OS_Byte 166 has returned the base address of the MOS variables (OS_Bytes &A8-&FF) in all versions of the OS since the original BBC Micro. Somebody broke it in high-vector versions of RO5. This module ensures OS_Byte 166 (and 167) return the correct values in all versions of RISC OS, and also write-protects the value as it is informational, not functional.