STB6100_K_NF_MSB
nfrac = ((regs[STB6100_K] & STB6100_K_NF_MSB) << 8) | regs[STB6100_NF_LSB];
regs[STB6100_K] = (regs[STB6100_K] & ~STB6100_K_NF_MSB) | ((nfrac >> 8) & STB6100_K_NF_MSB);