Based on the IBM Datamaster's display system, the standard IBM MDA card uses the Motorola 6845display controller and is equipped with four kilobytes of video memory. The MDA's high character resolution is a feature meant to facilitate business and wordprocessing use: Each character is rendered in a box of 9×14 pixels, of which 7×11 depicts the character itself. Some characters, such as the lowercase "m", are rendered eight pixels across. The MDA has the following character display attributes: Invisible, underline, normal, bright, reverse video, and blinking. Some of these attributes can be combined, so that e.g., bright, underlined text can be produced. The theoretical total screen display resolution of the MDA is 720×350 pixels. This number is arrived at through calculating character width by columns of text and character height by rows of text. However, the MDA cannot address individual pixels; it only works in text mode, limiting its choice of display patterns to 256 characters. The character patterns are stored in ROM on the card and the character set cannot be changed from the hardware code page437. The only way to simulate "graphical" screen content is through ASCII art. Because of the lack of pixel-addressable graphics, MDA owners cannot play most graphics-based games. At least one game, IBM's One Hundred And One Monochrome Mazes, requires MDA. Code page 437 has 256 characters, including the standard 95 printable ASCII characters from, and the 33 ASCII control codes are replaced with printable graphic symbols. It also includes another 128 characters like the aforementioned characters for drawing forms. Some of these shapes appear in Unicode as box-drawing characters. The characters are also used in early PC games such as early BBS door games, or games like Castle Adventure by Kevin Bales. IBM's original MDA includes a parallel printer port, thus avoiding the need for a separate parallel interface on computers fitted with an MDA.
Early versions of the MDA board have hardware capable of outputting red, green and blue TTL signals on the normally unconnected video connector pins, theoretically allowing an 8-color display with a suitable monitor. The registers also allow the monochrome mode to be set on and off. No published software exists to actually control the feature.
Clone boards
Other boards offer MDA compatibility, although with differences on how attributes are displayed or the font used.
3270 PC
Amstrad PPC / PC20
Control Systems Artist 1
Tamarack Microelectronics TD3088A3
Competing adapters
There were two commonly available competing display adapters:
For PC users requiring bitmapped graphics and/or color, IBM offered its Color Graphics Adapter, launched at the same time as MDA. CGA was much less popular than MDA at first; the lower resolution of its text mode characters and absence of a printer port, which was included on the original MDA card, made CGA cards less attractive for business use. PC Magazine reported in June 1983 that while "the IBM monochrome display is absolutely beautiful for text and wonderfully easy on the eyes, but is limited to simple character graphics. Text quality on displays connected to the color/graphics adapter... is at best of medium quality and is conducive to eyestrain over the long haul".
Introduced in 1982, the non-IBM Hercules Graphics Card offers both an MDA compatible high resolution text mode and a monochrome graphics mode. It can address individual pixels, and display a black and white picture of 720×348 pixels. This resolution is better than even the highest monochrome resolution CGA cards can offer. Thus, even without a color capability of any kind, the Hercules adapter's offer of monochrome graphics without sacrificing MDA-equivalent text quality made it a more desirable choice for many.
