The MDA is one of the original video systems shipped with the IBM-PC
circa 1981.  It has been mostly replaced by EGA, VGA, and SVGA systems.
TECH Help! covers the following MDA and general video topics:

Video Services ..... INT 10H video services
Video Modes ........ includes MDA modes

Video Memory Layouts accessing MDA video memory
BIOS Data Area ..... includes video-specific variables in low memory

Screen Attributes .. codes determine "colors" for text mode

CGA I/O Ports ...... I/O port addresses; video controller registers
MDA Pin Outs ....... connectors on MDA cards

INT 1dH ............ video initialization table

The MDA provides NO graphics support and is limited to a single text mode
(80x25).  Its video memory begins at b000:0000 and it supports only one
"page" of video.

A popular variant known as Hercules and MonoGraphics also exists.  These
versions do support a 720x348 2-color graphics mode and they work with
the same low-cost video display as the MDA.

  Testing for an MDA  
Use INT 11H (equipment list) and check bits 4 & 5.  If (w AND 30H) is 30H,
then you are running on an MDA or compatible and video memory begins at
b000:0; otherwise, its a CGA, EGA, VGA or other color system (text-mode
video memory begins at b800:0).

See Also: CGA
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Monochrome Display Adapter (MDA)