DEC Radix-50

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.

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	.	$	%

PDP-11, VAX^[2]

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.

References

1. ^ Frank Durda IV. "RADIX50 Character Code Reference". 2004. http://nemesis.lonestar.org/reference/telecom/codes/radix50.html
2. ^ 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