// 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: // Zero x/y if zx/zy Mux16(a=x, b=false, sel=zx, out=x1); Mux16(a=y, b=false, sel=zy, out=y1); // Negate x/y if nx/ny Not16(in=x1, out=notx); Not16(in=y1, out=noty); Mux16(a=x, b=notx, sel=nx, out=x2); Mux16(a=y, b=noty, sel=ny, out=y2); // x+y if f Mux16(a=false, b=y2, sel=f, out=y_add); Add16(a=x2, b=y_add, out=preout0); // x&y if !f Mux16(a=y2, b=true, out=y_and); And16(a=preout0, b=y_and, out=preout1); // Negate out if no Not16(in=preout1, out=notpreout); Mux16(a=preout1, b=notpreout, sel=no, out=preout2); // Output zr Or8Way(in=preout2[0..7], out=w0); Or8Way(in=preout2[8..15], out=w1); Or(a=w0, b=w1, out=zr); // Output ng And(a=preout2[0], b=preout2[0], out=ng); // Output out And(a=preout2, b=preout2, out=out); }