WordStar is a word processor application that had a dominant market share during the early- to mid-1980s. It was published by MicroPro International, and written for the CP/M operating system but later ported to DOS. Although Seymour I. Rubinstein was the principal owner of the company, Rob Barnaby was the sole author of the early versions of the program. Starting with WordStar 4.0, the program was built on new code written principally by Peter Mierau.