how does adb/dbx work?

[Ken Goldman]

> I was wondering if somebody could explain how adb (and dbx) executes programs.
> Does it have a software simulator that interprets the instructions in the
> object file of the program being debugged?  Or does it use the VAX hardware
> to execute the program, say by hardware single-stepping. 

Maybe I can help with some PDP11 insight.  First, hardware single step
is out of the question in a multi-user system.  The way the PDP11
debuggers work is by replacing the instruction to be traced with a
trap instruction.  Then the software runs full speed until it hits the
trap, at which point you enter the debug software.  At that point you
can examine registers, memory, etc.

The debugger I used was for assembly language.  I suppose that a symbolic
debugger understands the symbol tables and such.