PDP11 I/O page memory map

[Noel Chiappa]

    > From: Paul Koning

    > RTI/RTT are used in the debugger, so they need to work in user mode.
    > They refuse to raise your privilege level, though.

I understand that it has uses, but by specifying the 'failure' mode in User
mode (when the contents of the current or previous modes is not User) to be
'ignore', rather than 'trap', that's one more thing that makes the PDP-11
non-virtualizable. (This choice, to ignore, instead of trap, has the same
issue in other places where it's done that way, e.g. RESET.)

     Noel