Understanding BootROM

BootROM is a hardware component that contains the startup (boot) programs, which are the first processes that run when the computer is turned on. These programs include the Power-On Self Test (POST) and Open Firmware.

POST

The Power-On Self Test checks basic hardware components for functionality. POST sets components such as the processor, random access memory (RAM), and crucial hardware interfaces to states expected by the system. In addition, POST verifies that the RAM is functioning to operating specifications.

If the system fails POST, the following error beeps occur:

• One beep No RAM installed

• Two beeps Incompatible RAM types

• Three beeps No good memory banks

• Four beeps No good boot images in BootROM

• Five beeps Processor is not usable

Error beeps may vary, depending upon the computer model. If no sound is present, some models will flash an LED to indicate a failure. Search the Knowledge Base for the specific hardware error messages.

If the system passes POST, you will hear the normal startup chime.

Open Firmware

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.

Open Firmware also checks whether startup modifier keys have been pressed, and takes the appropriate action.

Startup Modifier	Action
Command-Option-O-F	Start up in Open Firmware
Command-Option-P-R	Reset parameter RAM
Command-Option-Shift-Delete	Bypass startup volume
Option	Open the Startup Manager
X	Force Mac OS X startup
C	Start up from an optical disc
mouse or trackpad button	Eject optical disc
N	Start up from a network server
R	Reset PowerBook screen
T	Start up in Target Disk Mode
Shift	Start up in Safe mode
Command-V	Start up in Verbose mode
Command-S	Start up in Single-User mode