psrinfo and/or prtdiag shows only 3 of 4 CPU's installed on
a Sun Enterprise [TM] e250/450     

                    

Set the upa-noprobe-list to 0 :

  At the OK prompt....

  OK setenv .upa-noprobe-list 0
  OK reset-all


That's it, all 4 cpu's should now show up with psrinfo or prtdiag.

NOTE:
  Depending on the version of OBP on an e250/450, the upa-noprobe-mask
  variable may be named differently.  It may be seen in one of the
  following forms:

    upa-noprobe-list
    upa-noprobe-mask

  On the latest versions of OBP it is a hidden variable

    .upa-noprobe-mask

  You will need to use printenv -a at the OK prompt to see this
  variable with the most current version of OBP.  Although the
  name of the variable has changed a couple of times, its' function
  remains the same.

If printenv -a does not return anything then try running the following command from the ok prompt:
ok printenv .upa-noprobe-mask  
Note: This variable (upa-noprobe-list) can also be viewed and changed from Solaris using the eeprom command.