Video Display Standards

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The very first video adapter was an MDA (Monochrome Display Adapter). MDA has a resolution of 720x350 pixels and is capable of supporting text, but not graphics or color. CGA (Color Graphics Adapter) and EGA (Enhanced Graphics Adapter) were the early display standards for monitors. CGA was capable of producing just two colors with a resolution of 640x200. EGA could produce 16 colors with a resolution of 640x350. These video technologies were used in the early to mid-1980s. If you worked for extended periods using these technologies in the 1980s, your eyesight might be the worse for it now.

Video Graphics Array (VGA)

VGA is the standard for all graphic devices, such as monitors and video cards. It has a resolution of 640x480 with a color depth of 16 colors, or 320x200 with 256 colors. VGA uses analog signals, as opposed to the digital signals used by its predecessors, CGA and EGA. The original implementation of the VGA standard introduced the following video subsystem standards to assist with processing and ultimately produce a better image:

• Frame buffer. The frame buffer is a memory buffer used to store data before it is displayed. The amount of information stored before it is sent to the display is called a frame.

• Graphics command language. With VGA, the CPU manages all the work of producing an image. Graphics command language introduced a set of simple commands that alleviated some of the CPU's video-related tasks.

• CRT controller. The CRT controller produces signals that control and reset the electron guns inside the monitor.

• Sequencer. The sequencer is basically a timer on the video adapter. It loads display addresses into memory and operates with a 16-bit internal counter.

• Serializer. The serializer is used to serialize data held in video memory before it is sent to an attribute controller.

• Attribute controller. This houses a color template that determines the value, or color, of a pixel.

• Display memory. A bank of 256KB DRAM separated into four 64KB color planes, its function is to store screen-displayed information.

• Graphics controller. The graphics controller carries out logical functions and calculations on data being placed in display memory.

VGA introduced many advantages, including these subsystems. Unfortunately, it didn't use resources fast enough to support the demands of GUIs and applications that were hungry for new standards. VGA circuitry is directly tied to the processor. It relies on the processor to do most of its work, which takes a toll on the processor's performance.

8514/A

The IBM standard of 8514/A was introduced at the same time as the VGA standard. It provided three new graphics modes that enabled higher resolution and color, and was well suited for IBM's proprietary Micro Channel Bus. The 8514/A standard provided some processing capabilities. It supported a resolution of 1024x768 and 256 colors in graphics mode.

Extended Graphics Adapter

Extended Graphics Adapter (XGA) cards were introduced in later IBM PS/2 model computers. The XGA adapters use either 512K or 1MB of VRAM and are capable of bus mastering using IBM's Micro Channel Architecture (MCA). Using 1MB of VRAM, XGA supports a graphics resolution of 1024x768 and 256 colors, or 640x480 using high color (16 bits). This standard would be followed by SXGA (Super Extended Graphics Adapter, capable of 1280x1024) and UXGA (Ultra Extended Graphics Adapter, capable of 1600x1200).

Super VGA and Ultra VGA

Today's video cards are called Super VGA (SVGA) cards. Technically, SVGA and Ultra VGA (UVGA) are not distinct video standards; they are words that describe a video card's capability to achieve higher resolution and colors. Manufacturers of SVGA and UVGA video cards each provide their own sets of instructions and software drivers to maximize the performance of the cards they produce. A group of graphic and video card manufacturers known as the Video Electronics Standards Association (VESA) has standardized video rules. SVGA was originally developed by VESA as competition to IBM's proprietary 8514/A and XGA technologies. (XGA was introduced by IBM in 1990 as a replacement for their old 8514/A video standard, capable of displaying 640x480-1024x768.) All varieties of SVGA support a palette of 16 million colors. The basic SVGA resolutions are 800x600, 1024x768, 1280x1024, and 1600x1200.

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