| Member
Join Date: Dec 2005
Posts: 127
| How Your Video Card Works & Common Video Card Myths
Two things I wrote a few weeks ago. Enjoy.
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I've seen all too many people ask why having X amount of RAM on Y card for playing Z game is overkill or underkill, or just having no idea how cards work. So... here's a little guide for ye all. My source is the nVidia GPU Programming Guide, so it may be a tad bit different for ATI cards. Accurate as of 7/08/05.
Ok, first, your CPU sends data to your video card to be cached in RAM. This is what it's there for, to hold pre and post-processed data.
Next, Geometry and Commands are sent to the Pre-T&L Cache. Vertex Shading (T&L) then takes place, and the data is sent to the Post-T&L Cache. Then, this data is sent to Triangle Setup and Rasterization.
Now, what about Textures? They're sent to the... take a guess... Texture Cache. Fragment Shading and Raster Operations are then executed on those Textures and semi-processed Geometry from the Triangle Setup and Rasterization stages.
All this is then sent to the frame buffer, which is located within your VRAM. Now what? The RAMDAC sends this info out through your VGA/DVI port, and the rest is for another guide.
That's the basic overview of how at least an nVidia card works. I would love it if this were pinned, but I doubt that'll happen.
And now, an FAQ: Q: How much VRAM should I have to play Doom 3? BattleField 2? Half-Life 2? Quake?
A: While Quake runs just fine on 64 megs of VRAM, at least 128 megs of VRAM should be used if you wish to smoothly play games such as Doom 3, BattleField 2, and Half-Life 2 (Which will be referred to as D3, BF2, and HL2 respectively from now on) at medium to high texture settings on most maps. 256 megs will allow you to play a bit more smoothly at higher settings, due to having more RAM to cache all the raw data in. Q: Why are there cards with 512 megabytes, or even 640 megabytes of RAM then?
These cards are usually workstation cards. A workstation is usually a very high end computer used to make money. Most are for high end A/V work, CAD, etc. You'll usually see $10k+ applications on these machines. Cards with these huge amounts of RAM usually run on Quadro or Realizm cores, and will cost you a VERY pretty penny or two. Two Quadros in SLI is a scene you'll find in many a high end workstation, where millions of poly's are being rendered every second. Q: What's this SLI you speak of?
A: Quote: |
Originally Posted by nVidia An NVIDIA SLI system includes a PCI Express motherboard that supports two physical connectors that are capable of having two NVIDIA-based PCI Express graphics cards plugged into them. Joined by the NVIDIA SLI connector, the two graphics cards power one monitor, delivering earth-shattering PC performance. Screenshot courtesy of 3DMARK03.
**The NVIDIA SLI connector is only used with GeForce 6600 GT products and above. | Basically, it allows you to use two PCI-E x16 cards (both running at x8) to process data at the same time. The work load is divided among the two at either 60/40, or X/Y axise.
For ATI cards, there's another technology called Crossfire.
Q: What's Crossfire?
A:
Quote: |
Originally Posted by ATI To build your own latest generation multi-GPU system, start with any existing Radeon® X800 or Radeon® X850 graphics card and a CrossFire Ready motherboard, such as those based on the ATI Radeon® Xpress 200 CrossFire chipset. Then add a Radeon CrossFire Edition co-processor board and plug in the external cable to unite multi-GPU power. |
So, it's basically ATI's version of SLI. Except for it has an arguably better rendering more (AFR, or Alternate Frame Rendering).
Q: PCI-E or AGP?
A: Currently, PCI-E is the most futuresafe (even though it isn't... but you didn't hear that from me). Right now, there's really no point to go PCI-E unless you want an extremely high end video card (7800 or X1), or SLI/Crossfire (The latter of which is rumored to be AGP compatible.
And, I'll squash the rumors now: As of the writing of this guide, AGP has not yet bottlenecked video cards, up to the 6800Ultra. So, the next person to say "OMG U NEED XTRA BANDWIDTH!" will be pointed to this bolded section. And no, WINDOWS VISTA DOES NOT REQUIRE 16 LANES OF TRAFFIC IN PCI-E!
