Ask HN: Oldest code still in common use on PCs?

Was just wondering about this today. My guess might be grep or BC which date back to the earliest unix versions... As a second question, I wonder what the oldest code that's in use across each operating system might be, for example in Windows how far back is there legacy code? MS-DOS 1.0 somewhere in there for legacy? Thought it would be a fun brain exercise for the day...