This is an erroneous assumption with regard to DOS. You might find the
following two links informative with regard to the origins of the term "DOS"
which, by the way, predate the PC and go back to the IBM mainframe era when
they did have disk drives by the way:
http://inventors.about.com/library/weekly/aa033099.htm
http://www.windowsreinstall.com/install/msdos/msdos/ods/historyofdos.html
The OP was referring to MS-DOS which, by the way, has only been used on
systems with disk drives. MS-DOS was based on QDOS, an acronym that
actually meant, "Quick and Dirty Operating System" and the term, obviously,
had nothing to do with disk drives at all. Those systems that did not have
hard drives, booted from a floppy disk that then loaded the necessary
information into memory but that system was not DOS nor was it called DOS.
In the IBM context, it did mean Disk Operating System but those computers on
which it was used had disk drives.
In the context of the OP, the "DOS" to which it is referring is MS-DOS, a
command based operating system designed for the purpose of being able to
enter simply commands to perform computer based tasks. Microsoft purchased
the rights to QDOS, renamed it MS-DOS as in Microsoft Disk Operating System,
made some modifications and presented it to IBM to use on their first
personal computers for the precise purpose of providing a simplified
interface in which the average user, in an office, with some training, could
easily function.