AGP - Frequently Asked Questions
(Accelerated Graphics Port)

- Questions -


1-1 : What is AGP?
1-2 : What are the key benefits of AGP?
1-3 : What are AGP 1x and AGP 2x?
1-4 : What is SBA?
1-5 : What is Pipelining?
1-6 : What is DIME?
1-7 : What is "AGP memory"?
1-8 : What is GART?
1-9 : Which applications will benefit from AGP?
1-10 : Will AGP replace PCI?
1-11 : Must I have a Pentium II for AGP?
1-12 : What are the main benefits of an Intel® Pentium® II / 440LX AGPset system?
1-13 : Why is AGP faster compared to PCI?
1-14 : Should I have 2 MB, 4 MB or more Frame Buffer Memory on my AGP graphics card?
1-15 : Which kind of memory should be on the motherboard?
1-16 : Which operating system supports AGP today?
1-17 : Can I use my PCI graphics card with an AGP motherboard?
1-18 : Does AGP also speed up MS-DOS applications?
1-19 : Must the system BIOS support AGP?
1-20 : What's next in the evolution of AGP?
1-21 : Can I use my Monster 3D with an AGP graphic card?
1-22 : Should I buy an AGP graphics card now?
1-23 : Will special applications for AGP graphics come up?
1-24 : What are the software requirements for an AGP-based system?
1-25 : How can I verify my system for the AGP software requirements?


- Answers -


1-1 : What is AGP?
AGP (Accelerated Graphics Port) is a dedicated 66 MHz high-speed interface between the motherboard chipset and the graphics controller on the graphics subsystem. The AGP interface specification allows the graphics subsystem access to dynamically allocated portions of system main memory. From the other side, the CPU can access this so-called AGP memory much faster than sending data over the PCI bus into the frame buffer of the graphics subsystem. This system memory can be used for storing large texture maps and executing texture manipulation commands by the graphics subsystem's direct memory execute (DIME).

1-2 : What are the key benefits of AGP?

• The peak bandwidth of AGP is four times higher (AGP 2X) than on the PCI bus.
• Direct execution of texture maps from system memory vs. requiring textures to be pre-loaded into local video memory via the PCI bus.
• Less PCI bus congestion.
• Improved system concurrency for balanced PC performance. The system processor can perform other activities while the graphics chip is accessing texture data in system memory.
• Saves one PCI slot.

1-3 : What are AGP 1x and AGP 2x?
The AGP is basically twice as fast as the PCI bus. The AGP specification specifies the 66MHz operation of AGP 1x. The AGP has the ability to transfer data on both the rising and falling edges of the clock signal. This doubles the data throughput once again. This operation is called AGP 2x and can transfer 528 MByte per second. AGP 2x is only functional with SBA. Intel® is currently working on an even higher performance mode for AGP, called AGP 4x mode. It will be incorporated into the next revision of the AGP specification and is targeted primarily at workstations and other high-performance platforms.

1-4 : What is SBA?
SBA means Side Band Addressing. In addition to the 32-bit data path, there is an 8-bit control channel. The AGP graphics card can send commands over the control channel even while the host transfers data over the data path. SBA only makes sense with pipelining.

1-5 : What is Pipelining?
Pipelining is a technique that allows sending of more than one data request sequentially before the first request can be served with data. On the PCI bus each request must be served with data before the next request can be sent. The time between request and answer is called latency time and is lost time on PCI. AGP pipelining uses this latency time for sending requests and is more efficient than PCI.

1-6 : What is DIME?
DIME (Direct Memory Execute) is the key to real AGP performance. DIME enables the graphic controller to execute commands directly in the system memory on the motherboard. These are mainly geometry transformations and filter operations for textures. The benefit of DIME is that no part of the frame buffer on the graphics card (off-screen memory) must be reserved for storing texture data. This part of the frame buffer can be used for higher resolution and / or more colors.

1-7 : What is "AGP memory"?
"AGP memory" is a portion of system memory dynamically allocated for quick access by the graphics controller. The CPU is still allowed to write graphics data into this memory area. Writing there can be done much faster than into the local frame buffer via the PCI bus. The latest applications may need anywhere from 2 MB up to 16MB or more for texture storage; by using AGP memory, they can get it.

1-8 : What is GART?
GART (Graphics Address Remapping Table) is a so-called built-in hardware in the motherboard chipset, to allow the graphics controller and its software to see a contiguous space in main memory, when in fact the pages are disjointed.

1-9 : Which applications will benefit from AGP?
All Direct3D applications will benefit from the faster AGP interface and the "Direct Memory Execute" of textures. But new applications written to exploit AGP should see noticeably speedier 3D with far richer texture content.

1-10 : Will AGP replace PCI?
No. PCI is a bus system and AGP is a point-to-point interface. Because of the higher clock rates on AGP, it is not possible to connect more then one device to the chip set. If you have heard that AGP is a PCI extension, this only reflects the fact that the AGP specification was based on the PCI specification.

1-11 : Must I have a Pentium II for AGP?
AGP was first available with Intel® Pentium II CPU and Intel® 440LX chipset. But now we see chipsets from other vendors, VIA AGP chipset ApolloVP3 for example, for socket 7 Pentium systems. Intel® will not provide a socket 7, Pentium processor based AGP solution.

1-12 : What are the main benefits of an Intel® Pentium® II / 440LX AGPset system?
The Intel® Pentium® II with the 440LX AGPset combination delivers the best performance available today and gets the most benefit out of AGP. The Pentium II processor with its Dual Independent Bus (DIB) architecture for the L2 cache and the memory subsystem provides 3 times more bandwidth.

1-13 : Why is AGP faster compared to PCI?

