Collections of system calls are often combined into more complex tasks and put into libraries. When using one of the functions defined in a library, you call a library function or make a library call. Even when the library routine is intended to access the hardware, it will make a system call long before the hardware is touched.

Each system call has its own unique identifying number. The kernel uses this number as an index into a table of system call entry points, which point to where the system calls reside in memory along with the number of arguments that should be passed to them.

When a process makes a system call, the behavior is similar to that of interrupts and exceptions. Like exception handling, the general purpose registers and the number of the system call are pushed onto the stack. Next, the system call handler is invoked, which calls the routine within the kernel that will do the actual work.

Although there are hundreds of library calls, each of these will call one or more systems calls. In total, there are about 150 system calls, all of which have to pass through this one point (referred to as a “call gate”) to ensure that user code moves up to the higher privilege level at a specific location (address)within the kernel. Therefore, uniform controls can be applied to ensure that a process is not doing something it shouldn’t.

As with interrupts and exceptions, the system checks to see whether a context switch should occur on return to user mode. If so, a context switch takes place. This is possible in situations where one process made a system call and an interrupt occurred while the process was in kernel mode. The kernel then issued a wake_up() to all processes waiting for data from the hard disk.

When the interrupt completes, the kernel may go back to the first process that made the system call. But, then again, there may be another process with a higher priority.