Question

I am uncompressing some data from double words.

unsigned char * current_word = [address of most significant byte]

My first 14 MSB are an int value. I plan to extract them using a bitwise AND with 0xFFFC.

int value = (int)( (uint_16)current_word & 0xFFFC );

My next 6 bits are a fractional value. Here I am stuck on an efficient implementation. I could extract one bit at a time, and build the fraction 1/2*bit + 1/4+bit + 1/8*bit etc … but that’s not efficient.

float fractional = ?

The last 12 LSB are another int value, which I feel I can pull out using bitwise AND again.

int other_value = (int) ( (uint_16)current_word[2] & 0x0FFF );    

This operation will be done on 16348 double words and needs to be finished within 0.05 ms to run at least 20Hz.

I am very new to bit operations, but I’m excited to learn. Reading material and/or examples would be greatly appreciated!

Edit: I wrote OR when I meant AND

Answer 1

Since you’re starting with [address of most significant byte] and using increasing addresses from there, your data is apparently in Big-Endian byte order. Casting pointers will therefore fail on nearly all desktop machines, which use Little-Endian byte order.

The following code will work, regardless of native byte order:

int value = (current_word[0] << 6) | (current_word[1] >> 2);
double fractional = (current_word[1] & 0x03) / 4.0 + (current_word[2] & 0xF0) / 1024.0;
int other_value = (current_word[2] & 0x0F) << 8 | current_word[3];