The Hollywood chipset includes a large register area including many miscellaneous controls. Some of these registers can be accessed by the powerPC. Address bit 23 (0x00800000) controls the permission: if it is set, then the registers are accessed with Starlet permission (full access). If it is clear, only the PPC subset of the registers is visible. From the PPC, the state of this bit is irrelevant, which suggests that it is forced to zero internally in one of the buses.
See also the MINI source code, especially [http://gitweb.bootmii.org/?a=viewblob&p=mini&h=354386107f79891ad3d37cc84ced4a32d3cbca76&hb=4a13e2d20752cd8e5996161f73b8a438bca4ae6c&f=hollywood.h hollywood.h].