Monochrome Display Adapter
Encyclopedia
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
's standard video display card and computer display standard
Computer display standard
Computer display standards are often a combination of aspect ratio, display resolution, color depth, and refresh rate.This article describes the different display standards for computer displays.-History:...
for the PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...
. The MDA did not have any pixel-addressable graphics modes. It had only a single monochrome
Monochrome
Monochrome describes paintings, drawings, design, or photographs in one color or shades of one color. A monochromatic object or image has colors in shades of limited colors or hues. Images using only shades of grey are called grayscale or black-and-white...
text mode
Text mode
Text mode is a kind of computer display mode in which the content of the screen is internally represented in terms of characters rather than individual pixels. Typically, the screen consists of a uniform rectangular grid of character cells, each of which contains one of the characters of a...
(PC video mode 7), which could display 80 columns by 25 lines of high resolution text characters or symbols useful for drawing forms.
The standard IBM MDA card was equipped with four kilobyte
Kilobyte
The kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 bytes or 1000 bytes, dependent upon context, in the fields of computer science and information...
s (kB) of video memory
Video memory
Video memory is a term generally used in computers to describe some form of writable memory, usually RAM, dedicated to the purpose of holding the information necessary for a graphics card to drive a display device...
. The MDA's high character resolution (sharpness) was a feature meant to facilitate business and wordprocessing use: Each character was rendered in a box of 9×14 pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
s, of which 8×14 made out the character itself (the other pixels being used for space between character columns and lines). Some characters, such as the lowercase "m", were rendered eight pixels across.
The MDA featured the following character display attributes: invisible, underline, normal, bright (bold), reverse video, and blinking; some of these attributes could be combined, so that e.g., bright, underlined text could be produced.
The theoretical total screen display resolution
Display resolution
The display resolution of a digital television or display device is the number of distinct pixels in each dimension that can be displayed. It can be an ambiguous term especially as the displayed resolution is controlled by all different factors in cathode ray tube , flat panel or projection...
of the MDA was 720×350 pixels. This number is arrived at through calculating character width (nine pixels) by columns of text (80) and character height (14 pixels) by rows of text (25). However, the MDA again could not address individual pixels; it could only work in text mode, limiting its choice of display patterns to 256 characters. Its character set is known as code page 437
Code page 437
IBM PC or MS-DOS code page 437 is the character set of the original IBM PC. It is also known as CP 437, OEM 437, PC-8, MS-DOS Latin US or sometimes misleadingly referred to as the OEM font, High ASCII or Extended ASCII....
. The character patterns were stored in ROM
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
on the card, and so could not be changed by software. The only way to simulate "graphical" screen content was through ASCII art
ASCII art
ASCII art is a graphic design technique that uses computers for presentation and consists of pictures pieced together from the 95 printable characters defined by the ASCII Standard from 1963 and ASCII compliant character sets with proprietary extended characters...
.
IBM's original MDA included a parallel printer port
Parallel port
A parallel port is a type of interface found on computers for connecting various peripherals. In computing, a parallel port is a parallel communication physical interface. It is also known as a printer port or Centronics port...
(hence its original name of "Monochrome Display and Printer Adapter"), thus avoiding the need for a separate parallel interface on computers fitted with an MDA.
Connector
Pin numbers (looking at socket):
| Pin | Function |
|---|---|
| 1 | Ground |
| 2 | Ground |
| 3 | Not Used |
| 4 | Not Used |
| 5 | Not Used |
| 6 | Intensity |
| 7 | Video |
| 8 | Horizontal Sync (+) |
| 9 | Vertical Sync (-) |
Signal
| Type | Digital, TTL |
|---|---|
| Resolution | 720h × 350v |
| H-freq | 18.432 kHz |
| V-freq | 50 Hz |
| Colors | 1 |
| Color Intensity | 2-4 |
Competing adapters
There were two commonly available competing display adapters:- For PC users requiring bitmapBitmapIn computer graphics, a bitmap or pixmap is a type of memory organization or image file format used to store digital images. The term bitmap comes from the computer programming terminology, meaning just a map of bits, a spatially mapped array of bits. Now, along with pixmap, it commonly refers to...
ped graphics and/or color, IBM offered its Color Graphics AdapterColor Graphics AdapterThe Color Graphics Adapter , originally also called the Color/Graphics Adapter or IBM Color/Graphics Monitor Adapter, introduced in 1981, was IBM's first color graphics card, and the first color computer display standard for the IBM PC....
(CGA, also CGA card), launched at the same time as their MDA. The lower resolution of its text mode characters (as compared to MDA) and absence of a printer port, which was included on the original MDA card, made CGA cards less attractive for business use. - Introduced in 1982, the non-IBM Hercules Graphics CardHercules Graphics CardThe Hercules Graphics Card was a computer graphics controller made by Hercules Computer Technology, Inc. which, through its popularity, became a widely supported display standard. It was common on IBM PC compatibles connected to a monochrome monitor . It supported one high resolution text mode and...
(also HGC) offered both an MDA compatible high resolution text mode and a monochrome graphics mode. It could address individual pixels and display a black and white picture of 720×348 pixels. This resolution was better than even the highest monochrome resolution CGA cards could offer. Thus, even without a color capability of any kind, the Hercules adapter's offer of monochrome graphics without sacrificing MDA-equivalent text quality made it a more desirable choice for many.