Operating systems offer processes running in User Mode a set of interfaces to interact with hardware devices such as the CPU, disks,and printers. Putting an extra layer between the application and the hardware has several advantages. First, it makes programming easier by freeing users from studying low-level programming characteristics of hardware devices. Second, it greatly increases system security, because the kernel can check the accuracy of the request at the interface level before attempting to satisfy it. Last but not least, these interfaces make programs more portable, because they can be compiled and executed correctly on every kernel that offers the same set of interfaces. Unix systems implement most interfaces between User Mode processes and hardware devices by means of system calls issued to the kernel.
UNIX
Unix (all-caps UNIX for the trademark) is a multitasking, multiuser computer operating system that exists in many variants. The original Unix was developed at AT&T's Bell Labs research center by Ken Thompson, Dennis Ritchie, and others.
MAC OSX is UNIX
Even Mac OSX is developed is a series of Unix-based graphical interface operating systems developed and marketed by Apple Inc.
LINUX / UNIX
Linux invented by Linus Torwalds , a finnish student as a project, which has grown to vast area of development where thousands of developers are been working upgrading the LINUX
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