Code page 437


Code page 437 is the character set of the original IBM PC. It is also known as CP437, OEM-US, OEM 437, PC-8, or MS-DOS Latin US. The set includes all printable ASCII characters as well as some accented letters, Greek letters, icons, and line-drawing symbols. It is sometimes referred to as the "OEM font" or "high ASCII", or as "extended ASCII".
This character set remains the primary set in the core of any EGA and VGA-compatible graphics card. As such, text shown when a PC reboots, before fonts can be loaded and rendered, is typically rendered using this character set. Many file formats developed at the time of the IBM PC are based on code page 437 as well.

Display adapters

The original IBM PC contained this font as a 9×14 pixels-per-character font stored in the ROM of the IBM Monochrome Display Adapter and an 8×8 pixels-per-character font of the Color Graphics Adapter cards. The IBM Enhanced Graphics Adapter contained an 8×14 pixels-per-character version and—for the CGA compatible modes—an 8x8 pixels-per-character version.. The VGA contained a 9×16 version.
All these display adapters have text modes in which each character cell contains an 8-bit character code point, giving 256 possible values for graphic characters. All 256 codes were assigned a graphical character in ROM, including the codes from 0 to 31 that were reserved in ASCII for non-graphical control characters.
Various Eastern European PCs used different character sets, sometimes user-selectable via jumpers or CMOS setup. These sets were designed to match 437 as much as possible, for instance sharing the code points for many of the line-drawing characters, while still allowing text in a local language to be displayed.

Alt codes

A legacy of code page 437 is the number combinations used in Windows Alt codes. A DOS user could enter a character by holding down the Alt key and entering the character code on the numpad and many users memorized the numbers needed for CP437. Although Microsoft Windows used different character sets such as CP1252, the original numbers were emulated so users could continue to use them; Microsoft added the ability to type a code from the Windows character set by typing 0 before the digits.

Character set

The following tables show code page 437. Each character is shown with its equivalent Unicode code point. A tooltip, generally available only when one points to the immediate left of the character, shows the Unicode code point name and the decimal Alt code. See also the notes below, as there are multiple equivalent Unicode characters for some code points.
Although the ROM provides a graphic for all 256 different possible 8-bit codes, some APIs will not print some code points, in particular the range 0-31 and the code at 127. Instead, they will interpret them as control characters. For instance, many methods of outputting text on the original IBM PC would interpret hex codes 07, 08, 0A, and 0D as BEL, BS, LF, and CR, respectively. Many printers were also unable to print these characters.
When translating to Unicode some codes do not have a unique, single Unicode equivalent; the correct choice may depend upon context.

History

The repertoire of code page 437 was taken from the character set of Wang word-processing machines, according to Bill Gates in an interview with Gates and Paul Allen that appeared in the 2 October 1995 edition of Fortune Magazine:

"... We were also fascinated by dedicated word processors from Wang, because we believed that general-purpose machines could do that just as well. That's why, when it came time to design the keyboard for the IBM PC, we put the funny Wang character set into the machine—you know, smiley faces and boxes and triangles and stuff. We were thinking we'd like to do a clone of Wang word-processing software someday."

According to an interview with David J. Bradley the characters were decided upon during a four-hour meeting on a plane trip from Seattle to Atlanta by Andy Saenz, Lew Eggebrecht and himself.
The selection of graphic characters has some internal logic:
  • Table rows 0 and 1, codes 0 to 31, are assorted dingbats. The isolated character 127 also belongs to this group.
  • Table rows 2 to 7, codes 32 to 126, are the standard ASCII printable characters.
  • Table rows 8 to 10, codes 128 to 175, are a selection of international text characters.
  • Table rows 11 to 13, codes 176 to 223, are box drawing and block characters. This block is arranged so that characters 192 to 223 contain all the right arms and right-filled areas. The original IBM PC MDA display adapter stored the code page 437 character glyphs as bitmaps eight pixels wide, but for visual enhancement displayed them every nine pixels on screen. This range of characters had the eighth pixel column duplicated by special hardware circuitry, thus filling in gaps in lines and filled areas. The VGA adapter allows this behaviour to be turned on or off.
  • Table rows 14 and 15, codes 224 to 254 are devoted to mathematical symbols, where the first twelve are a selection of Greek letters commonly used in physics.
Most fonts for Microsoft Windows include the special graphic characters at the Unicode indexes shown, as they are part of the WGL4 set that Microsoft encourages font designers to support. To draw these characters directly from these code points, a Microsoft Windows font called MS Linedraw replicates all of the code page 437 characters, thus providing one way to display DOS text on a modern Windows machine as it was shown in DOS, with limitations.
Code page 1055, also known as HP symbol set 0L, is a subset which includes the box-drawing, half-blocks, black circles, and black square, and moves them to the upper half; the space is also included.

Internationalization

Code page 437 has a series of international characters, mainly values 128 to 175. However, it only covers a few major Western European languages in full, including English, German and Swedish, and so lacks several characters important to many major Western European languages:
  • Spanish: Á, Í, Ó, and Ú
  • French: À, Â, È, Ê, Ë, Î, Ï, Ô, Œ, œ, Ù, Û, and Ÿ
  • Portuguese: Á, À, Â, Ã, ã, Ê, Í, Ó, Ô, Õ, õ, and Ú
  • Catalan: À, È, Í, Ï, Ò, Ó, and Ú
  • Italian: À, È, Ì, Ò, and Ù
  • Icelandic: Á, Ð, ð, Í, Ó, Ú, Ý, ý, Þ, and þ
  • Danish/Norwegian: Ø and ø. Character number 237, the small phi, could be used as a surrogate even though it may not render well. To compensate, the Danish/Norwegian and Icelandic code pages replaced cent sign with ø and the yen sign with Ø.
  • Most Greek alphabet symbols were omitted, beyond the basic math symbols.
Along with the cent, pound sterling and yen/yuan currency symbols, it has a couple of former European currency symbols: the florin and the peseta. The presence of the last is unusual, since the Spanish peseta was never an internationally relevant currency, and also never had a symbol of its own; it was simply abbreviated as "Pt", "Pta", "Pts", or "Ptas". Spanish models of the IBM electric typewriter, however, also had a single position devoted to it.
Later DOS character sets, such as code page 850, code page 852 and code page 737, filled the gaps for international use with some compatibility with code page 437 by retaining the single and double box-drawing characters, while discarding the mixed ones. All code page 437 characters have similar glyphs in Unicode and in Microsoft's WGL4 character set, and therefore are available in most fonts in Microsoft Windows, and also in the default VGA font of the Linux kernel, and the ISO 10646 fonts for X11.