How to replace bits in a bitfield without affecting other bits using c

Question

I wanted to replace bit/bits(more than 1) in a 32/64 bit data field without affecting other bits.Say for example:

I have a 64 bit register where bits 5&6 can take values 0,1,2,3.

5:6     0 0 0 1 1 0 1 1      

Now when i read the register i get say value 0x146(0001 0 10 0 0110).Now i want to change the value at bit position 5 and 6 to 01.(right now it is 10 which is 2 in decimal and i want to replace it to 1 e 01) without other bits getting affected and write back the register with only bits 5&6 modified.(so it become 126 after changing)

I tried doing this

reg_data=0x146 reg_data |= 1 << shift   (shift in this case is 5) 

If i do this value at bit positions 5& 6 will become 11(0x3) not 01(0x1) which i wanted.

• How do i go about doing read/modify/write?
• How do i replace only certain bit/bits in a 32/64 bit fields without affecting the whole data of the field using C?

Setting a bit is okay but more than one bit, i am finding it little difficult.

Any suggestions are highly appreciated.

Answer 1

Why don't you use a bitmask? Sort of like:

new_value = 0, 1, 2 or 3  // (this is the value you will set in) bit_mask = (3<<5)         // (mask of the bits you want to set) reg_data = (reg_data & (~bit_mask)) | (new_value<<5) 

This preserves the old bits and OR's in the new ones.