What interface does the operating system provide to application programs?

The interface provided by the operating system to the application program is the "system call". The operating system interface mainly includes command interface and program interface. The program interface, also called system call, is composed of a set of system call commands and provides a set of system call commands for user programs to use. System call is the interface between the application program and the system; it passes the application program's request to the kernel, calls the corresponding kernel function to complete the required processing, and returns the processing results to the application program.

The operating environment of this tutorial: Windows 7 system, Dell G3 computer.

The interface provided by the operating system to the application program is the "system call".

Operating system interfaces mainly include command interfaces and program interfaces, of which program interfaces are also called system calls.

The program interface (system call) consists of a set of system call commands and provides a set of system call commands for user programs to use.

System calls in the operating system

The set of all system calls provided by the operating system implementation is the program interface or application programming interface (Application Programming Interface, API). It is the interface between the application program and the system.

The main function of the operating system is to manage hardware resources and provide a good environment for application developers to make applications more compatible. To achieve this purpose, the kernel provides a series of predetermined functions. Multi-kernel functions are presented to users through a set of interfaces called system calls. The system call passes the application request to the kernel, calls the corresponding kernel function to complete the required processing, and returns the processing result to the application program.

Modern operating systems usually have multi-tasking functions, which are usually implemented by processes. Because the operating system quickly switches execution between each process, everything will appear to be simultaneous. At the same time, this also brings about many security issues. For example, a process can easily modify the data in the process's memory space to cause another process to anomaly or achieve some purposes. Therefore, the operating system must ensure that each process can execute safely. The solution to this problem is to add base registers and limit registers to the processor. The contents of these two registers use hardware to limit the address of the memory accessed by the memory access instruction. In this way, when the system switches processes, the contents of these two registers can be written to the address range assigned to the process, thereby avoiding malware.

In order to prevent user programs from modifying the contents of the base register and limit register to access other memory spaces, these two registers must be accessed through some special instructions. Usually, the processor has two modes: "user mode" and "kernel mode". A tag bit is used to identify which mode it is currently in. Some instructions such as modifying the contents of the base address register can only be executed in kernel mode. When in user mode, the hardware will directly skip this instruction and continue to execute the next one.

Similarly, for security reasons, some I/O operation instructions are restricted to only kernel mode execution, so it is necessary for the operating system to provide interfaces to provide applications with functions such as reading data from a certain location on the disk. Interfaces, these interfaces are called system calls.

When the operating system receives a system call request, it will cause the processor to enter kernel mode to perform instructions such as I/O operations and modify the contents of the base address register. After processing the system call content, the operation The system will return the processor to user mode to execute user code.

The difference between system calls and ordinary calls

A system call is essentially a procedure call, but it is a special procedure call that is different from that in a general user program There is a clear difference between the procedure calls.

• The running status is different

  The running status is different. The calling process and the called process of a system call run in different states, while ordinary procedure calls generally run in the same state.

• The calling method is different

  The calling method is different. System calls must first enter the system core through the soft interrupt mechanism before they can be directed to the corresponding command handler. Ordinary procedure calls can directly transfer from the calling process to the called process.

• Return to question

  Return to question. In a system that uses preemptive scheduling, when the system call returns, the scheduling analysis must be re-analyzed—whether a higher-priority task is ready. Ordinary procedure calls directly return to the calling process to continue execution.

For more related knowledge, please visit the FAQ column!

The above is the detailed content of What interface does the operating system provide to application programs?. For more information, please follow other related articles on the PHP Chinese website!