RADIX-50

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RADIX-50, commonly called Rad-50 or RAD50, is a character encoding created by Digital Equipment Corporation for use on their PDP series and DECsystem-10 and DECSYSTEM-20 computers. RADIX-50's 40-character repertoire (050 in octal) allows up to 3 characters to be encoded and packed into 16 bits (PDP-11) or 6 characters plus flag information into one 36-bit word (PDP-6, PDP-10, DECsystems). The 40 characters are (in order) space, uppercase A-Z, $ (dollar sign), . (period), "undefined", and 0-9. Some applications may substitute another character such as % (percent) for the undefined code.

Strings are encoded big-endian, with the first character in the most significant position. For example, the string "ABC", with character values 1, 2, and 3, would be encoded as (1*40 + 2) * 40 + 3 = 1683. When there are fewer than three characters, they are padded with trailing spaces. 16-bit encoded values range from 0 (three spaces) to 63999 ("999").

The use of Rad-50 was the source of the filename size conventions used by the PDP-11 operating systems. Using Rad-50 encoding, six characters of filename could be stored in two sixteen-bit words while three more characters of extension (filetype) could be stored in a third sixteen-bit word. The period that separated the filename and extension was implied (not stored and always present). Rad-50 was also commonly used in the symbol tables of the various PDP-11 programming languages.

[edit] References

• Compaq Computer Corporation. "Compaq Fortran 77 Language Reference Manual, Appendix B.3: Radix-50 Constants and Character Set". 1999. http://www.helsinki.fi/atk/unix/dec_manuals/cf77au/olrm0398.htm