If I knew which chip you were talking about I could help you. If you are talking about the '51 you are out in left field.
In the '51 the 4 register banks occupy direct RAM addresses 0-1f (bank 0 0-7, bank 1 8-f etc). The instruction set address the registers in the CURRENTLY SELECTED BANK, no nibble or such. The bank is selected by the RB0 and RB1 bits. In other words if bank 1 is selected, accessing r2 or accessing direct RAM address 0xa will yeld the same result.

Have fun,

Erik

Ps You REALLY need to read the 3 80C51_FAM.. documents. They are found at http://www.semiconductors.philips.com/products/...other.html