View source for IBM Monochrome Display Adapter

{{Short description|IBM PC graphic adapter and display standard}}
{{Infobox GPU
| name = Monochrome Display Adapter
| image = IBM PC Original Monochrome Display and Parallel Printer Adapter.jpg
| caption = IBM PC original MDA and parallel printer adapter
| codename = 
| created = {{Start date and age|1981}}
| discontinued = 1984
| transistors = 
| architecture = [[Motorola 6845]]
| entry = IBM MDA, Control Systems Artist&nbsp;1, Hitachi&nbsp;HD6845SP, UMC&nbsp;UM6845 
| midrange = 
| highend  = 
| predecessor = 
| successor   = [[Hercules Graphics Card]], [[Enhanced Graphics 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&nbsp;columns by 25&nbsp;lines of high-resolution text characters or [[Semigraphics|symbols useful for drawing forms]].

==Hardware design==
The original IBM MDA was an 8-bit [[ISA card]] with a [[Motorola 6845]] display controller, 4&nbsp;KB of [[RAM]], a [[D-subminiature|DE-9 output port]] intended for use with an [[IBM 5151|IBM monochrome monitor]]. A [[parallel port]] for attachment of a printer is also included, avoiding the need to purchase a separate card.<ref name="Elliott_2005_MDA" />

== Capabilities ==
[[File:MDA Codepage 437 (Complete).png|thumb|Characters of code page 437]]
[[File:Image rendered in MDA text mode using semigraphic blocks.png|thumb|right|Image rendered in MDA text mode using [[semigraphics]] blocks]]
The MDA was based on the [[IBM System/23 Datamaster]]'s display system,<ref name="bradley199009">{{cite news|author=Bradley, David J.|date=September 1990|title=The Creation of the IBM PC|pages=414–420|work=BYTE|url=https://archive.org/stream/byte-magazine-1990-09/1990_09_BYTE_15-09_15th_Anniversary_Summit#page/n451/mode/2up|access-date=2 April 2016}}</ref> 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&nbsp;×&nbsp;14 [[pixel]]s, of which 7&nbsp;×&nbsp;11 depicts the character itself and the other pixels provide space between character columns and lines.<ref>{{Cite book |url=https://minuszerodegrees.net/oa/OA%20-%20IBM%20Monochrome%20Display%20and%20Printer%20Adapter.pdf |title=IBM Monochrome Display and Printer Adapter |publisher=IBM}}</ref> Some characters, such as the lowercase "m", are rendered eight pixels across{{cn|date=March 2025}}.

The theoretical total screen [[display resolution]] of the MDA is 720&nbsp;×&nbsp;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]] [[code page 437|437]]. The only way to simulate "graphics" is through [[ASCII art]], obtaining a low resolution 80&nbsp;×&nbsp;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.<ref name="Elliott_2005_MDA">{{cite web |title=Monochrome Display Adapter: Notes |author-first=John C. |author-last=Elliott |date=2005-11-06 |url=http://www.seasip.info/VintagePC/mda.html |access-date=2016-11-23 |url-status=live |archive-url=https://web.archive.org/web/20161123133459/http://www.seasip.info/VintagePC/mda.html |archive-date=2016-11-23}}</ref>

{| class="wikitable"
! style="background:#efefef;" | Attribute 
! style="background:#efefef;" | Display
|-
| Invisible || style="color: black; background-color: black" | <samp>Invisible</samp>
|-
| Normal || style="color: #ffffff; background-color: black" | <samp>Normal</samp>
|-
| Underline || style="color: #ffffff; background-color: black" | <samp><u>Underline</u></samp>
|-
| Bright || style="color: #ffffff; background-color: black" | <samp>'''Bright'''</samp>
|-
| Bright underline || style="color: #ffffff; background-color: black" | <samp>'''<u>Bright underline</u>'''</samp>
|-
| Reverse video || style="color: black; background-color: #ffffff" | <samp>Reverse video</samp>
|-
| Invisible reverse || style="color: #ffffff; background-color: #ffffff" | <samp>Invisible reverse</samp>
|}

Early versions of the MDA board have hardware capable of outputting red, green and blue [[Transistor–transistor logic|TTL signals]] on the normally unconnected [[D-subminiature|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.<ref name="wilton1987">{{cite book|last=Wilton|first=Richard|title=Programmer's Guide To PC And PS/2 Video Systems|date=1987|publisher=Microsoft Press|isbn=1-55615-103-9|page=51}}</ref><ref name="Elliott_2005_MDA" /><ref name="mzdg1">{{cite web |title=IBM 5150 - early version |url=http://www.minuszerodegrees.net/5150/early/5150_early.htm |access-date=2014-08-30 |publisher=minus zero degrees (-0º) IBM 51xx PC Family Computers}}</ref><ref name=":1">{{Cite web |title=IBM: MDA / CGA / EGA |url=https://www.minuszerodegrees.net/mda_cga_ega/mda_cga_ega.htm |access-date=2022-11-17 |website=minus zero degrees (-0º) IBM 51xx PC Family Computers}}</ref>

