Then 0 ... 0xFF = data byte or -1 (0xFFFF) = buffer empty.<pre>
In fact, you could just say that any non-zero value in the high-byte indicates buffer empty.

Be careful with sign extension here:
the signed char value 0xff represents -1; if you assign this value to a signed int, you will still get -1, but as an int that'll be 0xffff