Move from Time Base
In 32-bit compilation mode, returns the lower word of the time base register. In 64-bit mode, returns the entire doubleword of the time base register.
unsigned long __mftb (void);
In 32-bit mode, this function can be used in conjunction with the__mftbu built-in function to read the entire time base register. In 64-bit mode, the entire doubleword of the time base register is returned, so separate use of __mftbu is unnecessary
It is recommended that you insert the __fence built-in function before and after the __mftb built-in function.