It is also possible to combine the values of output pins 7 (''Video)'' and 6 (''Intensity)<ref name=":0" /><ref name="auto" /><ref name=":1" />,'' to generate four brightness levels. ''Video'' corresponds to 2/3 [[Luma (video)|luminance]] and ''Intensity'' to 1/3 luminance),<ref>{{Cite web |title=RGB Classic Games - Character attributes |url=https://www.classicdosgames.com/tutorials/characterattributes.html |access-date=2022-05-06 |website=www.classicdosgames.com}}</ref> but the actual display of these levels is monitor-dependent:<ref name="auto">{{Cite web |last=Elliott |first=John |date=June 8, 2020 |title=Monochrome Display Adapter Notes |url=https://www.seasip.info/VintagePC/mda.html |website=John Elliott's homepage}}</ref>
{| class="wikitable"
! colspan="2" |Output pin values
! rowspan="2" |Displayed<br/>brightness
|-
!'''7 – Video'''
!6 – High intensity
|-
|0
|0
| style=" background-color: #000" | 
|-
|0
|1
| style="background-color: #666" | 
|-
|1
|0
| style="background-color: #AAA" | 
|-
|1
|1
| style="background-color: #fff" | 
|}

== Use ==
[[File:IBM PC 5150.jpg|thumb|[[IBM 5151]] monitor driven by a Monochrome Display Adapter (MDA)]]
The MDA was released alongside the IBM [[Color 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.<ref>{{Cite web |last=Elliott |first=John |date=April 24, 2011 |title=Dual-Head operation on vintage PCs |url=https://www.seasip.info/VintagePC/dualhead.html |access-date=2020-08-16 |website=John Elliott's homepage}}</ref> Some software like [[Lotus 1-2-3]] supports using both cards at the same time.<ref name="derfler198303">{{cite magazine |url=https://books.google.com/books?id=7wCiNAUEuAMC&pg=RA1-PA187 |title=A Program You Can Count On |magazine=[[PC Magazine]] |date=March 1983 |access-date=2013-10-21 |author-last=Derfler |author-first=Frank J. Jr. |pages=187 |volume=1 |issue=10}}</ref>

Because of the lack of pixel-addressable graphics, MDA owners were unable to play PC games released with graphics support. However, [[Text mode|textmode games]] were released for the PC (including [[text adventures]]) and at least one game, IBM's ''One Hundred And One Monochrome Mazes'', requires MDA.<ref name="manes19840124">{{cite news|author=Manes, Stephen|date=1984-01-24|title=It's Not Easy Being Green|page=391|work=PC Magazine|url=https://books.google.com/books?id=kSzKzjWHeVEC&pg=PA391|access-date=24 October 2013}}</ref> Box-drawing characters made the production of rudimentary graphics practical for early PC game titles, including [[BBS door]] games or titles such as ''[[Castle Adventure]]''.

Another use for the MDA was as a secondary display for [[debugging]]. Applications like [[SoftICE]]<ref>{{Cite book |url=http://softice1.free.fr/docs/NuMega%20-%20Using%20SoftICE.pdf |title=Using SoftICE |publisher=Compuware Corporation |year=1998 |pages=12}}</ref> and the Windows debugger<ref>{{Cite news |last=Glass |first=Brett |date=April 1, 1991 |title=The AT's Shortcomings Force Single-Monitor Programming |pages=62 |work=InfoWorld |url=https://books.google.com/books?id=0FAEAAAAMBAJ&q=debugger%20MDA%20monitor&pg=PA62}}</ref> 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.

=== Disadvantage ===
{{expert|Computing|talk=Disadvantage|date=December 2023}}
A typical 8-bit monochrome card could turn the 16-bit 8 MHz ISA bus into an 8-bit 4 MHz [[PC 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 normal [[VGA]] card for debugging purposes, this resulted in slow VGA performance. Microsoft recommended in its ''Writing HOT Games for Microsoft Windows'' (1994) to remove the monochrome card in such a setup for maximum speed of the VGA card.<ref>{{Cite book |url=https://www.tech-insider.org/windows/research/acrobat/940929-a.pdf |title=Writing HOT Games for Microsoft Windows – The Microsoft Game Developers' Handbook |publisher=Microsoft Windows Multimedia |year=1994 |pages=17}}</ref>

== Reception ==
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".<ref name="dievendorff19811007">{{cite book | title=IBM Personal Computer Questions and Answers | publisher=IBM | author=Dievendorff, Dick | year=1981 | url=http://bitsavers.trailing-edge.com/pdf/ibm/pc/pc/ | pages=25}}</ref> 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.<ref>{{Cite news |last=Gibson |first=Steve |date=September 19, 1988 |title=IBM's EGA Unified Backward-Compatible CGA, MDA, Hercules Standards |pages=49 |work=InfoWorld |url=https://books.google.com/books?id=mDoEAAAAMBAJ&q=IBM+MDA+more+popular+than+CGA&pg=PA49}}</ref> 

