1) Layered view of a computer system

the os is a piece of software that is setting between the applications programs and the hardware

the role of the os is

1. Protect the hardware from misuse by runaway the applications
2. provide application with simple and uniform mechanisms

2) Abstractions provided by an operating system

files are abstraction for I/O devices virtual memory is an abstraction for the main memory + I/O devices Processes are abstraction for processor + main memory + I/O devices