 IBM PC original MDA and parallel printer adapter | |
| Release date | 1981; 45 years ago (1981) |
|---|---|
| Discontinued | 1984 |
| Architecture | Motorola 6845 |
| Cards | |
| Entry-level | IBM MDA, Control Systems Artist 1, Hitachi HD6845SP, UMC UM6845 |
| History | |
| Successor | Hercules Graphics Card,Enhanced Graphics Adapter |
TheMonochrome Display Adapter (MDA, alsoMDA card,Monochrome Display and Printer Adapter,MDPA) isIBM's standardvideo display card andcomputer display standard for theIBM PC introduced in 1981. The MDA does not have anypixel-addressable graphics modes, only a singlemonochrometext mode which can display 80 columns by 25 lines of high-resolution text characters orsymbols useful for drawing forms.
The original IBM MDA was an 8-bitISA card with aMotorola 6845 display controller, 4 KB ofRAM, aDE-9 output port intended for use with anIBM monochrome monitor. Aparallel port for attachment of a printer is also included, avoiding the need to purchase a separate card.[1]
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The MDA was based on theIBM System/23 Datamaster's display system,[2] and was intended to support business andword processing use with its sharp, high-resolution characters. Each character is rendered in a box of 9 × 14pixels, of which 7 × 11 depicts the character itself and the other pixels provide space between character columns and lines.[3] Some characters, such as the lowercase "m", are rendered eight pixels across[citation needed].
The theoretical total screendisplay 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 inROM on the card, and this character set cannot be altered from the built-inhardware code page437. The only way to simulate "graphics" is throughASCII 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 printableASCII 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 asbox-drawing characters.
There are several attribute values - bit flags that can be set on each character on the screen. These areinvisible, underline, normal, bright (bold), reverse video, andblinking.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.[1]
| Attribute | Display |
|---|---|
| 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 board have hardware capable of outputting red, green and blueTTL signals on the normally unconnectedDE-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.[4][1][5][6]
It is also possible to combine the values of output pins 7 (Video) and 6 (Intensity)[7][8][6], to generate four brightness levels.Video corresponds to 2/3luminance andIntensity to 1/3 luminance),[9] but the actual display of these levels is monitor-dependent:[8]
| Output pin values | Displayed brightness | |
|---|---|---|
| 7 – Video | 6 – High intensity | |
| 0 | 0 | |
| 0 | 1 | |
| 1 | 0 | |
| 1 | 1 | |
The MDA was released alongside the IBMColor Graphics Adapter, and can be installed alongside the CGA in the same computer. A command included with PC DOS permits switching the primary display between the CGA and MDA cards.[10] Some software likeLotus 1-2-3 supports using both cards at the same time.[11]
Because of the lack of pixel-addressable graphics, MDA owners were unable to play PC games released with graphics support. However,textmode games were released for the PC (includingtext adventures) and at least one game, IBM'sOne Hundred And One Monochrome Mazes, requires MDA.[12] Box-drawing characters made the production of rudimentary graphics practical for early PC game titles, includingBBS door games or titles such asCastle Adventure.
Another use for the MDA was as a secondary display fordebugging. Applications likeSoftICE[13] and the Windows debugger[14] permitted the simultaneous use of an MDA and another graphics card, with the MDA displaying a debugger interface while the other card was showing the primary display.
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A typical 8-bit monochrome card could turn the 16-bit 8 MHz ISA bus into an 8-bit 4 MHzPC bus, which resulted in having the bus bandwidth cut by up to 75%. If the monochrome card was added to the PC as a second card besides a normalVGA card for debugging purposes, this resulted in slow VGA performance. Microsoft recommended in itsWriting HOT Games for Microsoft Windows (1994) to remove the monochrome card in such a setup for maximum speed of the VGA card.[15]
The author of an internal IBM publication stated in October 1981 that he had planned to purchase the CGA adapter but changed his mind after seeing its poor display quality. Describing MDA as beautiful, he observed that "you stare at text a whole lot more than you stare at color graphics".[16] MDA was more popular than CGA for business applications. The higher resolution of MDA's text and inclusion of a printer port made it more appealing for the business applications that were the focus of the original PC. However, dissatisfaction with its limitations quickly led to third parties releasing competing hardware.[17]
A well known example was theHercules Graphics Card. Introduced in 1982, it offered both an MDA-compatible high resolution text mode and a monochrome graphics mode. The founder ofHercules Computer Technology, Van Suwannukul, created the Hercules Graphics Card so that he could work on hisdoctoral thesis on anIBM PC using theThai alphabet, which was impossible at the low resolution of CGA or the fixed character set of MDA.[18] It could address individual pixels, and displayed a black and white picture of 720 × 348 pixels. This resolution was superior to the CGA card, yet offered pixel-addressable graphics, so despite lacking color capability, the Hercules adapter's offer of high resolution bitmap graphics combined with MDA-grade text quality made it a popular choice, which was even shipped with manyclones.[19]
MDA cards used a DE-9 output port intended for a digital TTL monitor, like the IBM monochrome monitor.
| Pin | Function |
|---|---|
| 1 | Ground |
| 2 | Ground |
| 3, 4, 5 | Not used |
| 6 | Intensity |
| 7 | Video |
| 8 | Horizontal sync (+) |
| 9 | Vertical sync (−) |
The signal had the following specifications:
Other boards offer MDAcompatibility, although with differences on how attributes are displayed or the font used.[1][21]
