[转]How Graphics Cards Work

原文地址:http://computer.howstuffworks.com/graphics-card.htm

by Jeff Tyson and Tracy V. Wilson
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1. Introduction to How Graphics Cards Work

The images you see on your monitor are made of tiny dots called pixels. At most common resolution settings, a screen displays over a million pixels, and the computer has to decide what to do with every one in order to create an image. To do this, it needs a translator -- something to take binary data from the CPU and turn it into a picture you can see. Unless a computer has graphics capability built into the motherboard, that translation takes place on the graphics card.

A graphics card's job is complex, but its principles and components are easy to understand. In this article, we will look at the basic parts of a video card and what they do. We'll also examine the factors that work together to make a fast, efficient graphics card.

Think of a computer as a company with its own art department. When people in the company want a piece of artwork, they send a request to the art department. The art department decides how to create the image and then puts it on paper. The end result is that someone's idea becomes an actual, viewable picture.

A graphics card works along the same principles. The CPU, working in conjunction with software applications, sends information about the image to the graphics card. The graphics card decides how to use the pixels on the screen to create the image. It then sends that information to the monitor through a cable.
Creating an image out of binary data is a demanding process. To make a 3-D image, the graphics card first creates a wire frame out of straight lines. Then, it rasterizes the image (fills in the remaining pixels). It also adds lighting, texture and color. For fast-paced games, the computer has to go through this process about sixty times per second. Without a graphics card to perform the necessary calculations, the workload would be too much for the computer to handle.

The graphics card accomplishes this task using four main components:

A motherboard connection for data and power
A processor to decide what to do with each pixel on the screen
Memory to hold information about each pixel and to temporarily store completed pictures
A monitor connection so you can see the final result

Next, we'll look at the processor and memory in more detail.

2. Processor and Memory

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Graphics cards take data from the CPU and turn it into pictures. Find out the parts of a graphics card and read expert reviews of graphics cards.
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Like a motherboard, a graphics card is a printed circuit board that houses a processor and RAM. It also has an input/output system (BIOS) chip, which stores the card's settings and performs diagnostics on the memory, input and output at startup. A graphics card's processor, called a graphics processing unit (GPU), is similar to a computer's CPU. A GPU, however, is designed specifically for performing the complex mathematical and geometric calculations that are necessary for graphics rendering. Some of the fastest GPUs have more transistors than the average CPU. A GPU produces a lot of heat, so it is usually located under a heat sink or a fan.

In addition to its processing power, a GPU uses special programming to help it analyze and use data. ATI and nVidia produce the vast majority of GPUs on the market, and both companies have developed their own enhancements for GPU performance. To improve image quality, the processors use:

Full scene anti aliasing (FSAA), which smoothes the edges of 3-D objects
Anisotropic filtering (AF), which makes images look crisper

Each company has also developed specific techniques to help the GPU apply colors, shading, textures and patterns.

As the GPU creates images, it needs somewhere to hold information and completed pictures. It uses the card's RAM for this purpose, storing data about each pixel, its color and its location on the screen. Part of the RAM can also act as a frame buffer, meaning that it holds completed images until it is time to display them. Typically, video RAM operates at very high speeds and is dual ported, meaning that the system can read from it and write to it at the same time.

The RAM connects directly to the digital-to-analog converter, called the DAC. This converter, also called the RAMDAC, translates the image into an analog signal that the monitor can use. Some cards have multiple RAMDACs, which can improve performance and support more than one monitor. You can learn more about this process in How Analog and Digital Recording Works.

The RAMDAC sends the final picture to the monitor through a cable. We'll look at this connection and other interfaces in the next section.

3. Input and Output

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Photo courtesy of HowStuffWorks Shopper
This Radeon X800XL graphics card has DVI, VGA and ViVo connections.
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Graphics cards connect to the computer through the motherboard. The motherboard supplies power to the card and lets it communicate with the CPU. Newer graphics cards often require more power than the motherboard can provide, so they also have a direct connection to the computer's power supply.

Connections to the motherboard are usually through one of three interfaces:

PCI Express is the newest of the three and provides the fastest transfer rates between the graphics card and the motherboard. PCIe also supports the use of two graphics cards in the same computer.

Most graphics cards have two monitor connections. Often, one is a DVI connector, which supports LCD screens, and the other is a VGA connector, which supports CRT screens. Some graphics cards have two DVI connectors instead. But that doesn't rule out using a CRT screen; CRT screens can connect to DVI ports through an adapter. At one time, Apple made monitors that used the proprietary Apple Display Connector (ADC). Although these monitors are still in use, new Apple monitors use a DVI connection.

