2.7.2 Layered Approach
With proper hardware support, operating systems can be broken into pieces
that are smaller and more appropriate than those allowed by the original
MS-DOS and UNIX systems. The operating system can then retain much greater
control over the computer and over the applications that make use of that
computer. Implementers have more freedom in changing the inner workings
of the system and in creating modular operating systems. Under a topdown
approach, the overall functionality and features are determined and
are separated into components. Information hiding is also important, because
it leaves programmers free to implement the low-level routines as they see fit,
provided that the external interface of the routine stays unchanged and that
the routine itself performs the advertised task.