Special function registers:

Special Function Registers (SFRs) accesses are restricted in the following ways:
• User must not attempt to access any SFR locations not defined.
• Accesses to any defined SFR locations must be strictly for the functions for the
• SFRs.
• SFR bits labeled ‘-’, ‘0’ or ‘1’ can only be written and read as follows:
• ‘-’ Unless otherwise specified, must be written with ‘0’, but can return any value when read (even if it was written with ‘0’). It is a reserved bit and may be used in future derivatives.
– ‘0’ must be written with ‘0’, and will return a ‘0’ when read.
– ‘1’ must be written with ‘1’, and will return a ‘1’ when read.