How to extract 2 bytes from a word, and how to make a word from 2 bytes (in C)?

Question

I am trying to extract two bytes from a 16-bit word, and to make a 16-bit word from two bytes. This is what I have tried (byte = unsigned char, word = unsigned short):

Split grpix word into 2 bytes:

word grpix; // Assume that the value has been initialized

byte grpixl = grpix & 0x00FF;
byte grpixh = grpix & 0xFF00;

Make grpix word from 2 bytes

byte grpixh; // Assume that the value has been initialized
byte grpixl; // Assume that the value has been initialized

word grpix = grpixh;
grpix <<= 8;
grpix += grpixl;

For some reason, my code doesn't work as expected, and now I'm not sure if the "splitting" of the word is wrong, if the "making" of the word is wrong, or both... Could you give me some advice?

Solution 1

You're not shifting when you split the word. So if grpix is 0x1234, then grpixl gets the expected 0x34 but grpixh ends up as 0x1200. You should say

byte grpixh = grpix >> 8;

Of course, you're also ignoring any endianness concerns that may be present. You should probably convert your word to a known endian (with something like htons()) before attempting to split (and do the reverse conversion when joining).

Solution 2

Get to know: http://graphics.stanford.edu/~seander/bithacks.html for doing all manner of operations.

right_byte = short_val & 0xFF;
left_byte = ( short_val >> 8 ) & 0xFF

short_val = ( ( left_byte & 0xFF ) << 8 ) | ( right_byte & 0xFF );

I always do a &0xFF mask to assure I have no sign problems.

Comments

[Lily Ballard]

In a 16-bit word, the mask here is pointless, because the bottom 8 bits will be discarded by the shift already.

[Fernando Aires Castello]

I changed my code to this: byte grpixh = (grpix >> 8) & 0x00FF; and now it works :)