74181

The 74181 circuit is a 4-bit ALU.

Verilog equivalent, does not deal with carry out etc.:

module alu74181(
input [3:0]s,
input ci, M,
input [3:0] a, b,
output reg [3:0] y
);

reg [3:0] p, g;

always @(*) begin

p[0] <= ~(a[0] | (s[0] & b[0]) | (s[1] & ~b[0]));
p[1] <= ~(a[1] | (s[0] & b[1]) | (s[1] & ~b[1]));
p[2] <= ~(a[2] | (s[0] & b[2]) | (s[1] & ~b[2]));
p[3] <= ~(a[3] | (s[0] & b[3]) | (s[1] & ~b[3]));

g[0] <= ~((a[0] & ~b[0] & s[2]) | (a[0] & b[0] & s[3]));
g[1] <= ~((a[1] & ~b[1] & s[2]) | (a[1] & b[1] & s[3]));
g[2] <= ~((a[2] & ~b[2] & s[2]) | (a[2] & b[2] & s[3]));
g[3] <= ~((a[3] & ~b[3] & s[2]) | (a[3] & b[3] & s[3]));

y[0] = (p[0] ^ g[0]) ^ ~(~ci & ~M);
y[1] = (p[1] ^ g[1]) ^ ~((~ci & ~M & g[0]) | (~M & p[0]));
y[2] = (p[2] ^ g[2]) ^ ~((~ci & ~M & g[0] & g[1]) | (~M & p[1]) | (~M & p[0] & g[1]));
y[3] = (p[3] ^ g[3]) ^ ~((~ci & ~M & g[0] & g[1] & g[2]) | (~M & p[2] ) |. (~M & p[1] & g[2]) |. (~M & p[0] & g[1] & g[2]));
end
endmodule // alu74181