SFU 1.0 and 2.0 relied on the MKS Toolkit by MKS/PTC, and SFU 3.0 later adopted the Interix (OpenNT) subsystem, which Microsoft acquired from Softway Systems in 1999.

The original POSIX Subsystem (psxss.exe) was Cutler's team who implemented a minimal POSIX.1 subsystem server alongside Win32 and OS/2. This was done primarily to satisfy US Federal Government procurement requirements (FIPS 151-2).