Define a system call? List the different types of the system calls?

- A program can request for services from the kernel of the operating system through a system call. 

- These services include all the following:

1. Hardware related services such as data access from the HDD.
2. Creation and execution of processes
3. Communication with integral services of the kernel such as scheduling.

- An essential interface is provided by the system call that lies between the operating system and the process. 

- The modern microprocessor architecture consists of a security model specifying multiple levels of privileges for the execution of the software. 

- For example, a program has limitations of its own address space so even by accident it does not modify or access the programs that are executing or the OS. 

- This way the direct manipulation of the hardware devices (such as the network devices and frame buffer etc.) by the program will also be avoided. 

- However, this is not the only case.

- There are situations where the programs really need access to these devices. - This is why the system calls are made available to the programs. 

- System calls ensure that such operations are safely implemented and are well defined. 

- The level at which the operating system executes is the highest privilege level.

- Therefore, the applications requests for the devices to the operating system through the system calls. 

- The execution of the system calls is carried out through the interrupts that would automatically put the CPU at the required level of the privilege. 

- After this, the control is passed on to the kernel. 

- From here the kernel determines whether or not the requested service should be granted to the calling program. 

- If the program is granted the service it requested, a specific instruction set is executed by the kernel which cannot be directly controlled by the calling program.

- These instructions return the privilege level down to that of the program that invoked the call. 

- Finally, the control is returned to the calling program.

- Generally, an API or a library is provided between the operating system and the normal programs. 

- The purpose of this library is to provide wrapper functions for the calls. 

- The names of these functions are same as that of the system calls. 

- The purpose of these functions is to expose a subroutine through which the system call can be used.

- These functions also provide modularity to the system call. 

- But above all, the primary function of the wrapper is placing the arguments in their proper processor registers that are to be passed on to the system call.

- A unique system call number is set that will be called by the kernel. 

- In this way, the portability is increased by the help of this library.

- Making direct system calls in the code of the application is very difficult and so requires embedded assembly code.

- In systems based up on ex-kernel, library is as important as the intermediary as they provide resource management, abstractions and shields the application from a low level kernel. 

- A control transfer involving features specific to the architecture is required for the implementation of the system call. 

- A way of implementing this is by using a software trap. 

- RISC processors can only use the implementation technique via interrupts. 

- But there are some other additional techniques for processors with CISC architectures. 

- One example is of the following two sets developed independently by Intel and AMD respectively for serving the same purpose:

SYSRET/ SYSEXIT

SYSCALL/ SYSENTER

- These were actually control transfer instructions and very fast indeed.