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Thread: Console lock on PDP-11

  1. #1

    Default Console lock on PDP-11


    Reading the console documentation on PDP-11, I found this sentence about the LOCK :
    Disables all console controls except switch register
    Ok, but reading this "except switch register", I don't understand why switch register always enables in LOCK position . What is the point of that ? If all console controls are disabled ,then load adr,deposit etc are disabled. Is there something that we can do in LOCK position , or not at all ?.

  2. #2


    Yes. The CPU will still be able to read the contents of the switches at address 177570 (if I remember the address correctly).

  3. #3


    Yes, I am agree with you. So, operator can change few switches , CPU can read it. But after that ? I don't understand the utility . Maybe, it is useful for maintenance operation or something else...But I don't see what..

  4. #4
    Join Date
    Sep 2006
    Silicon Valley


    I would have to do some digging if PDP-11 Fortran supported it, but there were many minicomputer Fortrans that supported the SENSE statement that
    could read a set of front panel sense switches

  5. #5


    I see now, can be useful to modify behavior of some programs. In this case console is used as a secondary input device. Thanks.

  6. #6
    Join Date
    Mar 2017
    New Jersey, USA


    Bell Labs Unix would boot differently depending on the switch settings. Also, depending on the switch settings it would display different information on the front panel LEDs.

    See page 289 of

    I don't know whether the lock switch would also disable the front panel LEDs, but I am guessing it did not.



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