Components on Video Card
All video display adapters contain certain basic components, such as the following:
-
Digital-to-analog converter (DAC). Formerly a separate chip, the DAC is usually incorporated into the video processor/accelerator chip on recent chipsets. The DAC is not necessary on a purely digital subsystem (digital video card and display), but because most display subsystems have an analog VGA video card, an analog display, or both, video cards will continue to have DAC features for some time to come.
Figure below indicates the locations of many of these components on a typical video card. Note that the acronym GPU refers to the graphics processing unit.
Virtually all video adapters on the market today use chipsets that include 3D acceleration features. The following sections examine these components and features in greater detail.
Video BIOS
Video adapters include a BIOS that is similar in construction but completely separate from the main system BIOS. (Other devices in your system, such as SCSI adapters, might also include their own BIOS.) If you turn on your monitor first and look quickly, you might see an identification banner for your adapter's video BIOS at the very beginning of the system startup process.
Similar to the system BIOS, the video adapter's BIOS takes the form of a ROM (read-only memory) chip containing basic instructions that provide an interface between the video adapter hardware and the software running on your system.
The software that makes calls to the video BIOS can be a standalone application, an operating system, or the main system BIOS. The programming in the BIOS chip enables your system to display information on the monitor during the system POST and boot sequences, before any other software drivers have been loaded from disk.
The video BIOS also can be upgraded, just like a system BIOS, in one of two ways. The BIOS uses a rewritable chip called an EEPROM (electrically erasable programmable read-only memory) that you can upgrade with a utility the adapter manufacturer provides.
On older cards, you might be able to completely replace the chip with a new one—again, if supplied by the manufacturer and if the manufacturer did not hard solder the BIOS to the printed circuit board. Most recent video cards use a surface-mounted BIOS chip rather than a socketed chip.
A BIOS you can upgrade using software is referred to as a flash BIOS, and most current-model video cards that offer BIOS upgrades use this method. Video BIOS upgrades (sometimes referred to as firmware upgrades) are sometimes necessary in order to use an existing adapter with a new operating system, or when the manufacturer encounters a significant bug in the original programming.
Occasionally, a BIOS upgrade is necessary because of a major revision to the video card chipset's video drivers. As a general rule, the video BIOS is a component that falls into the "if it ain't broke, don't fix it" category. Try not to let yourself be tempted to upgrade just because you've discovered that a new BIOS revision is available. Check the documentation for the upgrade, and unless you are experiencing a problem the upgrade addresses, leave it alone.
Video Processor
The video processor, or chipset, is the heart of any video adapter and essentially defines the card's functions and performance levels. Two video adapters built using the same chipset often have many of the same capabilities and deliver comparable performance.
Also, the software drivers that operating systems and applications use to address the video adapter hardware are written primarily with the chipset in mind. You often can use a driver intended for an adapter with a particular chipset on any other adapter using the same chipset.
Of course, cards built using the same chipset can differ in the amount and type of memory installed, so performance can vary. Since the first VGA cards were developed, several main types of processors have been used in video adapters.
Video Memory
Most video adapters rely on their own onboard memory that they use to store video images while processing them; although the AGP specification supports the use of system memory for 3D textures, this feature is seldom supported now that video cards routinely ship with 32MB, 64MB, or more of onboard memory.
Many low-cost systems with onboard video use the universal memory architecture (UMA) feature to share the main system memory. In any case, the memory on the video card or borrowed from the system performs the same tasks. The amount of memory on the adapter or used by integrated video determines the maximum screen resolution and color depth the device can support.
You often can select how much memory you want on a particular video adapter; for example, 32MB, 64MB, and 128MB are common choices today. Although adding more memory is not guaranteed to speed up your video adapter, it can increase the speed if it enables a wider bus (from 64 bits wide to 128 bits wide) or provides nondisplay memory as a cache for commonly displayed objects.
It also enables the card to generate more colors and higher resolutions and, for AGP cards (see the following), allows 3D textures to be stored and processed on the card, rather than in slower main memory.
SGRAM, SDRAM, DDR, and DDR-II SDRAM—which are derived from popular motherboard memory technologies—have replaced VRAM, WRAM, and MDRAM as high-speed video RAM solutions. Their high speeds and low production costs have enabled even inexpensive video cards to have 16MB or more of high-speed RAM onboard.
SDRAM
Synchronous DRAM (SDRAM) is the same type of RAM used on many current systems based on processors such as the Pentium III, Pentium 4, Athlon, and Duron. The SDRAMs found on video cards are usually surface-mounted individual chips; on a few early models, a small module containing SDRAMs might be plugged into a proprietary connector.
