Wikipedia
SX-Windows is a discontinued non-preemptive multitasking operating system, using the event driven paradigm. It runs on top of the Human 68k DOS operating system which resembled MS-DOS, similar to the relationship between Windows 3.1 and MS-DOS.
SX-Window had a standard window system for Sharp Corporation's X68000 series in Japan. It was introduced for X68000 in 1989, and came preinstalled on the X68000 EXPERT model. It was developed by Hudson. The look and feel of the GUI is like that of the NeXTSTEP operating system, and its API is similar to the Macintosh Toolbox. It has a garbage collection system without MMU unit of MPU, but it was difficult to program because all pointers derived from handles become invalid once any API is called. Although the X68000 was very powerful for game software, this GUI was very slow as no hardware acceleration card was supported for this window system. Only few applications and games were developed for this system. The final release was 3.1.