If ToD0: /7 check to make sure bs is a valid bit position in zong (you may assert this is true) 1/ check to make sure len is a valid length for a subfield that starts at bs (you may assert this is true) 1/. Make a mask that has len bits, night justified II Use mask to turn off any bits to the left of the bitfield values in new II Shift both the new value and the mask to the correct bit position 1/ Turn off atz bits in *val under the mask by bitwise anding with mask 1/ Or *val with new. (new has zeros for all non bit field positions, 50 won't affect val)