Most people use only one of their two monitor connections. People who need to use two monitors can purchase a graphics card with dual head capability, which splits the display between the two screens. A computer with two dual head, PCIe-enabled video cards could theoretically support four monitors.

In addition to connections for the motherboard and monitor, some graphics cards have connections for:

TV display: TV-out or S-video
Analog video cameras: ViVo or video in/video out
Digital cameras: FireWire or USB

Some cards also incorporate TV tuners. Next, we'll look at how to choose a good graphics card.

4. Choosing a Good Graphics Card

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Photo courtesy of HowStuffWorks Shopper
Some cards, like the ATI All-in-Wonder, include connections for televisions and video as well as a TV tuner.
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A top-of-the-line graphics card is easy to spot. It has lots of memory and a fast processor. Often, it's also more visually appealing than anything else that's intended to go inside a computer's case. Lots of high-performance video cards are illustrated or have decorative fans or heat sinks.

But a high-end card provides more power than most people really need. People who use their computers primarily for e-mail, word processing or Web surfing can find all the necessary graphics support on a motherboard with integrated graphics. A mid-range card is sufficient for most casual gamers. People who need the power of a high-end card include gaming enthusiasts and people who do lots of 3-D graphic work.

A good overall measurement of a card's performance is its frame rate, measured in frames per second (FPS). The frame rate describes how many complete images the card can display per second. The human eye can process about 25 frames every second, but fast-action games require a frame rate of at least 60 FPS to provide smooth animation and scrolling. Components of the frame rate are:

Triangles or vertices per second: 3-D images are made of triangles, or polygons. This measurement describes how quickly the GPU can calculate the whole polygon or the vertices that define it. In general, it describes how quickly the card builds a wire frame image.
Pixel fill rate: This measurement describes how many pixels the GPU can process in a second, which translates to how quickly it can rasterize the image.

The graphics card's hardware directly affects its speed. These are the hardware specifications that most affect the card's speed and the units in which they are measured:

GPU clock speed (MHz)
Size of the memory bus (bits)
Amount of available memory (MB)
Memory clock rate (MHz)
Memory bandwidth (GB/s)
RAMDAC speed (MHz)

The computer's CPU and motherboard also play a part, since a very fast graphics card can't compensate for a motherboard's inability to deliver data quickly. Similarly, the card's connection to the motherboard and the speed at which it can get instructions from the CPU affect its performance.

For more information on graphics cards and related topics, check out the links on the following page.

5. Lots More Information

The video card

The contents:

An introduction to the video card

The video card supports the CPU

About RAM on the video card

RAMDAC or digital?

Heavy data transport

Three components in a videocard

The video card is just as important as the screen – and more often overlooked. During the years 1999-2001 the overall quality of video adapters have been improved. Earlier there was some very lousy products in the market. Follow my articles to know more of the video adapter!

A video card is typically an adapter, a removable expansion card in the PC. Thus, it can be replaced!

The video card can also be an integral part of the system board This is the case in certain brands of PCs and is always the case in lap tops. I have a clear preference for a replaceable video card in my stationary PC. However modern motherboard may include good integrated video chip sets. You just have to know which ones!

Regardless of whether it is replaceable or integrated, the video adapter consists of three components:

A video chip set of some brand (ATI, Matrox, Nvidia, S3, Intel, to name some of the better known). The video chip creates the signals, which the screen must receive to form an image.

Some kind of RAM (EDO, SGRAM, or VRAM, which are all variations of the regular RAM). Memory is necessary, since the video card must be able to remember a complete screen image at any time. Using AGP, the video card may use the main memory of the motherboard.

A RAMDAC - a chip converting digital/analog signals. Using Flat panel monitors, you do not need a the function of a RAMDAC.

The video card supports the CPU

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The video card provides a support function for the CPU. It is a processor like the CPU. However it is especially designed to control screen images.

You could produce a PC without a video controlling chip and leave this work to the CPU. However, the CPU would be constantly occupied running the software that should generate screen images.

RAM on the video card

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Video cards always have a certain amount of RAM. This RAM is also called the frame buffer. Today video cards hold plenty of RAM, but before it was more important:

How much RAM? That is significant for color depth at the highest resolutions.

Which type RAM? This is significant for card speed.

Video card RAM is necessary to keep the entire screen image in memory. The CPU sends its data to the video card. The video processor forms a picture of the screen image and stores it in the frame buffer. This picture is a large bit map. It is used to continually update the screen image.

