Running Mac OS X Tiger

Chapter 5. System Startup and Login

Usually, the first thing you do when you sit down at your computer is either hit the Power-On button or—for those who leave their machines on all the time—log in. Most of the time, you probably don’t think much about what happens behind the Apple logo and spinner as the system boots up or about the events that happen between the time you enter your username and password and the time Mac OS X’s desktop appears.

Booting a Mac is not a single-step process; over 100 programs take part in the process of transforming your machine from an inert collection of plastic and metal into a running system. In general, though, the process can be broken down into three major steps. First, the hardware powers up and organizes itself. Next, the hardware launches the operating system, and then the operating system finally starts up. This chapter removes the veil of mystery and shows what goes on behind the scenes as your Mac starts up—from the time you press that Power-On button, right up to the login screen.

The Hardware Boot Process

When you press the Power-On button on your Mac, a small hardware program embedded in the main logic board of your machine, known as the POST (Power-On Self Test) , is activated. The first thing the POST does is power up and then initializes the CPU. Next the POST performs some tests on the core system components, such as the memory, to make sure the system can boot. If everything checks out OK, the POST starts a program called Open Firmware ...