Revision history   OpenPOWER Library

 3.3. Reserved Fields and Registers

 All  reserved fields must be set to zero.

 A specific instantiation of the CAIA must handle reserved bits as follows:

  •  Ignore the reserved bits on writes and return zeros for the reserved bits on reads; or

  •  All   reserved registers must be initialized to zero, unless otherwise indicated.

 Software is not required to initialize reserved registers. An implementation must decode all reserved registers for both reads and writes. The handling of reserved register values in a specific instantiation of the CAIA is implementation dependent. For each reserved register, an implementation takes one of the following actions:

  •  Ignore the value on writes and return zeros for reads of the register.

  •  Maintain the state of the reserved register.

 Fields and registers that are currently defined as reserved might become defined in future versions of the CAIA. If a reserved register or register field is defined in a future version of the CAIA, a state of zero should be defined in a manner that is backwards compatible with previous versions of the CAIA.


 An implementation of the CAIA must never use reserved fields or registers for an implementation-dependent purpose. All defined, reserved, and implementation-dependent registers must be decoded for both reads and writes. Furthermore, the range of addresses for a given area in the memory map must be a multiple of at least 4 KB (that is, the smallest page addresses). An implementation should consider the range to be a multiple of a larger supported page size to minimize the number of page table entries required to map the address range. See Appendix A, Memory Maps for more details.

loading table of contents...