POSIX compatibility was mainly driven by FIPS 151-2, which required US government agencies to only buy operating systems which were POSIX compliant. Quite a num

POSIX compatibility was mainly driven by FIPS 151-2, which required US government agencies to only buy operating systems which were POSIX compliant. Quite a number of proprietary operating systems during the 90s - e.g. System/390, OS/400, VMS, Non Stop, OS 2200, etc. gained POSIX compliance for the same reason. Of course, being POSIX compliant is very differnt to providing a usable Unix-style programming environment.