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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:
// Put you code here:
Mux16 (a=x, b[0..15]=false, sel=zx, out=x1);
Not16 (in=x1, out=notx1);
Mux16 (a=x1, b=notx1, sel=nx, out=x2);
Mux16 (a=y, b[0..15]=false, sel=zy, out=y1);
Not16 (in=y1, out=noty1);
Mux16 (a=y1, b=noty1, sel=ny, out=y2);
Add16 (a=x2, b=y2, out=x2py2);
And16 (a=x2, b=y2, out=x2ay2);
Mux16 (a=x2ay2, b=x2py2, sel=f, out=xy);
Not16 (in=xy, out=notxy);
Mux16 (a=xy, b=notxy, sel=no, out=out, out[0..7]=out1, out[8..15]=out2, out[15]=msbout);
Or8Way (in=out1, out=z1);
Or8Way (in=out1, out=z2); //some potential pitfall w.r.t. subbusing see https://www.coursera.org/learn/build-a-computer/discussions/weeks/2/threads/9VYr3LzkEeeK2BJ0oEsgKA
Or (a=z1, b=z2, out=z);
Not (in=z, out=zr);
And (a=msbout, b=true, out=ng);
}
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