The amount of RAM

Older video cards were typically available with 1, 2, 4 or more MB RAM. How much is necessary? That depends primarily on how fine a resolution you want on your screen. For ordinary 2D use, 16 bit colors are "good enough." Let us look at RAM needs for different resolutions:

Resolution	Bit map size with 16 bit colors	Necessary RAM on the video card
640 x 480	614,400 bytes	1 MB
800 x 600	960,000 bytes	1.5 MB
1024 x 768	1,572,864 bytes	2 MB
1152 x 864	1,990,656 bytes	2.5 MB
1280 x 1024	2,621,440 bytes	3 MB
1600 x 1200	3,840,000 bytes	4 MB

Note that the video RAM is not utilized 100% for the bit map. Therefore, 1 MB is not enough to show a 800 x 600 picture with 16 bit colors, as the above calculation could lead you to believe.

Today video cards come with 4 MB, 8 MB or more RAM.

Using ordinary RAM, you saw speed improvements of the graphics card using 4 MB instead of 2 MB, if the resolution only was 800 x 600 or 1024 x 768. In this case data can be written to and read from the RAM simultaneously - using different RAM cells. With only 2 MB RAM, data sometime had to wait for a free cell.

3D - lots of RAM

Supporting the demand for high quality 3D performance many new cards come with a frame buffer of 16 or 32 MB RAM. And they use the AGP interface for better bandwidth and access to the main memory.

VRAM

Briefly, in principle all common RAM types can be used on the video card. Most cards use very fast editions of ordinary RAM (SDRAM or DDR).

Some high end cards (like Matrox Millennium II) earlier used speciel VRAM (Video RAM) chips. This was a RAM type, which only was used on video cards. In principle, a VRAM cell is made up of two ordinary RAM cells, which are "glued" together. Therefore. you use twice as much RAM than otherwise. VRAM also costs twice as much. The smart feature is, that the double cell allows the video processor to simultaneously read old and write new data on the same RAM address. Thus, VRAM has two gates which can be active at the same time. Therefore, it works significantly faster.

With VRAM you will not gain speed improvements increasing the amount of RAM on the graphics controller. VRAM is already capable of reading and writing simultaneously due to the dual port design.

UMA and DVMT

On some older motherboards the video controller was integrated. Using SMBA (Shared Memory Buffer Architecture) or UMA (Unified Memory Architecture ) parts of the system RAM were allocated and used as frame buffer. But sharing the memory was very slow and the standards never became very popular.

A newer version of this is found in Intel chip set 810 and the better 815, which also integrates the graphics controller and use parts of the system RAM as frame buffer. Here the system is called Dynamic Video Memory Technology (D.V.M.T.).

The RAMDAC

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All traditional graphics cards have a RAMDAC chip converting the signals from digital to analog form.

CRT monitors work on analog signals. The PC works with digitized data which are sent to the graphics adapter. Before these signals are sent to the monitor they have to be converted into analog output and this is processed in the RAMDAC:

The reccommandation on a good RAMDAC go like this:

External chip, not integrated in the VGA chip

Clock speed: 250 - 360 MHz.

Heavy data transport

The original VGA cards were said to be "flat." They were unintelligent. They received signals and data from the CPU and forwarded them to the screen, nothing else. The CPU had to make all necessary calculations to create the screen image.

As each screen image was a large bit map, the CPU had to move a lot of data from RAM to the video card for each new screen image.

The graphic interfaces, like Windows , gained popularity in the early nineties. That marked the end of the "flat" VGA cards. The PC became incredibly slow, when the CPU had to use all its energy to produce screen images. You can try to calculate the required amount of data.

A screen image in 1024 x 768 in 16 bit color is a 1.5 MB bit map. That is calculated as 1024 x 768 x 2 bytes. Each image change (with a refresh rate of 75 HZ there is 75 of them each second) requires the movement of 1.5 MB data. That zaps the PC energy, especially when we talk about games with continual image changes.

Furthermore, screen data have to be moved across the I/O bus. In the early nineties, we did not have the PCI and AGP buses, which can move large volumes of data. The transfer went through the ISA bus, which has a very limited width (read in module 2b about the buses). Additionally the CPUs were 386’s and early 486’s, which also had limited power.

posted @ 2009-02-24 22:42 -> 阅读(534) 评论(0) 收藏举报

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[转]How Graphics Cards Work

The video card

Three components in a videocard

The video card supports the CPU

RAM on the video card

The amount of RAM

3D - lots of RAM

VRAM

UMA and DVMT

The RAMDAC

Heavy data transport

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