how does adb/dbx work?

[Guy Harris]

> > 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.

Depends on what you mean by "hardware single step" and "out of the
question".  The PDP-11, VAX, M68000, and several other processors have a
"trace bit" in the processor status {word|longword} which causes the
processor to trap at the end of the current instruction.  This is perfectly
usable in a multi-user system, considering the UNIX debuggers use it.
Running a program by single-stepping it is, however, slow as molasses in
January, whether single-user or multi-user (consider how many instructions
are executed by the kernel and the debugger per instruction executed by the
debuggee).

	Guy Harris