Editing IBM Monochrome Display Adapter (section)

== 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" | 
|}