IBM Monochrome Display Adapter

Lore & Background

The MDA was based on the display system of the IBM System/23 Datamaster and was designed to support business and word processing applications with its high-resolution characters. Each character was rendered in a 9×14 pixel box, with 7×11 pixels depicting the character itself. The card used a Motorola 6845 display controller, 4 KB of RAM, and a DE-9 output port intended for an IBM monochrome monitor. It also included a parallel printer port, eliminating the need for a separate card. The MDA could display 256 bitmap characters from the built-in code page 437, which included printable graphic symbols and box-drawing characters, but the character set could not be altered. Attribute flags allowed settings such as underline, bright (bold), reverse video, and blinking, some of which could be combined. Early versions of the MDA board had hardware capable of outputting red, green, and blue TTL signals on normally unconnected DE-9 pins, theoretically enabling an 8-color display with a suitable monitor, but no widely published software controlled this feature. The card could also combine video and intensity signals to generate four brightness levels, though actual display was monitor-dependent.

Reader's Guide

The MDA was released alongside the IBM Color Graphics Adapter (CGA) and could be installed in the same computer. A PC DOS command allowed switching the primary display between the two cards, and some software, such as Lotus 1-2-3, supported using both simultaneously. Because the MDA lacked pixel-addressable graphics, owners could not play most PC games with graphics support, though text-mode games (including text adventures and IBM's One Hundred And One Monochrome Mazes) were released. Box-drawing characters enabled rudimentary graphics in early titles like Castle Adventure. The MDA also saw use as a secondary display for debugging, with applications like SoftICE and the Windows debugger showing a debugger interface on the MDA while the primary display ran on another card. However, a typical 8-bit monochrome card could reduce the 16-bit 8 MHz ISA bus to an 8-bit 4 MHz bus, cutting bandwidth by up to 75%; Microsoft recommended removing the monochrome card when used alongside a VGA card for maximum VGA performance. The MDA was more popular than CGA for business applications due to its higher text resolution and built-in printer port. Dissatisfaction with its limitations led to third-party alternatives, most notably the Hercules Graphics Card (1982), which offered both an MDA-compatible text mode and a monochrome graphics mode with 720×348 pixel-addressable resolution.

Did You Know?

• The MDA cannot address individual pixels; it only displays text characters from a fixed ROM character set.
• The MDA was based on the IBM System/23 Datamaster's display system.
• The Hercules Graphics Card was created because its founder needed to display the Thai alphabet, which was impossible with the MDA's fixed character set.