Units in Borland's Pascal were similar to Modula-2's separate compilation system. In 1987, when Turbo Pascal 4 was released, Modula-2 was making inroads as an educational language which could replace Pascal. Borland, in fact, had a Turbo Modula-2 compiler, but this product ran only on CP/M and was never ported to DOS (its user interface was almost identical to that of Turbo Pascal 1–3). Instead of porting their Modula-2 compiler to DOS, Borland chose to implement separate compilation in their established Pascal product.