Are you using TI's code composer studio? I didn't know you could do that in it.
The 'u' at the end will be specifying that the literal value (0x100) is to be stored as an unsigned integer. I have seen that use in C# programming.
It is the 0x preceding the literal value that specifies hexadecimal, 0b preceding it would indicate binary (as 0b100, or possibly 0b100u to also indicate unsigned int).
See here
http://www.blackwasp.co.uk/CSharpNumericLiterals.aspx