Re: 11/40 misbehaviour

From: John Santos (JOHN_at_egh.com)
Date: 08/05/04

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    Date: Wed, 4 Aug 2004 23:26:35 -0400

    On 4 Aug 2004, Rich Alderson wrote:

    > I'm working on an 11/40 running RSX which is somewhat flaky: It keeps getting
    > T04 halts, which the manual tells me indicate that the trap bit (4) of the PSW
    > is getting set, but that RSX has no handler for this. It happens fairly often,
    > but not predictably.
    >
    > I don't have a set of 11/40 diagnostics, only 11/34--some will run, most will
    > not. Where can I find RX01 disk images with 11/40 diags? Or does anyone have
    > a feel for what might be causing the T04 halts?

    Are you sure it is a PSW bit 4 (T bit, indicates trace trap), and not a
    trap through the vector at 4? Debuggers use the trace trap in conjunction
    with the RTT instruction to single-step. They set the "T" bit in the saved
    PSW, then RTT. One instruction executes and then traps again. The debugger
    has to set up the trap handler for the trace trap (at the vector at 14) and
    proceed accordingly. Getting spurious trace-traps is very weird. Either
    you have bad memory in the kernel stack area (which is setting bit 4 in the
    saved PSW from various random interrupts), or the PS hardware itself is
    flakey and is reading or writing the bit 4 incorrectly.

    Trap through 4 could be anything... T4 is Timeout and other errors,
    according to my PDP 11/40 Processor Handbook. A Unibus timeout could
    be caused by incorrect device configuration, but I think the device
    setup code at boot time should try to protect you against this. (It
    does on RSTS, by disabling non-existent devices, but I'm not nearly
    as familiar with RSX.) Memory could be going out to lunch, and failing
    to respond to reads or writes. Or the bus cable(s), jumper(s) or
    terminators could be bad, and randomly changing an address bit causing
    a memory or device address to be wrong.

    Have you tried all the standard things, i.e. reseating boards, checking
    PS voltages, etc.?

    If that doesn't help, try paring down the system to the minimal
    working configuration (CPU, console terminal, enough memory to run,
    and the system disk) and then add things back one at a time until
    the problem recurs.

    I haven't seen PDP-11/40 diags in years. We used to have a tape
    (9-track, 800 bpi) of them, but it is long gone.

    HTH. 

    -- 
John Santos
Evans Griffiths & Hart, Inc.
781-861-0670 ext 539
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