Texas Instruments TMS320C28x Reference Manual page 653

Moving to a C28x Object
mode (OBJMODE == 0), the upper bits of XARn registers may contain
nonzero values. You MUST zero out the upper bits of the XARn registers
when switching from OBJMODE == 1 to OBJMODE == 0.
It is recommended that you not switch modes frequently in your code.
-
Typically, you will select the appropriate operating mode at boot time and
stick to one mode for the whole program.