In 80’s, Microsoft’s DOS was the dominated OS for PC
• Apple MAC was better, but expensive
• UNIX was much better, but much, much more expensive.
Only for minicomputer for commercial applications
• People was looking for a UNIX based system, which is
cheaper and can run on PC
• Both DOS, MAC and UNIX were proprietary, i.e., the
source code of their kernel is protected
• No modification is possible without paying high license
fees