80 CHAPTER 2 Elements of Computer System Organization particularly dangerous instructions. The remaining features of the hardware layer (in particular, most of the instruction repertoire of the underlying processor) pass through the operating system layer for use directly by the application layer, as in Figure 2.16. Thus, the dangerous instructions can be used only by the operating sys- tem layer, while all of the remaining instructions can be used by both the operating system and application layers. Conceptually, a designer could set things up so that the operating system layer intercepts every invocation of the hardware layer by the appli- cation layer and then explicitly invokes the hardware layer. That design would slow a heavily used interface down unacceptably, so in the usual implementation the appli- cation layer directly invokes the hardware layer, completely bypassing the operating system layer. Operating systems provide bypass for performance reasons, but bypass is not unique to operating systems, nor is it used only to gain performance. For example, the Internet is a layered communication system that permits bypass of most features of most of its layers, to achieve flexibility. In this section we examine two examples of layered computer system organiza- tion: the hardware layer at the bottom of a typical computer system and one part of the operating system layer that creates the typical application programming interface known as the file system.