Texas Instruments TMS320C3x - Manual (Page 82)

Reserved Bits and Compatibility
3-19 CPU Registers
3.3 Reserved Bits and Compatibility
To retain compatibility with future members of the ’C3x family of microprocessors,
reserved bits that are read as 0 must be written as 0. You must not modify the
current value of a reserved bit that has an undefined value. In other cases, you
should maintain the reserved bits as specified.