Technical information

CHAPTER 5
System Software
82 Mac OS ROM Image File Contents
RTAS 5
RTAS (Run-Time Abstraction Services) can be thought of as a BIOS (basic
input/output system). RTAS code handles hardware accesses needed by an
operating system, making it possible for multiple operating systems to get
hardware services without having to know the specifics. RTAS handles
hardware-specific services such as NV-RAM (containing parameter RAM), time
services (the real-time clock), PCI configuration cycles, power management, and
the code needed to restart and shutdown.
RTAS is relocateable code; its location in RAM is determined by the operating
system, and it remains functional after the operating system boots.
Note
RTAS is part of the system software. It is not needed by
applications, which use operating system APIs.
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Mac OS ROM Image File Contents 5
The Mac OS ROM image file (also called the bootinfo file) contains three main
components, each of which is made up of smaller components: the pieces that
are part of the bootinfo specification, the Trampoline code, and the Mac OS
ROM Image itself.
Most changes needed for a new CPU occur in the boot ROM, not in the bootinfo
file. Changes to the Mac OS ROM Image should be limited to new manager
software and support for hardware that is common to many Macintosh
computers (ATA interface modules, user interface modules, and the like).
The bootinfo file exists on the boot device and has a localizable name.
Identification information that leads to the file’s path is stored in NV-RAM and
the search algorithm for a usable bootinfo file parallels the search mechanism
across SCSI, ATA, and so forth, used in the former startup disk routine. By
default, the file is located by using the directory ID of the “blessed folder” in the
boot block of each HFS or HFS Plus partition, and then searching for a file with
a file type of 'tbxi'. Searching by file type is done to allow localization of the
file. Nonlocalized, the name of the bootinfo file is “Mac OS ROM”.