30 years working experience with operating systems.
All the way back to when Windows was a MS-DOS GUI.
OS's: MS-DOS, every Windows version since 3.0, Novell Netware, HP-UX, Banyan-Vines, AIX, OS/2 1.3 (Lan Manager), OS/2 2/3, Linux, and a little bit z/OS (IBM Mainframe).