Part I · How it works

1. How the kernel works

1. 1.1What does the kernel do?The kernel is the one program with full control of the hardware. It shares the CPUs, the memory and the devices among all the others.
2. 1.2How the kernel shares the CPUsThere are more tasks wanting a CPU than CPUs, so the scheduler gives each a short turn, then switches: a context switch.
3. 1.3How virtual memory worksEvery process sees its own memory, as if it were alone: the addresses it uses are virtual.
4. 1.4Interrupts: how devices get attentionA device that needs the CPU (a packet arrived, a disk finished, a key was pressed) raises an interrupt: the CPU stops what it's doing and runs the kernel's handler for that device.
5. 1.5How a program starts: fork and execA new process starts as a copy of an existing one: fork() duplicates the process that calls it.