Article 2892 of dec.notes.technology.dechips:
Title: A way to stop the system clock off ? 
Reply Title: Try this (on Rawhide, anyway..)

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Note 703.1            A way to stop the system clock off ?                1 of 1
MAY30::CUMMINS                                       31 lines   9-JUL-1997 09:51
                                -< Try this.. >-
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   Rawhide uses a free-running clock (on the motherboard). The clock cannot
   be disabled, but the interrupt can be disabled by masking it off in ICSR
   (ICSR:<22>, since the interval timer comes in on IRQ #2).
   
   UNIX PAL does a read/modify/write to ICSR when enabling superpage mode,
   floating-point enable, etc. In addition, console/PAL provides a deposit
   IPR function on Rawhide. One of the many IPRs that can be written/read
   at console is ICSR. The deposit will update the PAL impure area copy of
   ICSR, but this will be used on a subsequent boot. Naturally, ICSR must
   be modified on each CPU in an SMP configuration.
   
   The following command sequence is only applicable on Rawhide platforms
   (Dodge, Durango, Wrangler, Tincup, and DaVinci).
   
   P00>>>edit nvram           # executed on power-up/reset by console
   *10 for i in `ps|grep idle|find 5`;do d ipr:18 c140220000 &p$i;done 
   *exit
   P00>>>nvram                # will execute NVRAM script
   
   Let me know if questions.
   
   Finally, Lee Parker was asking similar questions for an outside customer
   (Encore) roughly a year ago. He would very likely be of help here... ELF
   listing follows:
   
   Common Name:   LEE PARKER 
   Search Surname:  PARKER,  Lee William E. Parker,  Lee  Search Given Name:  LEE,
   WILLIAM,  WILLIAM E,  William E. Lee,  William E. Parker  DTN: 343-1663,  
   343-1663  Telephone:  (770)343-1663  Intrnl Mail Addr:  ALF1-3/U26  
   Location:  ALF  Node:  decatl  Username:  parker  Org Unit:  MCS,  
   Realtime Expertise Center,  Atlanta CSC  Position:  UNIX Systems Support Consultant