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The OpenBoot Architecture


Explain boot PROM fundamentals, including OpenBoot Architecture Standard


The OpenBoot Device Tree In this section, pay close attention to the OpenBoot device tree. You're likely to see this topic on the exam.

The OpenBoot architecture provides an increase in functionality and portability compared to the proprietary systems of some other hardware vendors. Although this architecture was first implemented by Sun Microsystems as OpenBoot on SPARC (Scaleable Processor Architecture) systems, its design is processor independent. The following are some notable features of OpenBoot firmware:

  • Plug-in device drivers A device driver can be loaded from a plug-in device such as an SBus card. The plug-in device driver can be used to boot the operating system from that device or to display text on the device before the operating system has activated its own software device drivers. This feature lets the input and output devices evolve without changing the system PROM.

  • The FCode interpreter Plug-in drivers are written in a machine-independent interpreted language called FCode. Each OpenBoot system PROM contains an FCode interpreter. This enables the same device and driver to be used on machines with different CPU instruction sets.

  • The device tree Devices called nodes are attached to a host computer through a hierarchy of interconnected buses on the device tree. A node representing the host computer's main physical address bus forms the tree's root node. Both the user and the operating system can determine the system's hardware configuration by viewing the device tree.

    Nodes with children usually represent buses and their associated controllers, if any. Each such node defines a physical address space that distinguishes the devices connected to the node from one another. Each child of that node is assigned a physical address in the parent's address space. The physical address generally represents a physical characteristic that is unique to the device (such as the bus address or the slot number where the device is installed). The use of physical addresses to identify devices prevents device addresses from changing when other devices are installed or removed.

  • The programmable user interface The OpenBoot user interface is based on the programming language Forth, which provides an interactive programming environment. It can be quickly expanded and adapted to special needs and different hardware systems. Forth is used not only by Sun but also utilized in the OpenFirmware boot ROMs provided by IBM, Apple, and Hewlett-Packard.


Forth If you're interested in more information on Forth, refer to American National Standards Institute (ANSI) Standard X3.215-1994 (see

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