Computer build from first principals

/**
 * The ALU (Arithmetic Logic Unit).
 * Computes one of the following functions:
 * x+y, x-y, y-x, 0, 1, -1, x, y, -x, -y, !x, !y,
 * x+1, y+1, x-1, y-1, x&y, x|y on two 16-bit inputs, 
 * according to 6 input bits denoted zx,nx,zy,ny,f,no.
 * In addition, the ALU computes two 1-bit outputs:
 * if the ALU output == 0, zr is set to 1; otherwise zr is set to 0;
 * if the ALU output < 0, ng is set to 1; otherwise ng is set to 0.
 */

// Implementation: the ALU logic manipulates the x and y inputs
// and operates on the resulting values, as follows:
// if (zx == 1) set x = 0        // 16-bit constant
// if (nx == 1) set x = !x       // bitwise not
// if (zy == 1) set y = 0        // 16-bit constant
// if (ny == 1) set y = !y       // bitwise not
// if (f == 1)  set out = x + y  // integer 2's complement addition
// if (f == 0)  set out = x & y  // bitwise and
// if (no == 1) set out = !out   // bitwise not
// if (out == 0) set zr = 1
// if (out < 0) set ng = 1

CHIP ALU {
    IN  
        x[16], y[16],  // 16-bit inputs        
        zx, // zero the x input?
        nx, // negate the x input?
        zy, // zero the y input?
        ny, // negate the y input?
        f,  // compute out = x + y (if 1) or x & y (if 0)
        no; // negate the out output?

    OUT 
        out[16], // 16-bit output
        zr, // 1 if (out == 0), 0 otherwise
        ng; // 1 if (out < 0),  0 otherwise

    PARTS:
    // zx
    And16(a=x, b=false, out=zxTrue);
    Mux16(a=x, b=zxTrue, sel=zx, out=zerox);

    // nx
    Not16(in=zerox, out=notx);
    Mux16(a=zerox, b=notx, sel=nx, out=negx);

    // zy
    And16(a=y, b=false, out=zyTrue);
    Mux16(a=y, b=zyTrue, sel=zy, out=zeroy);

    // ny
    Not16(in=zeroy, out=noty);
    Mux16(a=zeroy, b=noty, sel=ny, out=negy);

    // f
    And16(a=negx, b=negy, out=andfxy);
    Add16(a=negx, b=negy, out=addfxy);
    Mux16(a=andfxy, b=addfxy, sel=f, out=fxy);

    // no
    Not16(in=fxy, out=notfxy);
    Mux16(a=fxy, b=notfxy, sel=no, out=out);
}

/**
 * 16-bit incrementer:
 * out = in + 1 (arithmetic addition)
 */

CHIP Inc16 {
    IN in[16];
    OUT out[16];

    PARTS:
    HalfAdder(a=in[0],  b=true, sum=out[0],  carry=c0);
    HalfAdder(a=in[1],  b=c0,   sum=out[1],  carry=c1);
    HalfAdder(a=in[2],  b=c1,   sum=out[2],  carry=c2);
    HalfAdder(a=in[3],  b=c2,   sum=out[3],  carry=c3);
    HalfAdder(a=in[4],  b=c3,   sum=out[4],  carry=c4);
    HalfAdder(a=in[5],  b=c4,   sum=out[5],  carry=c5);
    HalfAdder(a=in[6],  b=c5,   sum=out[6],  carry=c6);
    HalfAdder(a=in[7],  b=c6,   sum=out[7],  carry=c7);
    HalfAdder(a=in[8],  b=c7,   sum=out[8],  carry=c8);
    HalfAdder(a=in[9],  b=c8,   sum=out[9],  carry=c9);
    HalfAdder(a=in[10], b=c9,   sum=out[10], carry=c10);
    HalfAdder(a=in[11], b=c10,  sum=out[11], carry=c11);
    HalfAdder(a=in[12], b=c11,  sum=out[12], carry=c12);
    HalfAdder(a=in[13], b=c12,  sum=out[13], carry=c13);
    HalfAdder(a=in[14], b=c13,  sum=out[14], carry=c14);
    HalfAdder(a=in[15], b=c14,  sum=out[15], carry=false);
}

Finished the HDL code for the logic, starting work on the ALU.