As you have learned throughout this book, Mac OS X was an entirely new operating system rather than an evolution of Mac OS 9. As such, applications created for previous versions of the Mac OS won't work under Mac OS X?well, not directly anyway. Rather than forcing you to either obtain a Mac OS X?compatible version of all the applications you use (which might or might not be available) or to go without, Apple includes the Classic environment in Mac OS X. As you learned in Chapter 1, "Mac OS X: Foundations," and Chapter 6, "Installing and Using Mac OS X Applications," the Classic environment enables you to run Classic versions of applications.
A Classic application is one that was created for Mac OS 9.2 or earlier versions of the Mac OS.
The Classic environment is provided solely to enable you to run applications that were created for earlier versions of the Mac OS. After you have obtained Mac OS X versions of all the applications you use, you will no longer have any reason to use the Classic versions and thus, the Classic environment. Classic is really a bridge between the past (previous versions of the Mac OS) and the present and future (Mac OS X).
The Classic environment is actually a Mac OS X application that emulates Mac OS 9.2; you run Classic applications from within this emulated environment. The configuration of the Classic environment is the configuration of the Mac OS 9.2 installation you choose to use. The Classic environment provides all the system resources of Mac OS 9.2, such as extensions and control panels, and you can customize the Classic environment just as you can Mac OS 9.2 itself.
Although you will use the Classic environment to run Classic applications most of the time, you might also run into situations in which you want to start up under Mac OS 9.2 directly (for example, to use applications that access a hardware device that doesn't work under Mac OS X). The ability to boot up under OS 9.2 depends on the specific Mac model you have. Many modern Mac models can't start under Mac OS 9, but if you have an older model or one specifically designed to boot under OS 9.2 it might be possible.
Classic, a Bridge to the Past
The Classic environment is analogous to the transition from 68K processors to the PowerPC processor. Applications written for the 68K processor could not run on PowerPC machines, and vice versa. For a period of time, most applications were provided in fat versions, which meant the application came in both a 68K and a PowerPC version. You could use the application installer to choose which version was installed (sometimes both were installed or only the one appropriate for the processor you were using). After a period of time, the 68K versions of applications were dropped because most people who were purchasing new software were also using a PowerPC machine.
You can expect a similar transition for Classic applications as Mac OS X versions are released. For a time, both versions will be available, but as more Mac users make the switch to Mac OS X, the development and distribution of applications compatible with Mac OS 9 or earlier versions of the OS will cease. Because Mac OS X has been around for a couple of years, this transition period is coming to an end as most major Mac applications have now been ported to or rewritten for Mac OS X.
The Classic environment mostly does its job well and will enable you to use applications that were designed for Mac OS 9. However, it does have limitations, some of which are the following:
No memory protection? Because the Classic environment is an emulation of Mac OS 9.2, Classic applications do not have protected memory. When one Classic application has a problem, it can (and usually does) affect other Classic applications you are running. In this respect, running applications under the Classic environment is just like running them under Mac OS 9.2 or earlier. When a Classic application crashes, it can take down the rest of the open Classic applications as well as the Classic environment itself.
The Classic application itself is a Mac OS X application and does benefit from Mac OS X's protected memory architecture. If the Classic environment crashes, it doesn't affect the Mac OS X Finder, Carbonized applications, or Cocoa applications that are running.
Emulated performance? When you run a Classic application, you are using three layers of software, which are Mac OS X, the Classic environment, and the Classic application. Because of this, the performance of a Classic application can be considerably slower than when running it under Mac OS 9.2. This speed differential is most noticeable on slower Mac hardware and while using processor-intensive applications (such as advanced graphics or video applications).
Spotty hardware support? Because they run in an emulated environment, Classic applications can sometimes have trouble accessing hardware with which they are supposed to work. When you are unable to access a hardware device through the Classic environment, your only options are to restart under Mac OS 9.2 (if you can) or to get Mac OS X?compatible software for the hardware you want to run.
All user accounts on your Mac share the same Classic environment. However, each user account can access the Classic pane of the System Preferences utility and so can change the Classic environment to suit his purposes. (The Classic pane does not require Administrator authentication.)