Code Page 437 - Difference From ASCII

Difference From ASCII

Code page 437 is based on ASCII, with the following modifications:

• Codes from the C0 control range (00_hex to 1F_hex) and ASCII DEL (7F_hex) can assume their original function as control characters, but also function as graphic symbols when displayed in text mode from the screen buffer. For example, DEL and most of the C0 control codes are displayed graphically in a screen editor like MS-DOS Editor or when written to the screen via the DOS type command; by writing directly to the screen memory rather than using the DOS or BIOS character-output functions, an MS-DOS application can display all of them on the screen. The graphics include smiling faces, card suits, and musical notes. Code 127 (7F_hex), DEL, shows as a "house". This behavior is not specific to code page 437, but shared by all DOS code pages and so-called Windows OEM code pages, which generally resemble code page 437.
• The high-bit range, 128 to 255 (80_hex to FF_hex), is mapped to various symbols: a few European characters (accented Latin letters, etc.) in no particular order and not sufficient for representation of most Western European languages; box drawing characters; mathematical symbols; and a few Greek letters commonly used in mathematics and physics.

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."

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 (except character 127, 7F_hex), codes 32 to 126 (20_hex to 7E_hex), are the standard ASCII printable characters.
• Table rows 8 to 10 (8_hex to A_hex), codes 128 to 175 (80_hex to AF_hex), are a selection of international text characters.
• Table rows 11 to 13 (B_hex to D_hex), 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) of the rows 12 and 13 (C_hex and D_hex) have all right arms (except 217 (D9_hex)) or right filled areas (except 221 (DD_hex)), and this is due to the following technical reason: 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 (eight plus an additional gap). Thus characters with connections at their right edge had to duplicate their eighth pixels to avoid visual interruptions in the designs built up with them. This pixel extension is done by special hardware circuitry, and only this character subset is affected.
• Table rows 14 and 15 (E_hex and F_hex), codes 224 to 255 (E0_hex to FF_hex) are devoted to mathematical symbols, where the first twelve are a selection of Greek letters commonly used in physics. Characters 244 (F4_hex) and 245 (F5_hex) are the upper and lower portion of an italic long S, the symbol used as the integral sign (∫), and they can be extended with the character 179 (B3_hex), the vertical line of the box drawing block. Characters 249 (F9_hex) and 250 (FA_hex) are almost indistinguishable: the first is slightly larger than the second, which resembles the typographic middle dot (·). The character 255 (FF_hex) is merely blank, and acts as a kind of non-breaking space in order to arrange math formulae.