Yes, I know it’s old and retired and nobody cares, but I still have to support a couple Alphas running Tru64 and I always seem to forget this.

To display the status of the CPUs, use the psrinfo command. This will tell you which CPUs are online or offline.

# psrinfo
0 on-line since 01/27/2011 09:35:36
1 on-line since 01/27/2011 09:35:36
2 on-line since 01/27/2011 09:35:36
3 on-line since 01/27/2011 09:35:36
4 on-line since 01/27/2011 09:35:36
5 on-line since 01/27/2011 09:35:36
8 on-line since 01/27/2011 09:35:36
9 on-line since 01/27/2011 09:35:36
12 on-line since 01/27/2011 09:35:36
13 off-line since 01/27/2011 09:53:52

To put a CPU offline or online use the psradm command.

Offline:
# psradm -f CPU NUMBER

For example, when I turn off CPU 13 listed above, I used the following:
# psradm -f 13

Online:
# psradm -n CPU NUMBER