DEC/PDP Character Codes

DEC/PDP, The now defunct Digital Equipment Corporation, used three main character sets on its computers. The main one was of course the standard seven-bit ASCII, the other two were six bit codes.

DEC Six-bit, was simply the 64 basic ASCII characters, with codes from 32-95 (decimal) or 20-5F (hexadecimal) or 40-77 (octal), in exactly the same order, but with 32 (decimal) subtracted from their codes, so they ran from 0 to 63. The missing characters were control codes and lower case letters (and oddities like curly brackets), and as sixbit was mostly used when interacting with the operating system to specify a file name or something of that nature, those characters were not missed. (Most operating systems do not give files case-sensitive names, or allow invisible letters in them). Six bits was a good character size because the really important DEC computers (PDP-10 and such) had 36 bit memory words, and six characters (with a three character extension) was considered to be plenty for a file name.

DEC Squoze was a 40-character code, containing only letters, digits, and a couple of other bonus characters. Strings in Squoze are computed base-40, which means that a 6 character string would only occupy 32 bits. That meant that in the standard 36 bit memory word, there would be enough space for a 6 character file name together with four bits of flags or other information.

DEC Sixbit Character Table, Octal

__0 __1 __2 __3 __4 __5 __6 __7
0_
 
!
"
#
$
%
&
'
1_
(
)
*
+
,
-
.
/
2_
0
1
2
3
4
5
6
7
3_
8
9
:
;
<
=
>
?
4_
@
A
B
C
D
E
F
G
5_
H
I
J
K
L
M
N
O
6_
P
Q
R
S
T
U
V
W
7_
X
Y
Z
[
\
]
^
_


DEC Sixbit Character Table, Decimal

__0 __1 __2 __3 __4 __5 __6 __7 __8 __9
0_
 
!
"
#
$
%
&
'
(
)
1_
*
+
,
-
.
/
0
1
2
3
2_
4
5
6
7
8
9
:
;
<
=
3_
>
?
@
A
B
C
D
E
F
G
4_
H
I
J
K
L
M
N
O
P
Q
5_
R
S
T
U
V
W
X
Y
Z
[
6_
\
]
^
_


DEC Sixbit Character Table, Hexadecimal

__0 __1 __2 __3 __4 __5 __6 __7 __8 __9 __A __B __C __D __E __F
0__
 
!
"
#
$
%
&
'
(
)
*
+
,
-
.
/
1__
0
1
2
3
4
5
6
7
8
9
:
;
<
=
>
?
2__
@
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
3__
P
Q
R
S
T
U
V
W
X
Y
Z
[
\
]
^
_




DEC Squoze Character Table

Decimal Octal Character
0 00 space
1 01 0
2 02 1
3 03 2
4 04 3
5 05 4
6 06 5
7 07 6
8 10 7
9 11 8
10 12 9
11 13 A
12 14 B
13 15 C
14 16 D
15 17 E
16 20 F
17 21 G
18 22 H
19 23 I
20 24 J
21 25 K
22 26 L
23 27 M
24 30 N
25 31 O
26 32 P
27 33 Q
28 34 R
29 35 S
30 36 T
31 37 U
32 40 V
33 41 W
34 42 X
35 43 Y
36 44 Z
37 45 .
38 46 $
39 47 %


Example of the squoze encoding of "FILE05" into the 32 bit number 1688472045:

'F' 'I' 'L' 'E' '0' '5'
16 19 22 15 1 5
((((16 ×40+19) ×40+22) ×40+15) ×40+1) ×40+5 = 1688472045