Original sentence: The defining features of the operating system were a Mach kernel, a BSD operating system layer (based on 4.4BSD), the Yellow Box object-oriented frameworks from OPENSTEP, the Blue Box compatibility environment for running "Classic" Macintosh applications (PowerPC version only), and a Java virtual machine
Edited sentence: The defining features of the operating system were a Mach microkernel, a BSD operating system layer (based on 4.4BSD), the Yellow Box object-oriented frameworks from OPENSTEP, the Blue Box compatibility environment for running "Classic" Macintosh applications (PowerPC version only), and a Java virtual machine