Display Adapters with Characteristics and Standards

The display adapter and the video monitor work together. The video adapter is a small card that fits into a spare slot of the microcomputer. In some microcomputers, the electronics for this card are integrated in the motherboard.

In conjunction with the monitor, the display adapters the data stream of the system bus and formats it into a data stream that the monitor can understand and display.

The basic type of video adapter used now are VGA (Video Graphics Array) and SVGA (Super VGA). They can display information in text-form or in graphical form.

The graphical standards for display adapter developed over the years are: MDA, HGA, MGA, CGA, EGA, VGA, MCGA, SVGA, XGA, 8514/A etc. the full description for these display standards are:

  • MDA : Monochrome Display Adapter
  • HGA : Hercules Graphics Adapter
  • CGA : Colour Graphics Adapter
  • EGA  : Enhanced Graphics Adapter
  • VGA : Video Graphics array
  • MCGA: Multi-Colour Graphics Array
  • SVGA: Super VGA
  • XGA : Extended Graphics Array

The SVGA includes higher resolution and better colour vibrancy than VGA and is the best display standard for high resolution graphics applications.

The 8514/A monitor and display adapters are primarily used for medical and seismic imaging, CAD/CAM presentations and other applications that require a very high resolution.

The characteristics of common display adapters are given  in Table. The present day display adapter supports all the modes of the preceding display adapters.

Adapter

Maximum Resolution / Colours

CGA

640 x 200/2

MDA

720 x 350/3

EGA

640 x 350/64

HGA

720 x 350/3

MCGA

640 x 480/2

VGA

640 x 480/262, 144

SVGA

up to 1024 x 768/256

8514/A

1024 x 768/262, 144

XGA

1024 x 786/262, 144

Above picture (a) shows a display system without using a graphics coprocessor and figure 4.10(b) shows the scheme using a graphics coprocessor. The speed of the graphics coprocessor limits the speed at which monitor can be refreshed.

Video controller cards also include video  RAM(or VRAM). VRAM is dual-ported, that is can send screenful of data to the monitor while at the same time receiving next screen full of data from the CPU.

display_adapters

PC Display System

The CPU sends information of every pixel on the screen to the video display adapter. At the resolution of 1024 x 768, there are 786432 pixels to control. For 256 colours, each pixel requires one byte of information.

But for maximum amount of colour each pixel require 24 bits(millions of colours possible) and the CPU must send 1024 x 768 x 24 bits, that is 1024 x 768 x 24 bytes which is actually 2,359,295 bytes , to the monitor for each screen.

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