OS/2 / NT
OS/2 uses the CONFIG.SYS file extensively for setting up its configuration, drivers and environment before the graphical part of the system loads.
In the OS/2 subsystem of Windows NT, what appeared as CONFIG.SYS to OS/2 programs was actually stored in the registry.
There are many undocumented or poorly documented CONFIG.SYS directives used by OS/2.
Read more about this topic: CONFIG.SYS