HDL for ALU

// This file is part of the materials accompanying the book

// "The Elements of Computing Systems" by Nisan and Schocken,

// MIT Press. Book site: www.idc.ac.il/tecs

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



/**

 * The ALU.  Computes a pre-defined set of functions out = f(x,y)

 * where x and y are two 16-bit inputs. The function f is selected

 * by a set of 6 control bits denoted zx, nx, zy, ny, f, no.

 * The ALU operation can be described using the following pseudocode:

 *     if zx=1 set x = 0       // 16-bit zero constant

 *     if nx=1 set x = !x      // Bit-wise negation

 *     if zy=1 set y = 0       // 16-bit zero constant

 *     if ny=1 set y = !y      // Bit-wise negation

 *     if f=1  set out = x + y // Integer 2's complement addition

 *     else    set out = x & y // Bit-wise And

 *     if no=1 set out = !out  // Bit-wise negation

 *

 * In addition to computing out, the ALU computes two 1-bit outputs:

 *     if out=0 set zr = 1 else zr = 0 // 16-bit equality comparison

 *     if out<0 set ng = 1 else ng = 0 // 2's complement comparison

 */



CHIP ALU {



    IN  // 16-bit inputs:

        x[16], y[16],

        // Control bits:

        zx, // Zero the x input

        nx, // Negate the x input

        zy, // Zero the y input

        ny, // Negate the y input

        f,  // Function code: 1 for add, 0 for and

        no; // Negate the out output



    OUT // 16-bit output

        out[16],



        // ALU output flags

        zr, // 1 if out=0, 0 otherwise

        ng; // 1 if out<0, 0 otherwise



    PARTS:

    Not(a=zx,out=notzx);
    And16(a=x,b=notzx,out=x1);
    Not(a=zy,out=notzy);
    And16(a=y,b=notzy,out=y1);
    Not(a=nx,out=notnx);
    Xor(a=x1,b=notnx,out=x2);
    Not(a=ny,out=notny);
    Xor(a=y1,b=notny,out=y2);
    Add16(a=x2,b=y2,out=addout);
    And16(a=x2,b=y2,out=andout);
    Mux16(a=andout,b=addout,sel=f,out=out1);
    Not(a=no,out=notno);
    And16(a=out,b=notno,out=out);
    Not(a=out[15],out=ng);

}