QMS 2425 Reference page 146

Also known as System Use, this non-configurable client is the amount
of RAM used to run the printer's operating system. It's never
increased or decreased. The system memory subtracted from the
total amount of RAM identifies the amount of RAM available for all the
other memory clients.
You can add one internal IDE hard disk (standard on the Executive
Model) and up to three external SCSI hard disks to your QMS 2425/
2425 TURBO Print System. These hard disks serve as secondary
storage places for such items as downloaded fonts, emulations, and
spooled data.
» Note: See FAQ 7191 for a list of each approved SIMM, its
manufacturers and part numbers, and the QMS products on which it
can be used. You can access QMS FAQs through the Internet at
http://www.qms.com/support/supportbase/ or through Q-FAX. (See
appendix A, "QMS Customer Support," to find out how to access Q-
FAX documents). FAQ numbers may be subject to change, so you
may want to obtain a directory of Q-FAX documents before requesting
a specific document.
Your printer supports virtual memory capabilities for all of the memory
clients except Disk Cache and Frame Buffer when a hard disk is
installed and disk swapping is enabled (Administration/Memory/
Enable Disk Swap menu). These capabilities extend the amount of
memory available for certain supported features, such as spooling
and collation.
» Note: Chunk collation begins automatically after 100 pages
regardless of how much memory is available to the Display List client.