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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: tools/builtIn/ALU.hdl
/**
* The ALU. 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.
* Which function to compute is determined by 6 input bits
* denoted zx, nx, zy, ny, f, no.
* The computed function's value is called "out".
* In addition to computing out, the ALU computes two
* 1-bit outputs called zr and ng:
* if out == 0, zr = 1; otherwise zr = 0;
* If out < 0, ng = 1; otherwise ng = 0.
* The 6-bit combinations (zx,nx,zy,ny,f,no) and
* their effect are documented in the book.
*/
// Implementation: the ALU manipulates the x and y
// inputs and then operates on the resulting values,
// as follows:
// if (zx == 1) sets x = 0 // 16-bit constant
// if (nx == 1) sets x = ~x // bitwise "not"
// if (zy == 1) sets y = 0 // 16-bit constant
// if (ny == 1) sets y = ~y // bitwise "not"
// if (f == 1) sets out = x + y // integer 2's-complement addition
// if (f == 0) sets out = x & y // bitwise And
// if (no == 1) sets out = ~out // bitwise Not
// if (out == 0) sets zr = 1
// if (out < 0) sets ng = 1
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
BUILTIN ALU;
}
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