NSS is "Named Saved System".
You can take a snap-shot of a running (or runnable) system on VM
which CP (the hypervisor part of VM) will store into a spool file.
You can then IPL that system by name, rather than boot by device.
The syntax of the IPL command is (gross simplification)
[hcp] ipl <device>
[hcp] ipl <device> [clear]
[hcp] ipl <name>
[hcp] ipl <name> [parm <parms>]
where 'hcp' is optional and would be how you issue CP commands
from Linux. If you're on a VM console, there is no 'hcp',
you might prefix with 'cp' instead on CMS, or you
might simply omit that prefix and let 'ipl'
be recognized as a hypervisor command.
NSS is "virtual ROM" for a named system.
When booting from NSS, the system to be booted comes up instantly,
rather than going through the motions of booting from device.
With care, portions of a Named Saved System can be marked
READ ONLY so that CP (the hypervisor portion of VM)
can share that storage among several virtual machines.
DCSS is related to NSS.
DCSS is a "Discontiguous Shared Segment",
can be read-only or read-write, can be shared or exclusive,
and appears to the guest operating system as attached storage.
(Need not be in the range of defined memory; that is, it can be
ABOVE the defined storage for your virtual machine.)
DCSS is not the same as NSS but is supported by the
same mechanisms within VM (CP). DCSSs are named, like NSSs,
but are "attached" by a DIAGNOSE code, not booted.
