A functional unit for a data processor comprises an input register to store a variable X; a first circuit, having an input connected to the input register and an output, to generate a value eX on its output; a second circuit, having an input connected to the input register and an output, to generate an output which is a value (tan h(X/2)+1)/2 on its output; a comparator, having an input connected to the input register and an output, to generate a line on its output based on a comparison between X and a constant; and a selector to select between inputs connected to the outputs of the first circuit and the second circuit, in response to the output of the comparator, and provide an output representing a value sigmoid(X).