how does adb/dbx work?

[John Bruner]

> This is true for breakpoints.  On the other hand, if you are
> "single-stepping" (on a PDP-11 or a Vax, at least), the debugger sets
> the "trace" bit in the subprocess's Program Status Word (PSW).  This bit
> causes a trace interrupt after the next instruction executes, returning
> control to the debugger.  This way, the debugger doesn't need to know
> all the possible places the program might end up after an instruction is
> executed (so it could stuff a trap in each possible location).

The "trace" bit can also be used by an intrusive debugger (one which
is linked with the program), since a program can use the "signal"
system call to catch trace traps and can set the T bit in its own
PS.  I used this once to implement a PC address trace package.
-- 
  John Bruner (S-1 Project, Lawrence Livermore National Laboratory)
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