Can a/some DBMS be installed as an OS Is there any DBMS that can be installed as an OS instead of as an app on a OS?
I wasn't able to find any.

Later edit, just to clarify:
I'm used to installing a DBMS (MySQL, MSSQL) as a server(ice) onto an OS(Windows, Ubuntu, etc.). I was trying to find out if there are some DBMS' that do NOT require an underlying OS and are 'installed' as the OS directly on hardware. I was asking here because googling didn't help.
Thanks for the answers I got, it seems that the answer is close to 'NO'
Yes. IBM i, formerly known as AS/400, is a platform with the operating system that has a relational DBMS as its integral part. Indeed, Db2 for i is in a way an alternative access method to the IBM i file system.