Disk operating system
From Computer History Wiki
A disk operating system (often DOS) is an operating system which adds support for secondary storage on disks, principally by adding a file system.
Many early computers, and early personal computers, went through a stage, early on, where the very earliest systems did not support disks, and then a DOS was created/released for them. Examples include the IBM System/360, the PDP-11, and the IBM PC and Apple II.
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