Which is oldest operating system??

Question

Guys which is oldest operating system of microsoft

Answer 1

Oldest operating system was invented by Chinese, it's called abacus.

Answer 2

MS DOS maybe? it has initial release on August 12, 1981, the first OS from windows series is the following https://en.wikipedia.org/wiki/Windows_1.0 released November 20, 1985.

Answer 3

It,s Abacus invented in China.

But oldest windows operating system is ☆ MS DOS ☆