"Hard" Deconfiguration

In two special cases of deconfiguring a subsystem (CPUs and memory), the OBP actually takes action beyond just creating an appropriate "status" property in the device tree. At the first moments after reset, the OBP must initialize and functionally configure (or bypass) these functions in order for the rest of the system to work correctly. These actions are taken based on the status of two NVRAM configuration variables, post-status and asr-status, which hold the override information supplied either from POST or via a manual user override (see "ASR User Override Capability").

CPU Deconfiguration

If any CPU is marked as having failed POST, or if a user chooses to disable a CPU, then the OBP will set the Master Disable bit of the affected CPU, which essentially turns it off as an active UPA device until the next power-on system reset.

Memory Deconfiguration

Detecting and isolating system memory problems is one of the more difficult diagnostic tasks. This problem is further complicated by the possibility of installing different capacity DIMMs within the same memory bank. (Each memory bank must contain four DIMMs of the same capacity.) Given a failed memory component, the firmware will deconfigure the entire bank associated with the failure.