No one can do all things, learn to be good at use what others already did.

• Most computers have two modes of operation: kernel mode and user mode. The OS is the most fundamental piece of software and runs in kermel mode(also called usperivosr mode). In this mode, OS has complete access to all the hardware and can execute any instruction th machine is capable of exectuing.
• The rest of the software runs in user mode, in which only a subset of machine instructions are available. In particular, those instructions that affect control of machine or do I/O are forbidden to user-mode program.
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Modern Operating System