8: Multitasking

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In many current systems, the CPU contains multiple cores, which means it can run several processes at the same time. In addition, each core is capable of “multitasking”, which means it can switch from one process to another quickly, creating the illusion that many processes are running at the same time.

The part of the operating system that implements multitasking is the “kernel”. In a nut or seed, the kernel is the innermost part, surrounded by a shell. In an operating system, the kernel is the lowest level of software, surrounded by several other layers, including an interface called a “shell.” Computer scientists love extended metaphors.

At its most basic, the kernel’s job is to handle interrupts. An “interrupt” is an event that stops the normal instruction cycle and causes the flow of execution to jump to a special section of code called an “interrupt handler”.

A hardware interrupt is caused when a device sends a signal to the CPU. For example, a network interface might cause an interrupt when a packet of data arrives, or a disk drive might cause an interrupt when a data transfer is complete. Most systems also have timers that cause interrupts at regular intervals, or after an elapsed time.

A software interrupt is caused by a running program. For example, if an instruction cannot complete for some reason, it might trigger an interrupt so the condition can be handled by the operating system. Some floating-point errors, like division by zero, are handled using interrupts.

When a program needs to access a hardware device, it makes a system call, which is similar to a function call, except that instead of jumping to the beginning of the function, it executes a special instruction that triggers an interrupt, causing the flow of execution to jump to the kernel. The kernel reads the parameters of the system call, performs the requested operation, and then resumes the interrupted process.