What does SCE mean in UNCLASSIFIED

A system call is how a computer program requests a service from the kernel of an operating system. It is the mechanism used for interfacing user applications with the operating system's services.

To make a system call, your application needs to use special instructions provided by your chosen operating system to make requests to its kernel. Depending on your programming language and environment, this can involve using APIs or libraries provided by your OS vendor, or writing assembly code directly.

System calls are useful for accessing resources that are not available through other means, such as memory management, file I/O operations, process control and inter-process communications. They also provide access to higher-level services like networking and security protection (such as firewalls).

No, different operating systems have different implementations and approaches to providing certain services via their respective kernels and therefore support different kinds of system calls. For example, Windows has different APIs than Linux does for interacting with its kernel services.

Generally speaking, yes - when properly implemented they can provide efficient and secure access to kernel resources without exposing any critical data or code execution paths to malware attacks or other malicious actors. However if an application inappropriately uses, or misuses them then serious security vulnerabilities may be present in an application's codebase.