MacOSXArchitecture

Development projects fall in one of three architectural layers in Mac OS X: the scripting layer, the application layer, or the kernel. Figure 2-2 illustrates this concept. As you can see from the figure, the scripting languages call primarily on the application layer (Carbon and Cocoa in the case of AppleScript, and the BSD environment in the case of UNIX and Shell scripts). The application-layer development environments include Carbon, Cocoa, and Java; each of these can call on Application Services and Core Services. QuickTime has some characteristics of Application Services and some characteristics of an application environment. BSD makes calls directly into the kernel. If you write kernel extensions or I/O Kit drivers, your code operates directly in the kernel.

Note: Although Perl is shown in the figure, it is representative of all UNIX scripting languages, including Perl, PHP, Python, Tcl, and Ruby. UNIX shells available in Mac OS X include tcsh, sh, csh, bash, and zsh.

Figure 2-2 : Mac OS X system architecture and development environments