hi,

Although the 8051 has a lot of different groups of instructions, there are only two instructions which are supposed for nibble support. They are

SWAP A

and

XCHD A,@Ri

First one interchanges the low- and high-order nibbles of the Accumulator. Second one exchanges the low-order nubble of the Accumulator with that of the internal RAM location indirectly pointed with the register R0 or R1.

There is one more instruction which processes nibbles:

DA A

But this instruction is from arithmetic group (here: BCD-arithmetic) and has nothing with bits` manipulation.

Regards,
Oleg