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