gtpm1m1uTPF V4R1 Migration Guide: 3.1 to 4.1

Modifying Low Storage

Low address protection is a hardware facility that protects the first 512 bytes of storage against any alteration by a program regardless of the storage key used by the program. Not even the TPF control program can modify low storage while low address protection is active. Any program attempting to store into the first 512 bytes of each I-stream will take a program check when low address protection is active. Low address protection guards the part of the system that is most likely to be modified by a storage corrupter.

To ensure the integrity of the system, keep low address protection turned on. If you have programs that modify the first 512 bytes of memory, use the new LCPCC macro to turn off low address protection, then on again.