Isletim sisteminin çalismasi sirasinda isletim sisteminin kendisinin de çagirdigi, sistem programcisinin da disaridan çagirabildigi islevlere “sistem islevleri” denir. Bu tür islevlere Windows sisteminde API (Application Programming Interface) islevleri, UNIX isletim sisteminde ise “sistem çagrilari” (system calls) denir.
An operating system can access a system’s hardware directly, but a user program is not given direct access to the hardware. This is done so that the kernel can keep the system safe and secure from malicious user programs. But often, a user program requires some information from the hardware (e.g., from a web camera to show you the picture), but it cannot get the information directly. So, it requests the operating system to supply it the information. This request is made by using an appropriate system call. yani hardware user arasinda iletisim yoktur. bu iletisimi kernel yapar. kernel guvenlik icin bu sekilde hardware ile user arasina girer. fakat bazen olur ki user hardware ile iletisim kurmak zorunda kalabilir. mesela web cam uzerinden data almak gibi. bu durumda user kernel’den bunu request eder. bu request de system call ile yapilir.
When a task or ISR makes a system call, the flow of control passes to the kernel to execute one of the system routines provided by the kernel. real-time concepts for embedded systems isimli kitap