SCI Debugger Official Documentation

The SCI Debugger

Decription

The SCI debugger is invoked by pressing Alt-D. The debugging window will pop up in the upper right corner of the screen, and you may start issuing commands to the debugger. To put the debugger away, just press Alt-D again.

The debugging window looks something like this:

     ---------------------------------
     |                               |
     |   current object              |
     |                               |
     |   opcode                      |
     |                               |
     |   acc:xxxx                    |
     |                               |
     |                               |
     |    Stack           Parms      |
     |     xxxx            xxxx      |
     |     xxxx            xxxx      |
     |     xxxx            ----      |
     |     xxxx            ----      |
     |     xxxx            ----      |
     |                               |
     ---------------------------------

Fields

The fields are:

current object	The name of the object whose method is currently executing.
opcode	The next pmachine opcode to be executed. While I won't go into the details of all the opcodes, the critical ones for debugging are 'send' and 'callk', and 'ret'.
send	Sends a message to the object whose ID is in the accumulator of the pmachine (acc in the window above). The selector and arguments of the message are on the stack, the first five elements of which are displayed in the 'Stack' column in the window. Sometimes a send is just a call to set or get the value of a property, in which case you will not enter a method.
callk	Calls the kernal routine displayed in the opcode.
ret	Returns to the caller of the current method or procedure.
acc	The value in the pmachine accumulator.
Stack	The first five elements on the pmachine stack. A ---- here indicates the end of the stack.
Parms	The first five parameters to the current method or procedure. A ---- here indicates the absence of a parameter.

Commands

Debugging Commands

Commands to the debugger are single-key commands, sometimes followed by a prompt for a number or string. The commands are:

Return or mouse click	Single step to the next pmachine instruction.
Tab	Step across a send or procedure call (not necessary in the case of callk). This is not always reliable. One of these days, I'll look into it.
q	Quit, exitting to DOS.
s	Toggle the send stack on/off. While on, the send stack updates when stepping in the debugger.
i	Inspect an object. You will be prompted for the object to inspect. Enter either the name of the object (such as 'ego', without the quotes or its ID (remember to preceed hex numbers with a '$'). The property names and values of the object will be displayed. Pressing 'i' again (while the inspector window is up) will prompt you for one of the object's selector names. It will then display the property corresponding to the selector either as a number/string pair or as another inspector window. Inspector windows can be nested three deep. To put away an inspector window, press any key but 'i'.
a	Inspect the object whose ID is in the accumulator. This is a quick way to see which object a send opcode is about to send to.
o	Display all objects in the heap. (Same as (ShowObjs FALSE).)
f	Display the free blocks in the heap. (Same as (ShowFree).)
g l t	Display the value of a global, local or temporary variable. You will be prompted for the number of the variable.