In 1990, GeoWorks released GEOS for IBM PC compatible systems, PC/GEOS. Also called GeoWorks Ensemble, it was incompatible with the Commodore and Apple versions but provided numerous enhancements, including scalable fonts and multitasking even on XT and AT-class PC clones. Being written directly in assembly language, it also provided much better performance than the relatively sluggish Microsoft Windows 3.0 on 386 and 486 PCs. GEOS was bundled with numerous PCs at the time, but like other GUI environments for the PC platform, such as GEM, it ultimately proved less successful in the marketplace than Windows. Some claim that Geoworks faded away because Microsoft threatened to withdraw supply of MS-DOS to hardware manufacturers who bundled Geoworks with their machines.
In December 1992 NEC and Sony bundled an OEM version of GeoWorks called the CD Manager with their respective CD-ROM players that sold as retail box add-on peripherals for consumers. The NEC Bundle retailed for around $500.00 with a 1x external CD Rom, SCSI Interface Controller, Labtec CD-150 amplified stereo speakers and 10 software titles.
The subsystem of GeoWorks was used by America Online for their DOS-based AOL connection and browsing software from the time of introduction on IBM compatible PCs until the late 1990s when America Online dropped development for graphical DOS in favor of Microsoft Windows. During that time, the popular single 3.5" disk that AOL was distributing on could be hacked to boot the GeoWorks graphical operating environment.
GeoWorks attempted to get third party developers but was unable to get much support due to expense of the developer kit — which ran $1,000 just for the manuals — and the difficult programming environment, which required a second PC networked via serial port in order to run the debugger.
Note: Even though GeoWorks is referred to as an Operating System it still requires DOS or other OS software in order to load.