I was wondering what grey colored boxes in register description layout indicate. For example, in ATmega8 specification, we can see that MCUCR register has boxes corresponding to bits 7:4 that are colored grey. Same goes for some bits in GICR, GIFR, MCUCSR, etc., in ATmega8. I see this type of color coding in other ATMEL specs as well. Can someone please explain the meaning of this coloring?