DEC Radix-50

RADIX-50, commonly called Rad-50, RAD50 or DEC Squoze, is a character encoding created by Digital Equipment Corporation for use on their DECsystem, PDP, and VAX computers. RADIX-50's 40-character repertoire (050 in octal) can encode six characters plus four additional bits into one 36-bit word (PDP-6, PDP-10/DECsystem-10, DECSYSTEM-20); three characters plus two additional bits into one 18-bit word (PDP-9, PDP-15); or three characters into one 16-bit word (PDP-11, VAX).

The actual encoding differed between the 36-bit and 16-bit systems.

Etymology

The name "SQUOZE" was borrowed from the scheme used in the SHARE 709 operating system for representing object code symbols; it packed six characters of a 50-character alphabet plus two additional flag bits into one 36-bit word.

36-bit systems

Radix-50 in 36-bit systems was commonly used in symbol tables for assemblers or compilers which supported six-character symbol names. This left four bits to encode properties of the symbol.

Radix-50 was not normally used in 36-bit systems for encoding ordinary character strings; file names were normally encoded as six 6-bit characters, and full ASCII strings as five 7-bit characters and one unused bit per 36-bit word.

PDP-6, PDP-10/DECsystem-10, DECSYSTEM-20[1]
Most
significant
bits		 Least significant bits
000 001 010 011 100 101 110 111
000 space 0 1 2 3 4 5 6
001 7 8 9 A B C D E
010 F G H I J K L M
011 N O P Q R S T U
100 V W X Y Z . $ %

18-bit systems

Radix-50 (called Radix 508 format) was used in Digital's 18-bit PDP-9 and PDP-15 computers to store symbols in symbol tables, leaving two extra bits per word ("symbol classification bits").[2]

16-bit systems

Some strings in DEC's 16-bit systems were encoded as 8-bit bytes, while others used Radix-50. In Radix-50, strings were encoded in successive words as needed, with the first character within each word located in the most significant position. For example, using the PDP-11 encoding, the string "ABCDEF", with character values 1, 2, 3, 4, 5, and 6, would be encoded as a word containing the value 1×402 + 2×401 + 3×400 = 1,683, followed by a second word containing the value 4×402 + 5×401 + 6×400 = 6,606. Thus, 16-bit words encoded values ranging from 0 (three spaces) to 63,999 ("999"). When there were fewer than three characters in a word, the last word for the string was padded with trailing spaces.

There were several minor variations of the encoding families. For example, the RT-11 operating system considered the character corresponding to value 011101 to be undefined, and some utility programs used that value to represent the * character instead.

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 a filename could be stored in two 16-bit words, while three more extension (file type) characters could be stored in a third 16-bit word. The period that separated the filename and its extension was implied (i.e., was not stored and always assumed to be present). Rad-50 was also commonly used in the symbol tables of the various PDP-11 programming languages.

PDP-11, VAX[3]
Most
significant
bits		 Least significant bits
000 001 010 011 100 101 110 111
000 space A B C D E F G
001 H I J K L M N O
010 P Q R S T U V W
011 X Y Z $ . % 0 1
100 2 3 4 5 6 7 8 9

References

  1. ↑ Frank Durda IV. "RADIX50 Character Code Reference". 2004. http://nemesis.lonestar.org/reference/telecom/codes/radix50.html
  2. ↑ Digital Equipment Corporation, PDP-9 Utility Programs--Advanced Software System--Programmer's Reference Manual, full text, Digital Equipment Corporation, 1968, Appendix 1.
  3. ↑ 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
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