CDC display code

Display code is the 6-bit character set used by many computer systems manufactured by Control Data Corporation, notably the CDC 6600 in 1964. The CDC 6000 series, and their followons, had 60 bit words. As such, typical usage packed 10 characters per word.

There were several variations of display code, notably the 63-character character set, and the 64-character character set. There were also 'CDC graphic' and 'ASCII graphic' variants of both the 63- and 64-character sets. The choice between 63- or 64-character character set, and between CDC or ASCII graphic was site-selectable. Generally, early CDC customers started out with the 63-character character set, and CDC graphic print trains on their line printers. As time-sharing became prevalent, almost all sites used the ASCII variant - so that line printer output would match interactive usage. Later CDC customers were also more likely to use the 64-character character set.

A later variation, called 6/12 display code, was used in the Kronos and NOS timesharing systems in order to support full ASCII capabilities. In 6/12 mode, an escape character (the circumflex, octal 76) would indicate that the following letter was lower case. Thus, upper case and other characters were 6 bits in length, and lower case characters were 12 bits in length.

The PLATO system used a further variant of 6/12 display code. Noting that lower case letters were most common in typical PLATO usage, the roles were reversed. Lower case letters were the norm, and the escape character preceded upper case letters.

The typical text file format used a zero-byte terminator to signify the end of each record. The zero-byte terminator was indicated by, at least, the final twelve bits of a 60-bit word being set to zero.^[1] The terminator could actually be anywhere from 12- to 66-bits long - depending on the length of the record. This caused an ambiguity in the 64-character character set, when a colon character needed to be the final character in a record. In such cases a blank character was typically appended to the record after the trailing colon.

Display code characters (64-character character set version)

ASCII CDC Binary Decimal Octal Graphic Graphic  Name  000 000 0 00 : : colon[2] 000 001 1 01 A A   000 010 2 02 B B   000 011 3 03 C C   000 100 4 04 D D   000 101 5 05 E E   000 110 6 06 F F   000 111 7 07 G G   001 000 8 10 H H   001 001 9 11 I I   001 010 10 12 J J   001 011 11 13 K K   001 100 12 14 L L   001 101 13 15 M M   001 110 14 16 N N   001 111 15 17 O O   010 000 16 20 P P   010 001 17 21 Q Q   010 010 18 22 R R   010 011 19 23 S S   010 100 20 24 T T   010 101 21 25 U U   010 110 22 26 V V   010 111 23 27 W W   011 000 24 30 X X   011 001 25 31 Y Y   011 010 26 32 Z Z   011 011 27 33 0 0   011 100 28 34 1 1   011 101 29 35 2 2   011 110 30 36 3 3   011 111 31 37 4 4

ASCII CDC Binary Decimal Octal Graphic Graphic  Name  100 000 32 40 5 5   100 001 33 41 6 6   100 010 34 42 7 7   100 011 35 43 8 8   100 100 36 44 9 9   100 101 37 45 + +   100 110 38 46 - -   100 111 39 47 * *   101 000 40 50 / /   101 001 41 51 ( (   101 010 42 52 ) )   101 011 43 53 $ $   101 100 44 54 = =   101 101 45 55     blank 101 110 46 56 , ,   101 111 47 57 . .   110 000 48 60 # ≡ equiv 110 001 49 61 [ [   110 010 50 62 ] ]   110 011 51 63 % % [3] 110 100 52 64 " ≠ not eq 110 101 53 65 _ → concat[4] 110 110 54 66 ! ∨ log OR 110 111 55 67 & ∧ log AND 111 000 56 70 ' ↑ super 111 001 57 71 ? ↓ sub[5] 111 010 58 72 < <   111 011 59 73 > >   111 100 60 74 @ ≤   111 101 61 75 \ ≥   111 110 62 76 ^ ¬ NOT 111 111 63 77 ; ;

1. ^ CDC often used the term byte to refer to 12-bit quantities, even though characters were only 6-bits long.
2. ^ Display code 00 has no associated graphic in the 63-character set
3. ^ Display code 63 is a : (colon) in the 63-character set
4. ^ In CDC PASCAL, this is the left brace, {, character
5. ^ In CDC PASCAL, this is the right brace, }, character