OpenBoot Architecture

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 that 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.

• 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.

• 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 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.

• Programmable user interface The OpenBoot user interface is based on the programming language Forth, which provides an interactive programming environment. It is a language that is used for direct communication between humans and machines. It can be quickly expanded and adapted to special needs and different hardware systems. You’ll see Forth used not only by Sun but also by other hardware vendors such as Hewlett-Packard.