ALUMODE Inputs

The 4-bit ALUMODE controls the behavior of the second stage adder/subtracter/logic unit. ALUMODE = 0000 selects add operations of the form Z + (W + X + Y + CIN). CIN is the output of the CARRYIN MUX (see Figure 1). ALUMODE = 0011 selects subtract operations of the form Z – (W + X + Y + CIN). ALUMODE = 0001 can implement –Z + (W + X + Y + CIN) – 1. ALUMODE = 0010 can implement –(Z + W + X + Y + CIN) – 1, which is equivalent to not (Z + W + X + Y + CIN). The negative of a two’s complement number is obtained by performing a bitwise inversion and adding one, for example, –k = not (k) + 1.