Thank you... I hope you learned something from this.
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Ok, I've been seeing too much crap again with people believing rumors about video cards and their functions.
Myth: The human eye can't detect over 30 FPS.
Fact: It varies.
Some people can not detect the difference between 30 FPS with their eyes alone. However, it's very easy to detect an FPS of 100+ when compared to 30 or even 60 FPS. If you're running Counter-Strike at 100 FPS, type in fps_max 30 in the console. You will almost definately see the difference. Also, for gaming, if you're only getting 30 FPS, that will mean that each frame will have a higher lapse of degrees when turning, making it not quite the best for games where quick reflexes are required.
Myth: The 6600GT has a locked pipeline quad.
Fact: No, it doesn't.
The 6600GT comes with 8 pipes (aka two pixel pipeline quads), not 12. Attempting to unlock it is like trying to turn a 7800GT into a GTX, or a 6800 into a 6800GT. They are different cores, and will therefor run differently. However, older Connect3D X800GTOs have been reported to unlock an extra pipeline quad, turning it into an X850XT PE. The same can be done with a Sapphire X800GTO2.
Myth: SLI will give you about twice your usual performance.
Fact: SLI can only offer up to 1.9x your usual graphics performance if a CPU bottleneck is eliminated, according to nVidia.
Why this is, I can't be sure. I personally feel that it's the fact that SLI uses SFR (Split Flame Rendering), so the load isn't equally distributed around both cards. Each card renders a part of a given frame. Crossfire's AFR (Alternate Frame Rendering) works by having each card render a different frame. For example: Card1 will render Frame 0, while Card2 renders Frame 1, then Card1 does Frame 2, Card2 Frame 3, etc. SLI was designed for workstations in the first place, not for gaming.
Myth: Crossfire works sort of like SLI, just throw in two (matched) cards.
Fact: You need a Crossfire master card for Crossfire, along with a board with the RX200 chipset.
A Crossfire master card (not to be confused with the credit card company) is a card without a standard BIOS that is designed to run a Crossfire setup. And no, Crossfire does not need two exact cards. You can run an X800 master card with an X700 for support rendering.
Myth: More VRAM = Better!
Fact: Not true, depending on what you're doing.
VRAM is used by a card to store commands, textures, and rendered materials before the RAMDAC sends the stuff out through your VGA/DVI port. Most games will only need 128 megs of VRAM at medium-high settings, and 256 megs of VRAM comes in handy with max settings, especially with textures. And the 7800GTX with 512 megs of VRAM is NOT better just because of the RAM. It comes significantly overclocked out of the box, at 550 Mhz Core and 850 Mhz (1.7 Ghz effective) Memory. And for those of you who are wondering about the 6200 series cards that have "128 Meg supporting" in the name, the mem controller uses system RAM as artificial VRAM when needed. HOWEVER, for reasons that I can't fully explain, having low amounts of system RAM will almost definately bottleneck higher end graphics cards, such as the 7800GT or GTX. 1.5 to 2 gigs will take away that bottleneck, and then it will be the CPU's job.
Myth: You need an FX-57 at 10 Duotrigintillion Hertz for that card to fully perform!
Fact: Not all cards will be bottlenecked by a CPU.
It's the CPU's job to send data to a video card to be rendered. However, increasing the resolution and upping settings such as Anti-Aliasing or Anisotropic Filtering will help decrease any bottleneck that the CPU may be imposing. Another way to decrease bottlenecks is more RAM, and a good resolution of 2048x1536 (1600x1200 works too). Another way is to end all non-crucial processes before gaming or benchmarking. This takes load off the CPU, and allows it to send more data to the card. And, another way to remove a CPU bottleneck: Don't buy top of the line cards. Few games actually need 6 pixel quads, and 7 vertex shaders to run reasonably well. Just because you can afford it, do you really need it? |