If you press control-shift-D at the system console, the system will enter CONSOLE DIAG mode. Here, all the CPU's are halted at softint level. They can process device interrupts but they cannot execute any thread-level code. The source for this stuff is in oz_knl_diag.c.
You are given this menu:
1=ShowProcs 10=ShowDate 15=ShowDevs 20=ThreadTrace 2=ShowThreads 11=PeekPoke 16=AbortThread 3=ShowKthreads 12=Exit 17=SuspendThread 4=ThreadLevel 13=DumpNpp 18=ResumeThread 5=Logon 14=DumpEvents 19=CallDebugger oz_knl_diag#Make a selection by typing the number and pressing Enter. 12 is the exit code that will return the system to normal operation.
Selection 4 is slightly special. It will put the diag routines in thread-level. So the other CPU's are now free to execute normally. The current cpu, however will present you with this menu:
1=SoftintLevel 4=CreateLnm 7=MountVol 10=CopyFile 2=RunImage 5=DisplayFile 8=ListDir 11=DismVol 3=ShowLogical 6=InitVol 9=CreateDir 12=Exit oz_knl_diag>Different options here. But 12 returns the console to normal operation. 1 returns back to the softint level menu (above).
These prompts were originally my startup routine before I had a command-line interpreter (shell).
One thing you can do that is very useful is if your startup script is messed up and you can't log on. You can start the shell by going into diag mode then: