connected via PCI or PCI -X, have higher requirements in terms of data transfer rate. Among other things, this motivation that the PCI -SIG has designed the standard PCI Express, the heir to both PCI and AGP, offering an even greater data transfer rate than AGP. Since mid- 2006, new motherboards were presented with AGP support barely even graphics cards for the AGP slot are rare.