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+// This file is part of www.nand2tetris.org

+// and the book "The Elements of Computing Systems"

+// by Nisan and Schocken, MIT Press.

+// File name: projects/02/ALU.hdl

+

+/**

+ * 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:

+    // zdx = (zx == 0) ? x : 0x0000; same with y

+    Mux16(a=x, b=false, sel=zx, out=zdx);

+    Mux16(a=y, b=false, sel=zy, out=zdy);

+    Not16(in=zdx, out=ndx);

+    Not16(in=zdy, out=ndy);

+    Mux16(a=zdx, b=ndx, sel=nx, out=px);

+    Mux16(a=zdy, b=ndy, sel=ny, out=py);

+    And16(a=px, b=py, out=xandy);

+    Add16(a=px, b=py, out=xplusy);

+    Mux16(a=xandy, b=xplusy, sel=f, out=fxy);

+    Not16(in=fxy, out=nfxy);

+    Mux16(a=fxy, b=nfxy, sel=no, out[15]=ng, out[0..7]=low, out[8..15]=high, out=out);

+    Or8Way(in=low, out=lowneq0);

+    Or8Way(in=high, out=highneq0);

+    Or(a=lowneq0, b=highneq0, out=outneq0);

+    Not(in=outneq0, out=zr);

+}