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Introduction

  The IBM type computers can generate text on the monitor attached to the
  video card in the computer. So what else is new? Many video cards also have
  the ability to switch to another mode called graphics mode. In this mode the
  screen is no longer 80 columns by 25 rows of characters, but is based on
  lighted points on the screen called pixels. The number of pixels on the
  screen depends on the type of graphics adapter you have in your computer.

  The graphics screen is a large video buffer that is refreshed between 50 and
  70 times a second, depending on which video mode is in use. Obviously, when
  a program changes the content of the display memory, the screen changes
  almost immediately. The actual amount of RAM required to display a graphics
  image is also dependent on the video mode in use.

  The Color Graphics Adapter (CGA) has 320 pixels across the screen and 200
  pixels down the screen (BIOS mode 4). Each pixel is a 2-bit memory location
  with four possible combinations and therefore can be one of four colors. CGA
  has built in two different sets of colors, called palettes. You are limited
  to the colors that are available within each palette.

  CGA also has the capability to display 640 pixels across and 200 pixels
  down, but only in two colors, black and white (BIOS mode 6). Each pixel is
  1-bit or two possible combinations representing two colors. The size of
  a CGA screen is 16 Kbytes.

  The Enhanced Graphics Adapter (EGA) is far more sophisticated with the
  ability to display 16 colors at the same time from a palette of 64 colors
  (BIOS mode 16). The EGA mode 16 has 640 pixels across and 350 pixels down
  the screen. Each pixel is 4 bits in memory or 16 possible combinations. The
  memory is arranged in 4-bit planes. EGA can also operate in 640 by 350 black
  and white (mode 15), 640 by 200 16-color (mode 14), and 320 by 200 16-color
  (mode 13). It also can operate in CGA mode.

  The Video Graphics Array (VGA) can operate similar to the EGA adapter except
  for the screen resolution which is 640 columns and 480 rows (mode 18). Also,
  it can select 16 colors from a palette of 256, but most EGA software will
  run unchanged.

  The MCGA adapter also was introduced with the IBM PS/2 machines and can
  generate 256 colors at a time from a possible 262,144 (mode 19). This
  adapter also can operate in 640 by 480 black and white (mode 17).

  The Hercules Graphics Apapter (HGC) was introduced in 1982 to add graphics
  to a monochrome monitor. It also duplicates the function of IBM original
  Monochrome Display Adapter which only displays text. Hercules has 720
  columns by 348 rows of pixels.

  These are the most common graphics modes on the market today, however, in
  the spirit of competition many adapters now support higher resolutions.
  Super VGA is almost an official standard and has a resolution of 800 columns
  by 600 rows of pixels in 16 colors.

  A CGA adapter image requires 16 Kbytes, while a VGA adapter in mode 18
  requires 153 Kbytes. The differences in the size of the screen memory can
  have implications for how much disk space is used when saving a picture or
  how fast the image is drawn to the screen.

  Another important point is the location of a pixel on the graphics screen.
  The upper left pixel on the graphics screen is position (0,0), while the
  lower right corner for VGA is (639,479). The positioning on a graphics
  screen is just reversed from the a SAY command for the Clipper text screen.
  On the graphics screen the first number is the horizontal pixel position
  from the left side of the screen, while the second number is the vertical
  pixel position from the top down.

  FlipperGraphics Library automatically detects the monitor type and display
  adapter when the initialization command FLIP_INIT() is issued. So this takes
  much of the concern away about which adapter any specific computer might
  have and allows your software to run on any supported machine. FLIPPER also
  provides methods for changing to another video mode, like from EGA to CGA.

               GRAPHMODE(1) && Change from EGA to CGA

  FLIPPER even has the ability to run in Super VGA or other specific video
  mode only supported on a particular video card. For example, the Paradise
  graphics card can be placed into 800 by 600 mode with 16 colors by issuing
  the following function:

               BIOS_MODE(800,600,1,22)  && Change to mode 22

  It is important to understand the limits and capabilities of each so you can
  add graphics to your applications that will communicate in a pleasing and
  effective way.