Below is the basic commands that the debugging tool supports.

 
        'A'  -  ?? - Error              'B'  -  ?? - Error
        'C'  -  ?? - Error              'D'  -  Display memory
        'E'  -  Enter                   'F'  -  Fill memory
        'G'  -  Go                      'H'  -  Hex math
        'I'  -  ?? - Error              'J'  -  ?? - Error
        'K'  -  ?? - Error              'L'  -  Load file.s19
        'M'  -  Move memory             'N'  -  ?? - Error
        'O'  -  ?? - Error              'P'  -  Toggle Printer Echo
        'Q'  -  Quit                    'R'  -  Register
        'S'  -  Search memory           'T'  -  Trace execution
        'U'  -  Unassemble              'V'  -  Video Reset
        'W'  -  ?? - Error              'X'  -  eXtended display mode
        'Y'  -  ?? - Error              'Z'  -  Place Cursor
        '?'  -  Display this screen

You can see this screen by just typing a '?' at the command prompt.  If you 
want a little more information type the command letter followed immediately
by a question mark, like U? will give you:
*u?
(U)nassemble machine code
u
u [start] [end]
u [start] L [length]
*
start = start address  
end   = end address
etc...

A typical session might go something like this:

c:ddt68                              Will load the debugger
c:ddt68 c                            Will load the debugger and CM1_C000.S19
c:ddt68 e                            Will load the debugger and ECU_8000.S19
*lmy.s19                             Load your S19 file
*r                                   Will display current registers and flags
*u                                   Unassemble a screen worth of code
*t                                   Trace one instruction
*g012A                               Run till you hit the address $012A
*d1000                               Display memory at $1000
*q                                   Quit