Karbosguide.com - Module 7b.1

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
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  • 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

    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

    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

    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 386s and early 486s, which also had limited power.


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    To learn more

    Read about monitors in Module 7a.

    Read about sound cards in Module 7c .

    Read about digital sound and music in Module 7d .

    Read about FPU work in 3D graphics.

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