Code Page 437

Code page 437 (CCSID 437) is the character set of the original IBM PC (personal computer). It is also known as CP437, OEM-US, OEM 437, PC-8, or DOS Latin US. The set includes all printable ASCII characters as well as some accented letters (diacritics), Greek letters, icons, and line-drawing symbols. It is sometimes referred to as the "OEM font" or "high ASCII", or as "extended ASCII" (one of many mutually incompatible ASCII extensions).

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 (MDA) and an 8×8 pixels-per-character font of the Color Graphics Adapter ( CGA) cards. The IBM Enhanced Graphics Adapter (EGA) contained an 8×14 pixels-per-character version, and 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 (see details), 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 keycodes. The user could enter a character by holding down the Alt key and entering the three-digit decimal Alt keycode on the numpad and many users memorized the numbers needed for CP437 (or for the similar code page 850). When Microsoft switched to their proprietary character sets (such as CP1252) and later Unicode in Windows, the original codes were retained; Microsoft added the ability to type a code in the new character set by typing the numpad 0 before the digits.

Character set

The following tables show code page 437. Each character is shown with its equivalent Unicode code point (when it is not equal to the character's code). 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 the codes for BEL, BS, CR and LF. Many printers were also unable to print these characters.

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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 (developer of the PC's ROM-BIOS) the characters were decided upon during a four-hour meeting on a plane trip from Seattle to Atlanta by Andy Saenz (responsible for the video card), Lew Eggebrecht (chief engineer for the PC) and himself.

The selection of graphic characters has some internal logic:
* Table rows 0 and 1, codes 0 to 31 (00_hex to 1F_hex), are assorted dingbats (complementary and decorative characters). The isolated character 127 (7F_hex) also belongs to this group.
* Table rows 2 to 7, codes 32 to 126 (20_hex to 7E_hex), are the standard ASCII printable characters.
* Table rows 8 to 10, codes 128 to 175 (80_hex to AF_hex), are a selection of international text characters.
* Table rows 11 to 13, codes 176 to 223 (B0_hex to DF_hex), are box drawing and block characters. This block is arranged so that characters 192 to 223 (C0_hex to DF_hex) 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.Joshua D. Neal
Attribute Controller Registers: Attribute Mode Control Register
Hardware Level VGA and SVGA Video Programming Information Page: bit 2 is Line Graphics Enable.
* Table rows 14 and 15, codes 224 to 254 (E0_hex to FE_hex) 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. (The monospaced raster font family Terminal was an early font that replicated all code page 437 characters, at least at some resolutions.) 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.

Internationalization

Code page 437 has a series of international characters, mainly values 128 to 175 (80_hex to AF_hex). However, it only covers a few major Western European languages in full, including English, German and Swedish, and so lacks several characters (mostly capital letters) 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 (ED_hex), the small phi (closed form), could be used as a surrogate even though it may not render well (furthermore, it tends to map to Unicode, and/or render in Unicode fonts, as the open-form phi or the closed-vertical-form phi, which are even further from the O with stroke). To compensate, the Danish/ Norwegian and Icelandic code pages ( 865 and 861) replaced cent sign (¢) with ø and the yen sign (¥) with Ø.
* Most Greek alphabet symbols were omitted, beyond the basic math symbols. (They were included in the Greek-language code pages 737 and 869. Some of the Greek symbols that were already in code page 437 had their glyphs changed from mathematical or scientific forms to match the actual use in Greek.)

Along with the cent (¢), pound sterling (£) and yen/ yuan (¥) currency symbols, it has a couple of former European currency symbols: the florin (ƒ, Netherlands) and the peseta (₧, Spain). 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 (DOS Latin-1), code page 852 (DOS Central-European) and code page 737 (DOS Greek), 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 (''e.g.'' horizontal double/vertical single). 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.

See also

* Alt code
* ANSI
* ASCII
* Semigraphical characters
* Atari ST character set, derived from code page 437

Notes

References

External links

IBM PC memory-mapped video graphics to Unicode
on official Unicode site

{{character encoding

437
Computer-related introductions in 1980