VGA is used for getting acceptable graphics quality. The term
Video Graphics Array (VGA) refers specifically to the display hardware first
introduced with the IBM PS/2 line of computers in 1987,[1] but through its
widespread adoption has also come to mean either an analog computer display
standard, the 15-pin D-subminiature VGA connector or the 640×480 resolution
itself. While this resolution has been superseded in the personal computer
market, it is becoming a popular resolution on mobile devices.
Video Graphics Array (VGA) was the last graphical standard
introduced by IBM that the majority of PC clone manufacturers conformed to,
making it today (as of 2009) the lowest common denominator that all PC graphics
hardware supports, before a device-specific driver is loaded into the computer.
For example, the MS-Windows splash screen appears while the machine is still
operating in VGA mode, which is the reason that this screen always appears in
reduced resolution and color depth.
VGA was officially superseded by IBM's XGA standard, but in
reality it was superseded by numerous slightly different extensions to VGA made
by clone manufacturers that came to be known collectively as "Super
VGA".
VGA Graphics has three modes of
operations:
1st Mode: This is the most common standars VGA operation,
where a maximum of 256 colors are selected out of 256 K color possibilities but
at any point only 16 color could be dislayed on the screen.
2nd Mode: It is known as 13-h mode. In this viedo frame
buffer is an 8 bit plane. The content of element is a between of 0 and 255.
This number directly index red, green and blue contents of corresponding color
register of D AC(Digital to Analog Conversion). The maximum screen resolution
in 13-h mode is 320X320 whereas displays maximum 16 colors through LUT(Look up
table).
3rd Mode: This is remain largely undocumented still recently,
but is getting popular and has close resemblance with 13-h mode.
Basic Components
VGA consists of several distinct functional blocks. Each has
a specific function, although there is some overlap between them. In some
cases, there are similar registers in two or more components that should be
programmed in tandem for normal operation.
1).Sequencer is the beating heart of VGA. It provides timing
signals that drive the other components. The sequencer also contains a plane
mask register (SR02) which can write protect any memory plane.
2).Graphics Controller takes care of various aspects of
memory access. It controls both memory reads and writes from the host (CPU) and
reads from the VGA output circuitry. The graphics controller supports several
read and write modes, some of them fairly complex but not necessarily very
useful.
3).Attribute Controller formats data for output. It handles
attributes such as blinking or intensity, as well as some aspects of color. The
attribute controller already existed on the EGA and supports 16-color palettes.
4).DAC, or Digital to Analog Converter, provides a 256-entry
color lookup table (CLUT or LUT) where each output color is defined by 18-bit
RGB values (6 bits per channel). The DAC is what really sets VGA apart from its
predecessors.
5).CRT Controller or CRTC drives the display and sets
important characteristics of the displayed mode such as horizontal and vertical
resolution, scan line length, or starting display offset.
No comments:
Write comments