Reading: book chapter 1. 5 to end of chapter 1: overview. Os is the juggler, providing each running program the illusion that it has complete control over machine, e. g. , it is the only one running, and it has its own cpu, memory, disk. Abstracts hardware with system call instructions so programmers don"t have to deal with hardware details. What"s the hardware interface? (the physical reality) What is the application interface? (nicer interface for programmers) Os hides the number of physical cpus from programs, so programs can be written as a nr. of concurrent threads, without knowing the # of physical cpus. The thread abstraction saves just enough state to stop a thread at any point of time and resume it later. Cpu state consisting of the cpu registers, including the program counter (pc) and stack pointer (sp) Each thread thinks it has its own set of cpu registers. Show where processor state is saved and restored.