for the people going "MOUSE DRIVERS?!":
DOS didn't come with native support for mice. The original PC didn't natively support them anyway. So when you bought a mouse, it'd come with a disk containing a TSR (terminate-and-stay-resident program, basically a background task (on a single-tasking OS)) that would listen for the mouse to send commands, and provided a new API for programs to query for mouse movement/position/clicks