DABUG III is labeled as from DAVID ANEAR 2/2/81

The following keys active when DABUG has not been enabled
CTRL+Left_Shift - pause BASIC using CTRL-C check vector routine
CTRL+Q - enable DABUG input hook, a (^^) character appears on bottom of screen

Once DABUG mode active (^Q), the follow commands are available:

 CTRL+U = cursor up / start screen editor (halftone block shows on-screen) 
          [no protection from driving beyond screen down through memory]
 CTRL+D = cursor down
          [no protection from driving beyond screen up through memory]
 CTRL+> = cursor right
 CTRL+< = cursor left
 CTRL+A = copy character at cursor to line buffer, move to next position
 CTRL+B = toggle 48x16 mode on OSI 600D
 RUB-OUT = backspace/screen clear
 Shift+O = true backspace
 
 ESC sets BASIC keyword select mode and puts a "=>" character on screen
 the next character typed selects a BASIC keyword. The mapping follows:
     A = ASC
     B = TAN
     C = SIN
     D = SGN
     E = RETURN
     F = FRE
     G = LEN
     H = VAL
     I = STR$
     J = STOP
     K = CONT
     L = LOAD
     M = LOG
     N = ATN
     O = DEF
     P = PEEK
     Q = NEXT
     R = RND
     S = SAVE
     T = RESTORE
     U = CHR$
     V = COS
     W = STEP
     X = LET 
     Y = READ
     Z = DIM
     0 = POKE
     1 = FOR
     2 = THEN
     3 = GOSUB
     4 = GOTO
     5 = INPUT
     6 = DATA
     7 = LEFT$
     8 = MID$
     9 = RIGHT$
     : = USR
     - = INT
   RUB = LIST
 RETURN = RUN
    LF = REM
 SPACE = PRINT
     > = NOT
     < = AND
     / = TAB(
     ; = END
     . = EXP
     , = SQR
     ? = SPC(
     + = CLEAR
   ESC = cancel keyword mode

When active in 48x16 mode, it also removes the graphics character from BASIC print routine - it masks the high bit & refuses to print anything higher than "z" or less than " "
