<P> As device management is a very OS - specific topic, these drivers are handled differently by each kind of kernel design, but in every case, the kernel has to provide the I / O to allow drivers to physically access their devices through some port or memory location . Very important decisions have to be made when designing the device management system, as in some designs accesses may involve context switches, making the operation very CPU - intensive and easily causing a significant performance overhead . </P> <P> In computing, a system call is how a process requests a service from an operating system's kernel that it does not normally have permission to run . System calls provide the interface between a process and the operating system . Most operations interacting with the system require permissions not available to a user level process, e.g. I / O performed with a device present on the system, or any form of communication with other processes requires the use of system calls . </P> <P> A system call is a mechanism that is used by the application program to request a service from the operating system . They use a machine - code instruction that causes the processor to change mode . An example would be from supervisor mode to protected mode . This is where the operating system performs actions like accessing hardware devices or the memory management unit . Generally the operating system provides a library that sits between the operating system and normal programs . Usually it is a C library such as Glibc or Windows API . The library handles the low - level details of passing information to the kernel and switching to supervisor mode . System calls include close, open, read, wait and write . </P> <P> To actually perform useful work, a process must be able to access the services provided by the kernel . This is implemented differently by each kernel, but most provide a C library or an API, which in turn invokes the related kernel functions . </P>

The operating system that a computer uses sometimes is called which of the following