# ALU Implemented

A project log for Computer build from first principals

Complete computer build from the logic to the compiler and operating system.

Michael Walker 06/26/2019 at 06:100 Comments
```/**
* 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);
}```