To achieve binary compatibility across all currently supported hardware platforms, the DDI interfaces were carefully designed around architectural abstractions. The underlying abstraction, the device tree, is an extension of the devinfo tree in the original SPARCstationTM design. Each node in the device tree is described by a device information structure or "dev_info node." The bottom-most nodes in the tree are termed leaf nodes. Most devices (such as disks and tape drives, framebuffers, I/O cards, and network interfaces) are examples of leaf devices that would be associated with leaf nodes. The associated device drivers are called leaf drivers.
The intermediate nodes in the tree are generally associated with buses (for example, SBus, SCSI, VME). These nodes are called nexus nodes and the drivers associated with them are called nexus drivers. Bus nexi are intended to encapsulate the architectural details associated with a particular element.
The device tree structure creates a formal parent-child relationship between nodes. This parent-child relationship is the key to platform architecture independence.
When a leaf driver requires a service that is platform dependent (for example, a DMA mapping), the system transparently converts the request into a call to its parent to provide the service. The service providers are always nexus drivers; each nexus driver can in turn pass the request to its parent in order to provide the service. This approach enables leaf drivers to operate regardless of the platform architecture.