Each register file access uses the five bits of the register number in the instruction word. For single-precision and integer accesses, the most significant four bits are in the Fm, Fn, or Fd field, and the least significant bit is the M, N, or D bit for each instruction format. For instructions with double-precision operands or destinations, the M, N, and D bit corresponding to a double-precision access must be zero. Figure 19.3 shows the register file. See the ARM Architecture Reference Manual for instruction formats and the positions of these bits.