IBM Monochrome Display Adapter

The Monochrome Display Adapter (MDA, also MDA card, Monochrome Display and Printer Adapter, MDPA) is IBM's standard video display card and computer display standard for the IBM PC introduced in 1981. The MDA does not have any pixel-addressable graphics modes, only a single monochrome text mode which can display 80 columns by 25 lines of high-resolution text characters or symbols useful for drawing forms.

The original IBM MDA was an 8-bit ISA card with a Motorola 6845 display controller, 4 KB of RAM, a DE-9 output port intended for use with an IBM monochrome monitor. A parallel port for attachment of a printer is also included, avoiding the need to purchase a separate card.

The MDA was based on the IBM System/23 Datamaster's display system, and was intended to support business and word processing use with its sharp, high-resolution characters. Each character is rendered in a box of 9 × 14 pixels, of which 7 × 11 depicts the character itself and the other pixels provide space between character columns and lines. Some characters, such as the lowercase "m", are rendered eight pixels across^{[citation needed]}.

The theoretical total screen display resolution of the MDA is 720 × 350 pixels, if the dimensions of all character cells are added up, but the MDA cannot address individual pixels to take full advantage of this resolution. Each character cell can be set to one of 256 bitmap characters stored in ROM on the card, and this character set cannot be altered from the built-in hardware code page 437. The only way to simulate "graphics" is through ASCII art, obtaining a low resolution 80 × 25 "pixels" screen, based on character positions.

Code page 437 has 256 characters (0-255), including the standard 95 printable ASCII characters from (32-126), and the 33 ASCII control codes (0-31 and 127) are replaced with printable graphic symbols. It also includes another 128 characters (128-255) like the aforementioned characters for drawing forms. Some of these shapes appear in Unicode as box-drawing characters.

There are several attribute values - bit flags that can be set on each character on the screen. These are invisible, underline, normal, bright (bold), reverse video, and blinking. Reverse video swaps the foreground and background colors, while blinking causes text to flash periodically. Some of these attributes can be combined, so that e.g. bright, underlined text can be rendered.

Early versions of the MDA board have hardware capable of outputting red, green and blue TTL signals on the normally unconnected DE-9 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 (widely) published software exists to actually control the feature.

It is also possible to combine the values of output pins 7 (Video) and 6 (Intensity), to generate four brightness levels. Video corresponds to 2/3 luminance and Intensity to 1/3 luminance), but the actual display of these levels is monitor-dependent: