RADIX-50, commonly called Rad-50 or RAD50, 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) allows up to 3 characters to be encoded and packed into 16 bits (PDP-11, VAX) or 6 characters plus flag information into one 36-bit word (PDP-6, PDP-10/DECsystem-10, DECSYSTEM-20).
The actual encoding differed between the 36-bit and 16-bit systems.
| 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 | . | $ | % |
| 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 |
Strings are encoded in successive words as needed, and the first character within each word is 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*40 + 2) * 40 + 3 = 1683 followed by a word containing the value (4*40 + 5) * 40 + 6 = 6606. Thus, 16-bit encoded values range from 0 (three spaces) to 63999 ("999"). When there are fewer characters in a word than it can hold, the string is 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 * 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 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 assumed to be present). Rad-50 was also commonly used in the symbol tables of the various PDP-11 programming languages.
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