Question: It seems that I must reset SDB ( SDBRST) every time when I enter PDM environment; especial in the situation that MP needs to unhalt PP. Why ?

Answer: SDBRST reloads the SDB communication software running on the SDB that allows the SDB to communicate to the debuggers/PC. In addition, resetting the SDB resets the C80, which then clears all interrupts/mem faults, etc. that may be pending in the previous pdm session. Thus it is a good idea to reset.