4.5. A Description of the Boot Process

Mac OS X system :
When the power to a Macintosh computer is turned on, the BootROM firmware is activated. BootROM (which is part of the computer’s hardware) has two primary responsibilities: it initializes system hardware and it selects an operating system to run. BootROM has two components to help it carry out these functions:

POST (Power-On Self Test) initializes some hardware interfaces and verifies that sufficient memory is available and in a good state.

On PowerPC-based Macintosh computers, Open Firmware initializes the rest of the hardware, builds the initial device tree (a hierarchical representation of devices associated with the computer), and selects the operating system to use.

On Intel-based Macintosh computers, EFI does basic hardware initialization and selects which operating system to use.

If multiple installations of Mac OS X are available, BootROM chooses the one that was last selected by the Startup Disk System Preference. The user can override this choice by holding down the Option key while the computer boots, which causes Open Firmware or EFI to display a screen for choosing the boot volume.

Reference:
http://developer.apple.com/mac/library/documentation/MacOSX/Conceptual/BPSystemStartup/Articles/BootProcess.html

Linux 9 system:
The beginning of the boot process varies depending on the hardware platform being used. However, once the kernel is found and loaded by the boot loader, the default boot process is identical across all architectures. This chapter focuses on the x86 architecture.

Reference:
http://www.redhat.com/docs/manuals/linux/RHL-9-Manual/ref-guide/s1-boot-init-shutdown-process.html#S3-BOOT-INIT-SHUTDOWN-OTHER-ARCHITECTURES

Windows system:

When the system is turned on, the BIOS runs self-test diagnostics to verify the system's hardware and memory. The system begins to boot automatically if no errors are found. If errors are found, error messages are displayed that describe recovery options.
The BIOS of additional hardware devices are run at this time.
The BIOS boot program tries to read the first physical sector from the boot device. This first disk sector on the boot device contains the master boot record mboot, which is loaded and executed. If no mboot file is found, an error message is displayed.

Reference:
http://docs.sun.com/app/docs/doc/806-4073/6jd67r98d?a=view