Add 128 to a signed char and the compiler is free to assume it is zero/false (because undefined behavior) OR assume the value is always greater than 127 (because undefined behavior). Or if it compiles it into machine code, the result may depend on register width since it may or may not store it back into memory. Resulting in a value either larger than 127 or mod 128 depending on register pressure since the compiler isn't obligated to AND 0xFF because Undefined Behavior.