Technical information

CHAPTER 5
System Software
Boot ROM Contents 81
POST Code 5
The Power-on Self Test (POST) software is executed when the computer first
boots. This encompasses many of the traditional Macintosh ROM operations
and is based on the hardware initialization code used in the past: setup and
initialization of the processor and ASICs, a boot beep, an error beep,
diagnostics, and transfer to Open Firmware.
A small debugging mini-nub is part of this section. It allows prodding and
poking with some MacsBug-like commands. Getting into the debugger nub is
not possible using standard user interface input, however.
Failure to boot in POST causes the error beep, optionally entering the mini-nub.
Open Firmware 5
The Open Firmware component of NewWorld is based on the CHRP version of
Open Firmware, also known as Open Firmware 3.0. This is the most robust and
full-featured Open Firmware used by Apple to date. This version of Open
Firmware contains several notable changes from the Open Firmware found in
the first and second generation PCI Macintosh computers:
■ Open Firmware is capable of reliably reading files from block devices.
■ Open Firmware builds an expanded device tree that holds every facet of
hardware information needed by an operating system.
■ Open Firmware contains code that mimics the StartSearch code in the Mac
OS ROM and provides a Mac-like user interface during startup.
■ Open Firmware creates an interrupt tree that is interlaced through the device
tree to provide a mechanism to describe the interrupt layout of the computer.
If a boot failure occurs in Open Firmware, Open Firmware attempts to provide
a Macintosh-like experience.
Mac OS 'ndrv' Drivers 5
The boot ROM may also contain Mac OS drivers that are hardware-specific and
needed at boot time; they are organized as 'ndrv' drivers. Drivers needed at
boot time (video drivers, network drivers, or disk drivers) need to be loaded
from the device tree.