Ficl Debugger

Ficl release 2.05 includes a simple step debugger for colon definitions and does> words. If you use it and can suggest improvements (or better yet if you write some), please let me know.

Using the debugger

debug <your word here>

If the word is unnamed, or all you have is an xt, you can instead use:

The debugger invokes see on the word, printing a crude source listing, then stops at the first instruction of the definition. There are four (case insensitive) commands you can use from here onwards:

I (step in)

If the next instruction is a colon defintion or does> word, steps into that word's code. If the word is a primitive, simply executes the word.

O (step over)

Executes the next instruction in its entirety

G (go)

Run the word to completion and exit the debugger

L (list)

Lists the source code of the word presently being stepped

Q (quit)

Abort the word and exit the debugger, clearing the stack

X (eXecute)

Interpret the remainder of the line as ficl words for their side effects. Any errors will abort the debug session and reset the VM. Usage example:

x drop 3 \ fix argument on stack

Anything else

Prints a list of available debugger commands

The on-step event

If there is a defined word named on-step when the debugger starts, that word will be executed before every step. As a guideline, this word should have no side effects. Its intended use is to display the stacks and any other VM state you're interested in, but you may have some better ideas. If so, please let me know. The default on-step is:

: on-step ." S: " .s cr ;

Other useful words for debugging and on-step

r.s ( -- )

Prints a represention of the state of the return stack non-destructively. You have to have a good understanding of the return stack side-effects of control words to make sense of it, but it does give an accurate representation of what's there. Example: DO .. LOOPs stack three parameters on the return stack: the loop count and limit, and the LEAVE target address.

.s ( -- )

Prints the parameter stack non-destructively

f.s ( -- )

Prints the float stack non-destructively (only available if FICL_WANT_FLOAT is enabled)

Debugger internals

The debugger words are mostly located in source file tools.c. There are supporting words (debug and on-step) in softcore.fr as well. There are two main words that make the debugger go: debug-xt and step-break. Debug-xt takes the xt of a word to debug (as returned by ', for example) checks to see if it is debuggable (not a primitive), sets a breakpoint at its first instruction, and runs see on it. To set a breakpoint, debug-xt replaces the instruction at the breakpoint with the xt of step-break, and stores the original instruction and its address in a static breakpoint record. To clear the breakpoint, step-break simply replaces the original instruction and adjusts the target virtual machine's instruction pointer to run it.

Step-break is responsible for processing debugger commands and setting breakpoints at subsequent instructions.

To Do