AGP - A New Interface for Graphic Accelerators

What's The Beef ?

So far so good, let's now summarize the benefits AGP is offering:

  • higher bandwidth than PCI, up to 4 times as high
  • no sharing of bandwidth with other components like in case of PCI
  • DIME, direct memory execution of textures
  • CPU accesses to system RAM can proceed concurrently with the graphics chip's AGP RAM reads
  • Allowing the CPU to write directly to shared system AGP memory when it needs to provide graphics data, such as commands or animated textures. Generally the CPU can more quickly access main memory than it can graphics local memory via AGP, and certainly faster than via the PCI bus.

Obviously it doesn't take a Pentium II to provide the needs for an AGP system. This is why Socket 7 systems with AGP (e.g. upcoming VIA Apollo VP3 chipset) will do just the same as the AGP provided by the 440LX chipset for Pentium II platforms.

Software Considerations

Unfortunately, getting an AGP board plus an AGP graphic accelerator won't be enough to take advantage of AGP's new performance. Nothing goes without a proper operating system which has to take care of particularly the DIME/GART part of the AGP benefits. The OS has to provide main memory for the AGP RAM and has to monitor that main memory is still enough for the running applications. This shall be achieved via DirectDraw of Memphis (Windows98) and Windows NT 5 . As long as these operating systems aren't out, nobody will be able to take advantage of the DIME and hence only half of the AGP benefits are used.

AGP - Some Critical Thoughts

The number one benefit from AGP is supposed to be the DIME feature, which is meant to save video RAM onboard the graphics adapter. There are some doubts however, where I'm wondering if this idea will turn out to be as wonderful as it sounds. We have learned that AGP offers a theoretical peak throughput of 528 MB/s using 'x2' mode and the next 'x4' mode is already planned. This mode would offer a throughput of about 1 GB/s, isn't that amazing? There is a little problem we easily forget though. This throughput is meant to transport data from main memory to the graphic accelerator. Now currently the maximum throughput of main memory to the CPU at 66 MHz bus clock is exactly these 528 MB/s. You certainly don't expect that the whole system is doing nothing while the graphic accelerator is accessing the main memory via DIME, do you? Whilst the graphics accelerator is doing its work, the CPU and other DMA using devices are accessing main memory just as well of course. Therefore AGP will never be able to get a throughput of 528 MB/s, since this is the whole bandwidth of main memory and thus it has to be shared with CPU and others . If you see it in a very simple statistical way you can't expect that AGP will get more of that main memory bandwidth than 50% = 264 MB/s. What is the 'x2' mode good for then? These above averaged 528 MB/s bandwidth of main memory are already only valid for SDRAM systems. EDO is considerably slower, let alone good old FPM. What AGP really needs is the 100 MHz bus!! This bus will offer 800 MB/s bandwidth with SDRAM and so AGP could get a good share of it. Hence there's not much value in going on about 'x1' or 'x2' mode AGP graphic cards currently, since there's simply no technical chance that data could be transfered at the speed 'x2' mode is offering in 66 MHz bus speed systems. What does this mean for us? 'Let's wait again!!' Let's wait for the 440BX or VIA Apollo VP4 chipset, both using 100 MHz system bus.

There's one other consideration as well. Modern VRAM or WRAM cards as well as RAMBUS RAM cards are offering a video memory (onboard, LFB or local memory) bandwidth of up to 1.6 GB/s (e.g. Number Nine Revolution 3D, 128 bit port WRAM). This is much more than even 'x4' mode will offer. These cards will be faster if they are using their local memory for texture processing rather than the much slower AGP RAM. This means that high end cards will work just as PCI cards in the past, only taking advantage of the higher data transfer speed of AGP, no DIME used. Intel thinks that this will be more expensive, but isn't it funny ... RAM prices are lower than ever. This should not really be a reason for a more expensive card.