This memory is designed to work with bus speeds up to 200MHz and provides performance just slightly slower than SGRAM. SDRAM is used primarily in current low-end video cards and chipsets such as NVIDIA's GeForce2 MX and ATI's RADEON VE.
SGRAM
Synchronous Graphics RAM (SGRAM) was designed to be a high-end solution for very fast video adapter designs. SGRAM is similar to SDRAM in its capability to be synchronized to high-speed buses up to 200MHz, but it differs from SDRAM by including circuitry to perform block writes to increase the speed of graphics fill or 3D Z-buffer operations. Although SGRAM is faster than SDRAM, most video card makers have dropped SGRAM in favor of even faster DDR SDRAM in their newest products.
DDR SDRAM
Double Data Rate SDRAM (also called DDR SDRAM) is the most common video RAM technology on recent video cards. It is designed to transfer data at speeds twice that of conventional SDRAM by transferring data on both the rising and falling parts of the processing clock cycle. Today's high-end and mid-range video cards based on chipsets such as NVIDIA's GeForce 4 and GeForce 3 Ti and ATI's RADEON 8000 and 7000 series use DDR SDRAM for video memory.
DDR-II SDRAM
The second generation of DDR SDRAM fetches 4 bits of data per cycle, instead of 2 as with DDR SDRAM. This doubles performance at the same clock speed. The first video chipset to support DDR-II was NVIDIA's GeForce FX, which became the top of NVIDIA's line of GPUs in late 2002.
RAM Calculations
The amount of memory a video adapter needs to display a particular resolution and color depth is based on a mathematical equation. A location must be present in the adapter's memory array to display every pixel on the screen, and the resolution determines the number of total pixels.
For example, a screen resolution of 1024x768 requires a total of 786,432 pixels. If you were to display that resolution with only two colors, you would need only 1 bit of memory space to represent each pixel. If the bit has a value of 0, the dot is black, and if its value is 1, the dot is white.
If you use 24 bits of memory space to control each pixel, you can display more than 16.7 million colors because 16,777,216 combinations are possible with a 4-digit binary number (224=16,777,216). If you multiply the number of pixels necessary for the screen resolution by the number of bits required to represent each pixel, you have the amount of memory the adapter needs to display that resolution.
Digital-to-Analog Converter
The digital-to-analog converter on a video adapter (commonly called a RAMDAC) does exactly what its name describes. The RAMDAC is responsible for converting the digital images your computer generates into analog signals the monitor can display. The speed of the RAMDAC is measured in MHz; the faster the conversion process, the higher the adapter's vertical refresh rate.
The speeds of the RAMDACs used in today's high-performance video adapters range from 300MHz to 500MHz. Most of today's video card chipsets include the RAMDAC function inside the 3D accelerator chip, but some dual-display-capable video cards use a separate RAMDAC chip to allow the second display to work at different refresh rates than the primary display.
The benefits of increasing the RAMDAC speed include higher vertical refresh rates, which allows higher resolutions with flicker-free refresh rates (72Hz–85Hz or above). Typically, cards with RAMDAC speeds of 300MHz or above display flicker-free (75Hz or above) at all resolutions up to 1920x1200. Of course, you must ensure that any resolution you want to use is supported by both your monitor and video card.
Video Bus
You've learned in this chapter that certain video adapters were designed for use with certain system buses. Earlier bus standards, such as the IBM MCA, ISA, EISA, and VL-Bus, have all been used for VGA and other video standards. Because of their slow performances, all are now obsolete; current video cards are made exclusively for either the PCI AGP, or PCI Express bus standard.
Video Driver
The software driver is an essential, and often problematic, element of a video display subsystem. The driver enables your software to communicate with the video adapter. You can have a video adapter with the fastest processor and the most efficient memory on the market but still have poor video performance because of a badly written driver.
Video drivers generally are designed to support the processor on the video adapter. All video adapters come equipped with drivers the card manufacturer supplies, but often you can use a driver the chipset maker created as well. Sometimes you might find that one of the two provides better performance than the other or resolves a particular problem you are experiencing.
Most manufacturers of video adapters and chipsets maintain Web sites from which you can obtain the latest drivers; drivers for chipset-integrated video are supplied by the system board or system vendor. A driver from the chipset manufacturer can be a useful alternative, but you should always try the adapter manufacturer's driver first.
Before purchasing a video adapter, you should check out the manufacturer's site and see whether you can determine how up-to-date the available drivers are.
At one time, frequent driver revisions were thought to indicate problems with the hardware, but the greater complexity of today's systems means that driver revisions are a necessity. Even if you are installing a brand-new model of a video adapter, be sure to check for updated drivers on the manufacturer's Web site for best results.
