IBM PowerPC 405GP User Manual page 85

Programming Note: A good coding practice is to perform the initial write to a register with
reserved fields as described, and to perform all subsequent writes to the register using a read-
modify-write strategy: read the register, use logical instructions to alter defined fields, leaving
reserved fields unmodified, and write the register.
Figure 3-1 on page 3-5 illustrates the registers in the user and supervisor programming models.
3-4
PPC405GP User's Manual Preliminary