1 08/22/83  New features in the Multics Communications System
 2 
 3 06/15/81 Multiple canonicalization algorithms:
 4 Multics now supports two different algorithms for converting a typed
 5 string into its canonical representation.  The algorithm in effect is
 6 controlled by a new mode named "can_type".
 7 
 8 The command
 9      set_tty -modes can_type=replace
10 causes only the last character typed in any column to appear in the
11 input line.  This is "replacement canonicalization" as described below
12 and is intended for use with most video display terminals (CRTs).
13 
14 The command
15      set_tty -modes can_type=overstrike
16 causes all characters typed in any column to appear in the input line.
17 This is "overstriking canonicalization" as described below and was the
18 only form of canonicalization present in previous versions of Multics.
19 
20 
21 Description:
22 The description of this mode as given in the write-up of tty_ in MPM
23 Communications is:
24 
25 can_type=overstrike, can_type=replace
26     specifies the method to be used to convert an input string to
27     canonical form.  Canonicalization is only performed when the I/O
28     switch is in "can" mode.  (Default is can_type=overstrike.)
29 
30 
31 overstriking canonicalization:
32 When can_type=overstrike is specified, the canonicalization algorithm
33 in use is equivalent to the only algorithm available in previous
34 Multics releases.  This algorithm assumes that the user is typing input
35 on a terminal which is capable of displaying several characters in a
36 single column.
37 
38 
39 replacement canonicalization:
40 When can_type=replace is specified, the canonicalization algorithm
41 assumes that the user is typing input on a terminal which is not
42 capable of displaying multiple characters in a single column.
43 (Examples of these terminals include most modern video (CRT)
44 terminals.)  Only the last character entered into a column is visible;
45 replacement canonicalization causes the canonical form of typed input
46 to contain only the last character entered in any column.
47 
48 For example with can_type=replace, typing:
49 
50     This is a tsetBBBest of tpying text.BBBBBBBBBBByp<LF>
51 
52 where B is a backspace character and <LF> is the line-feed character
53 will appear on the screen and be input as:
54 
55     This is a test of typing text.
56 
57 
58 
59 When using can_type=replace, it is not possible to overstrike a
60 character with the erase character.  In other words, it is not possible
61 to delete a character in the middle of a typed line without
62 repositioning to the character in question and retyping the rest of the
63 line.
64 
65 Therefore, the user may wish to disable the erase character when using
66 replacement canonicalization.  This may be accomplished by the command
67 line:
68     set_tty -edit \400
69 
70 
71 When using can_type=replace, the following assumptions are made about
72 the nature of the terminal being used:
73 
74 o Overstriking a character with any other printing character or a space
75   causes the first character to be erased.
76 
77 o Entering a tab character simply moves the cursor position to the next
78   tab stop (column 11, 21, etc.) without erasing any intervening
79   characters.
80 
81 
82 For more detail on this feature, refer to the "Typing Conventions"
83 section of the Multics Programmer's Manual -- Communications I/O (Order
84 No. CC92).