[Simh] Idle detection not working for TOPS20?

[Bob Armstrong]

  I set up a TOPS20 system with simh (simh v3.8.1 on Ubuntu, TOPS20 v4.1
clean install).  The installation went fine with no problems, but I cannot
get the idle detection feature in simh to work.  I say -

	pdp10
	sim> att rp0 tops20-4.1.rp06
	sim> set cpu tops20
	sim> set cpu idle tops20
	sim> show cpu
	CPU, idle enabled, stability wait = 20s, 1048KW, TOPS-20
	sim> b rp
	....

Everything boots and runs fine, but simh uses about 98% of the host CPU and
never goes down.  In an hour of wall clock time the pdp10 process uses about
58+ minutes of CPU time.  Clearly the idle detection isn't working - is it
broken, or am I doing something wrong?

  BTW, I find that I can break the emulation and usually I'll see 

	Simulation stopped, PC: 000003 (SOJG 2,3)

According to the simh doc, SOJG 2,3 running in AC3 is the expected TOPS20
idle job, so that part hasn't changed.

Thanks,
Bob Armstrong