Extracting fields with Shift/Mask

1. Shift the field down to the least significant bit of the word.
2. Mask out all the bits that are not part of the field.

Extracting fields with Shift/Mask Example

int hour = dtv.b03 & 0x3F;
int second = (dtv.bo3 >> 12) & 0x3F;
int month = dtv.b45 & 0xF;

Hour and second are 6 bit fields. Month is a 4 bit field. Second starts in bit 12. Hour and month start in bit 0.

Assigning Value to Bit Field

1. Clear the value in the existing field
   • Bits we want to change their value. Not touch anything else.
   • Reason: we need all the bits to be 0 where we want to put the results
2. Clear the extraneous (or sign) bits in the value to assign
   • Make sure the value is within the range
   • If its outside we want to clean it up
   • Make sure we don't have more than 4 bits
3. Shift the value over to the starting position
   • 0th bit of value lines up with starting position (low bit position) of where we're going
4. OR the value into the field

Example

// clear second
dtv.b03 &= 0xFFFC0FFF;
// 37 will fit into 6 bits - nothing extra to zap
dtv.b03 |= (37 << 12); // shift over and or in