Text: I didn't know anyone with a DOS system in the early '80s, but I knew plenty of people with Commodore 64s, TI 99-4As, Apple IIs, computers by Tandy, and even Timex--all with distinct and incompatible operating systems.
Entailment: He knew plenty of people that had a Commodore 64 in the '80s.
Contradiction: He knew many people with DOS.