What interface does the operating system provide to programmers?

The interface provided by the operating system to programmers is: system call. System calls are a collection of all system calls provided by the operating system implementation, that is, the program interface or Application Programming Interface (API); it is the interface between the application program and the system.

System call is the only interface provided by the operating system to programmers.

The main function of the operating system is to manage hardware resources and provide a good environment for application developers to make applications have better compatibility. To achieve this purpose, the kernel provides A series of multi-kernel functions with predetermined functions are presented to the user through a set of interfaces called system calls (system calls). The system call passes the application's request to the kernel, calls the corresponding kernel function to complete the required processing, and returns the processing result to the application.

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 many security issues. For example, a process can easily modify the data in the memory space of the process to cause another process to be abnormal or achieve some purposes. Therefore, the operating system must ensure that each process can be executed 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 instructions. 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.

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