The PCI bus is limited to a maximum bandwidth of 132Mbyte/s. This bandwidth is not available for the graphics subsystem alone. Other I/O devices such as fast SCSI and Ethernet controllers occupy 40MB/s and 12.5MB/s respectively. A 3D application, let's say a game, working with a resolution of 800x600 and 16-bit color depth (HighColor), writing a scene with 25 frame per second, providing bi-linear texture mapping and z-buffering, needs a PCI bandwidth of 350MB/s. In this case the PCI bus becomes hopelessly overloaded. The AGP interface between the add-on graphics accelerator and the AGPset provides a bandwidth of 528 MB/s (AGP 2x). This is really much more than the PCI bus can ever deliver. The 528 MB/s bandwidth is also available between the Intel® Pentium II processor and the AGPset and the System Memory and the AGPset.

1-14 : Should I have 2 MB, 4 MB or more Frame Buffer Memory on my AGP graphics card?
Let's look at the situation for a PCI graphics card first. For a today's typical PCI graphics card (2 MB or 4 MB), the table shows you the amount of local frame buffer (LFB) needed for different resolutions. A 2 MB card will not provide you enough texture RAM. A 4 MB card offers you a typical 800x600 resolution for 3D applications, but the remaining texture RAM is limited. Only with an expensive 8 MB graphics card does the remaining texture RAM become meaningful.
With AGP, textures will be stored in system memory. The selection of the amount of frame buffer for 3D only depends on the resolution used. In general, an inexpensive entry-level 2 MB AGP graphics card could be used for today's 3D applications with large, high-quality textures. Today's standard for higher 3D resolutions is a graphics card with 4 MB or 8 MB of local frame buffer.

1-15 : Which kind of memory should be on the motherboard?
With the Intel® 440LX AGPset on Pentium II motherboards, SDRAM is supported now. This allows faster data streaming between the Pentium II processor, AGP graphics accelerator and PCI devices. If you buy a new AGP motherboard, SDRAM is recommended.

1-16 : Which operating system supports AGP today?
Today, only Windows® 95, OEM Service Release 2.1 supports the AGP features. Windows NT 4.0 does not support AGP features, but if you wish to run NT 4.0 on an AGP motherboard with an AGP graphic card installed, you need to have Service Pack 3 (SP3) installed.

1-17 : Can I use my PCI graphics card with an AGP motherboard?
Yes, there is no limitation on using a PCI graphics card with an AGP motherboard. But the benefits of AGP can only be realized with an AGP graphics card.

1-18 : Does AGP also speed up MS-DOS applications?
MS-DOS does not support virtual addressing. So GART serves no purpose and some DOS application driver work would have to be done to turn on direct access to textures in system memory. The real benefit old applications will see is the faster AGP system speed.

1-19 : Must the system BIOS support AGP?
Yes, the BIOS on the motherboard must support AGP. Otherwise textures cannot be stored in system RAM. Check out your motherboard vendor's web site for the latest available BIOS version and flash it into your system.

1-20 : What's next in the evolution of AGP?
Today's AGP solution is already a milestone in 3D performance on a PC system. Nevertheless, AGP 4x will come soon. Intel® is working on doubling the speed of the AGP interface yet again. On the next generation of AGP motherboards we will see 100MHz SDRAM (today 66MHz) memories. This will speed up the overall system performance and the graphics controller to system memory bandwidth. By the end of the century, a new memory technology with a bandwidth never seen before will be on the motherboards.

1-21 : Can I use my Monster 3D with an AGP graphic card?
Yes of course you can. Your 3D applications accelerated for the Monster 3D will run as usual on the Monster. A Direct3D application will take all of the AGP benefits.

1-22 : Should I buy an AGP graphics card now?
If your system often runs up against its limits with games or other 3D applications, then it is time to break your limits now. Diamond Multimedia offers you today's best AGP graphics solutions. Select the graphics card of your choice. If you visit our web sites frequently, you will see very interesting AGP products.

1-23 : Will special applications for AGP graphics come up?
Yes, because applications will become optimized for AGP systems, taking full advantage of the AGP features, and will have large, high-quality textures. These applications will also run on non-AGP systems, because DirectX 5 provides a set of (texture) memory allocation mechanisms that work on all system configurations. The disadvantage for a non-AGP system will be a noticeable performance decrease.

1-24 : What are the software requirements for an AGP-based system?

• Microsoft® Windows® 95 OSR 2.1 installed
• The GART virtual device driver for your motherboard (e.g. VGART.VXD for Intel® 440LX chip set, or VIA_GART.VXD for VIA Apollo VP3 chipset)
• DirectX 5.0 installed.

1-25 : How can I verify my system for the AGP software requirements?

• To verify that you have OSR 2 installed
  1. From the START menu, point to SETTINGS and click on CONTROL PANEL.
  2. Double-click the SYSTEM control panel.
  3. On the GENERAL tab, check that the system version is 4.00.950B.
• To verify that you have OSR 2.1 installed
  1. From the START menu, point to SETTINGS and click on CONTROL PANEL.
  2. Double-click the ADD/REMOVE Programs control panel.
  3. In the list of installed software, check for USB Support. If it's in the list, you have OSR 2.1 installed.
• To verify the version of the VGART.VXD
  1. Use the Windows® Explorer to open the Windows folder and then the System folder.
  2. Locate the file VGART.VXD (or equivalent) and right-click on it to display the shortcut menu.
  3. Click on properties.
  4. Click on the Version tab (look for v.1.0.16 for VGART.VXD).
• To verify that you have DirextX 5.0 installed.
  1. From the START menu, point to SETTINGS and click on CONTROL PANEL.
  2. Double-click the ADD/REMOVE Programs control panel.
  3. Double-click on DirectX-Driver.
  4. Check you have DirectDraw and Direct3D version 4.05.00.0155 or later.

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