IBM Monochrome Display Adapter
| Company type | Hardware |
|---|---|
| Industry | Computer hardware |
| Founded | 1981 |
| Founder | IBM |
| Products | Monochrome video display card |
The Monochrome Display Adapter (MDA, also known as the MDA card or Monochrome Display and Printer Adapter (MDPA)) was IBM's primary video display card and computer display standard for the IBM PC upon its 1981 launch. The MDA does not support pixel-addressable graphics modes; it provides a single monochrome text mode capable of rendering 80 columns by 25 rows of crisp text characters or form symbols.
Hardware design
The initial IBM MDA was an 8-bit ISA card equipped with a Motorola 6845 display controller, containing 4 KB of RAM, and featuring a DE-9 output connector intended for use with the IBM monochrome monitor. Additionally, it incorporated an integrated parallel port for printers, removing the necessity for a separate printer interface card.
Capabilities
Derived from the display system used in the IBM System/23 Datamaster, the MDA was designed primarily for business applications and word processing, delivering sharp, high-resolution character output. Each character is rendered within a 9 × 14 pixel grid, with the glyph itself occupying 7 × 11 pixels. Certain characters, such as the lowercase "m," extend to eight pixels in width.
Although the theoretical screen resolution amounts to 720 × 350 pixels, the MDA lacks the capability to address individual pixels directly. Instead, it presents one of 256 predefined bitmap characters stored in ROM via hardware code page 437. Graphics are simulated through ASCII art techniques, effectively creating a low-resolution grid of 80 × 25 "pixels" defined by character cells.
Code page 437 encompasses standard printable ASCII characters alongside control codes replaced by graphic symbols and additional form-drawing characters, some of which align with Unicode box-drawing elements.
Character attributes available include invisible, underline, normal, bright (bold), reverse video, and blinking. The reverse video attribute inverts foreground and background colors, while blinking causes periodic flashing. These attributes can be combined—for example, bright text with underlining.
| Attribute | Appearance |
|---|---|
| Invisible | Invisible |
| Normal | Normal |
| Underline | Underline |
| Bright | Bright |
| Bright underline | Bright underline |
| Reverse video | Reverse video |
| Invisible reverse | Invisible reverse |
Early versions of the MDA hardware could output red, green, and blue TTL signals on DE-9 pins that were typically unused, potentially allowing an eight-color display when paired with compatible monitors. Although monochrome mode could be toggled via hardware registers, this functionality was not commonly exploited by software.
Output pins 7 (Video) and 6 (Intensity) can be combined to generate four distinct brightness levels by adjusting luminance ratios. The actual brightness perceived depends on the connected monitor:
| Output pin states | Resulting brightness | |
|---|---|---|
| 7 – Video | 6 – High intensity | |
| 0 | 0 | |
| 0 | 1 | |
| 1 | 0 | |
| 1 | 1 | |
Use
The MDA was introduced simultaneously with the IBM Color Graphics Adapter and could coexist with CGA within the same computer system. PC DOS provided a command that allowed users to toggle the primary display output between the CGA and MDA cards. Certain software applications, such as Lotus 1-2-3, were capable of utilizing both adapters concurrently.
Because the MDA lacked pixel-level graphics capabilities, it was unable to support most PC games that required graphical interfaces. Nevertheless, text-based games, including interactive fiction titles, were compatible, with IBM's One Hundred And One Monochrome Mazes specifically designed for MDA. The use of box-drawing characters facilitated rudimentary graphics in early PC software like BBS door games or Castle Adventure.
In addition, the MDA often functioned as a secondary monitor for debugging purposes. Debugging utilities such as SoftICE and Windows debuggers took advantage of running an MDA card alongside another graphics adapter—displaying debugging information on the MDA while the primary screen showed standard output.
Disadvantage
Standard 8-bit monochrome cards effectively reduced the 16-bit 8 MHz ISA bus to an 8-bit 4 MHz operation, resulting in up to a 75% decrease in data throughput. When paired with a VGA card during debugging sessions, this limitation led to diminished VGA performance. Microsoft recommended removing the monochrome adapter in such configurations to ensure optimal VGA speed.
Reception
An internal IBM report from 1981 highlighted the inferior display quality of the CGA adapter, emphasizing the MDA’s superior text sharpness. The MDA’s enhanced resolution combined with its built-in printer port made it more suitable for business-oriented tasks, which aligned with the primary market focus of the original IBM PC. However, these constraints encouraged third-party companies to create alternative hardware solutions.
A prominent example is the Hercules Graphics Card, introduced in 1982 by Hercules Computer Technology founder Van Suwannukul. Originally developed to support his doctoral research involving an IBM PC using the Thai alphabet, this card merged MDA-compatible high-resolution text with a monochrome graphics mode. It provided pixel-addressable graphics at a resolution of 720 × 348, exceeding CGA’s graphical capabilities despite lacking color support. This blend of detailed bitmap graphics and high-quality text made Hercules widely popular and led to its inclusion in numerous clone systems.
Specifications
MDA cards featured a DE-9 connector designed for digital TTL monitors such as the IBM monochrome display.
| Pin | Function |
|---|---|
| 1 | Ground |
| 2 | Ground |
| 3, 4, 5 | Unused |
| 6 | Intensity |
| 7 | Video |
| 8 | Horizontal sync (+) |
| 9 | Vertical sync (−) |
Technical details:
- Type: Digital TTL signaling
- Resolution: 720 × 350 pixels
- Horizontal frequency: 18.432 kHz
- Vertical frequency: 50 Hz
- Colors: Monochrome with two to four intensity levels depending on monitor
Clone boards
Various manufacturers produced boards compatible with MDA standards, primarily differing in font design and attribute rendering.
- 3270 PC
- Amstrad PPC / PC20
- Control Systems Artist 1
- Tamarack Microelectronics TD3088A3
See also
- Hercules Graphics Card
- Color Graphics Adapter
- Orchid Graphics Adapter
- Green-screen display
- Light pen (applicable to early MDA models)
- List of video connectors
- List of defunct graphics chips and card companies
- VGA text mode