The following Forth words can be used for debugging:

   debug <xt> - Mark word for debugging
   debug-off  - Unmark all words for debugging
   resume     - Return from subordinate Forth interpreter

The source debugger also implements the following commands when it has been activated:

   Up - Unmark current word for debugging, mark parent and continue
   Down - Mark next word for debugging
   Trace - Continue execution until end of word displaying
           debug information
   Rstack - Display contents of the Rstack
   Forth - Launch subordinate Forth interpreter

An example session:
0 > see boot 
: boot
  linefeed parse cr " platform-boot" $find if
  execute then
  2drop cr " Booting " type type cr "   ... not supported on this system." type cr
  ;
 ok
0 > debug boot 
Stepper keys: <space>/<enter> Up Down Trace Rstack Forth
 ok
0 > boot 
: boot  ( Empty ) 
00000000ffe26b08: linefeed  ( a ) 
00000000ffe26b10: parse  ( ffec6e24 0 ) 
00000000ffe26b18: cr 
 ( ffec6e24 0 ) 
00000000ffe26b20: (")  ( ffec6e24 0 ffe26b30 d ) 
00000000ffe26b40: $find  ( ffec6e24 0 ffe31710 ffffffffffffffff ) 
00000000ffe26b48: do?branch  ( ffec6e24 0 ffe31710 ) 
00000000ffe26b58: execute [sparc64] Booting file 'cdrom' with parameters ''
Not a bootable ELF image
Not a Linux kernel image
Not a bootable a.out image
Loading FCode image...
Loaded 5936 bytes
entry point is 0x4000
Evaluating FCode...
open isn't unique.
Boot load failed.
 ( Empty ) 
00000000ffe26b60: dobranch  ( Empty ) 
00000000ffe26bf8: (semis) 
[ Finished boot ]  ok
0 >