A well known example was the [[Hercules Graphics Card]]. Introduced in 1982, it offered both an MDA-compatible high resolution text mode and a monochrome graphics mode. The founder of [[Hercules Computer Technology]], Van Suwannukul, created the Hercules Graphics Card so that he could work on his [[doctoral thesis]] on an [[IBM PC]] using the [[Thai alphabet]], which was impossible at the low resolution of CGA or the fixed character set of MDA.<ref>{{Cite news |date=May 16, 1989 |title=The Resolution Time Line: Additional Pixels Enhance Quality |pages=96 |work=PC Magazine |url=https://books.google.com/books?id=2jkWJsu_9CoC&q=van+suwannukul+hercules&pg=RA1-PA96}}</ref>  It could address individual pixels, and displayed a black and white picture of 720&nbsp;×&nbsp;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 many [[Clone (computing)|clones]].<ref>{{Cite news |last=Ponting |first=Bob |date=June 26, 1989 |title=High-Resolution Standard is Latest Step in DOS Graphics Evolution |pages=48 |work=InfoWorld |url=https://books.google.com/books?id=lTAEAAAAMBAJ&q=infoworld+1982+hercules&pg=PT47}}</ref>

==Specifications==

[[File:Numbered DE9 female Diagram.svg|thumb|[[D-subminiature|DE-9 connector]], looking at back of PC]]MDA cards used a DE-9 output port intended for a digital TTL monitor, like the IBM monochrome monitor.

{| class="wikitable" style="font-size:small"
|+ MDA connector pin assignments<ref name=":0">IBM Personal Computer Hardware Library: Technical Reference (Revised edition, 1983).</ref><ref name="auto"/><ref name=":1" />
|-
! Pin
! Function
|-
| 1 || [[Ground (electricity)|Ground]]
|-
| 2 || Ground
|-
| 3, 4, 5 || Not used
|-
| 6 || Intensity
|-
| 7 || [[Raster scan|Video]]
|-
| 8 || [[Horizontal sync]] (+)
|-
| 9 || [[Refresh rate|Vertical sync]] (−)
|}

The signal had the following specifications:
* '''Type''': Digital, TTL
* '''Resolution''': 720&nbsp;×&nbsp;350
* '''Horizontal frequency''': 18.432&nbsp;kHz
* '''Vertical frequency''': 50&nbsp;Hz
* '''Colors''': Monochrome, with 2 to 4 intensity levels (depending on monitor)<ref name="auto" /><ref>There are four possible combinations of values for the 'Intensity' and 'Video' pins, but not all monitors will display them as four distinct intensity levels.</ref>

==Clone boards==
[[Image:KL MDA Unknown.jpg|thumb|280px|MDA Video card with Hitachi HD6845 (= Motorola MC6845)]]
Other boards offer MDA [[Computer compatibility|compatibility]], although with differences on how attributes are displayed or the font used.<ref name="Elliott_2005_MDA" /><ref>{{cite web|url=http://www.vgamuseum.info/index.php/component/content/article/59-vlasks-articles/index.php?option=com_customproperties&view=show&task=show&cp_made=&cp_bus=&cp_memsize=&cp_year=&cp_memory=&cp_family=&cp_cardtype=mda&cp_owner=&cp_directx=&cp_opengl=&cp_pipelines=&cp_manufacturer=&cp_process=&cp_text_search=|title=VGA Legacy|website=vgamuseum.info|archive-url=https://archive.today/20140629122632/http://www.vgamuseum.info/index.php/component/content/article/59-vlasks-articles/index.php?option=com_customproperties&view=show&task=show&cp_made=&cp_bus=&cp_memsize=&cp_year=&cp_memory=&cp_family=&cp_cardtype=mda&cp_owner=&cp_directx=&cp_opengl=&cp_pipelines=&cp_manufacturer=&cp_process=&cp_text_search=|archive-date=June 29, 2014|access-date=June 28, 2014|url-status=dead}}</ref>

*3270 PC
*Amstrad [[Amstrad PPC 512|PPC]] / PC20
*[[Control Systems (company)|Control Systems]] Artist 1
*[[Tamarack Microelectronics]] TD3088A3

==See also==
{{Commons category|IBM Monochrome Display Adapter}}
* [[Hercules Graphics Card]]
* [[Color Graphics Adapter]]
* [[Orchid Graphics Adapter]]
* [[Green-screen display]]
* [[Light pen]] (only in very early MDA cards)
* [[List of video connectors]]
* [[List of defunct graphics chips and card companies]]
* [[VGA text mode]]

==References==
{{Reflist}}

{{Computer display standard}}
{{IBM personal computers}}

[[Category:Monochrome Display Adapter]]
[[Category:IBM video hardware|Monochrome Display Adapter]]
[[Category:Computer display standards]]
[[Category:Graphics cards]]
[[Category:Computer-related introductions in 1981]]