Extended machine, kernel, system call

Operating system as an extended machine

The bare hardware (processor, memory, disk, devices) is difficult to program: the programmer must know controller registers, command codes, and interrupt handling. The OS hides these details and presents a simpler, cleaner abstract machine (the extended machine or virtual machine) with abstractions such as files, processes and address spaces. For example, a program calls read(fd, buf, n) and the OS handles seek, DMA, interrupts and errors. So the OS "extends" the hardware by adding functions that the hardware does not provide, and makes programming easier and more portable.

Kernel vs micro-kernel

Purpose of system call

A system call is the interface by which user programs request services of the kernel. Its purposes are:

• to let user programs perform privileged operations (I/O, file access, process creation) safely, since user code cannot execute them directly;
• to provide a controlled entry into kernel mode (using TRAP), protecting the OS and other users;
• to give a standard, hardware-independent interface (e.g. fork, open, read, write, exit) for application programs.

System calls fall in groups: process control, file management, device management, information maintenance and communication.