how does adb/dbx work? (hardware debugging facilities)

[Herb Chong - DCS]

In article <456 at ittvax> goldman at ittvax (Ken Goldman) writes:
>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.

just a side note here.  when using the PER debugger on an IBM VM/SP
system, the hardware PER facility is enabled.  you can specify an
address range where virtual storage modification, instruction fetch or
branching causes an interrupt.  the real machine operating system CP
handles this interrupt and gives the appearance of single step
execution to the users.  our 4341's have supported about 25 users
simultaneously running PER of programs while supporting an additional
80 or so users on the machine.  response was noticably degraded since
an interrupt was generated for every instruction executed within the
address ranges for each user using PER, and the virtual machine must
run in 370 EC mode which requires faking empty page and segment table
entries for the virtual DAT that goes on (see how messy it is to have
virtual machines).  370 EC mode is required because the control
registers that specify PER information are only available in EC mode.
BC mode only provides what a 360 does, a PSW.  this is of course
debugging at the assembly language level only, though you can use PER
on anything.  in other words, hardware single step is practical
provided your context switching includes switching of the hardware
control information.

Herb Chong...

I'm user-friendly -- I don't byte, I nybble....

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