[pups] 2.11BSD boot hangs.

John Holden johnh at psych.usyd.edu.au
Tue Feb 27 13:02:55 AEST 2001

On BA-23 boxes, there is a small, two lever DIP switch. Switch 1 in the
ON position (down) enables BEVENT on the Qbus. Without it enabled, there
will no line time clock interrupts generated, even if the LTC register at
777546 in enabled.

The second switch enables the 'restart' button when ON. Pressing 'restart'
starts a powerup sequence, running diagnostics and starting ODT or the
bootstrap (equivalent to cycling the power). 

On the earlier BA-11 series boxes for the LSI-11 (and /23), there was an
equivalent switch on the front panel labeled  "Aux". It could be used either to
enable the BEVENT or the remote switch for the cabinet power controller. The
11/23plus, 11/53/73 have programable LTC registers, so the switch is normally
left on. On the LSI-11,/2 and early 11/23, you would initially boot the machine
with it off, then enable it.

As for machines hanging without the LTC running, the problem is that the
scheduler (sched) never gets to run, since it sleeps waiting for the 'lbolt'
flag that is only ever set in the clock interrupt service routine. The timeout
queue also doesn't run, so only the internal 'init' process will ever get to run


You can run into problems with the BHALT line, which can be asserted by a line
break on the console line (if enabled), or on some DHV serial card emulations.
Turning off a terminal may be enough to halt the process if it generates a
serial break as the power goes down.

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