83 lines
2.5 KiB
Plaintext
83 lines
2.5 KiB
Plaintext
/**
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This file is part of www.nand2tetris.org
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and the book "The Elements of Computing Systems"
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by Nisan and Schocken, MIT Press.
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File name: projects/02/ALU.hdl
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*/
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/**
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* The ALU (Arithmetic Logic Unit).
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* Computes one of the following functions:
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* x+y, x-y, y-x, 0, 1, -1, x, y, -x, -y, !x, !y,
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* x+1, y+1, x-1, y-1, x&y, x|y on two 16-bit inputs,
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* according to 6 input bits denoted zx,nx,zy,ny,f,no.
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* In addition, the ALU computes two 1-bit outputs:
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* if the ALU output == 0, zr is set to 1; otherwise zr is set to 0;
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* if the ALU output < 0, ng is set to 1; otherwise ng is set to 0.
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*/
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/**
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Implementation: the ALU logic manipulates the x and y inputs
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and operates on the resulting values, as follows:
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if (zx == 1) set x = 0 // 16-bit constant
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if (nx == 1) set x = !x // bitwise not
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if (zy == 1) set y = 0 // 16-bit constant
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if (ny == 1) set y = !y // bitwise not
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if (f == 1) set out = x + y // integer 2's complement addition
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if (f == 0) set out = x & y // bitwise and
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if (no == 1) set out = !out // bitwise not
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if (out == 0) set zr = 1
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if (out < 0) set ng = 1
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*/
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CHIP ALU {
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IN
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x[16], y[16], /* 16-bit inputs */
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zx,
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nx,
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zy,
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ny,
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f,
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no;
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OUT
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out[16], /* 16-bit output */
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zr, /* 1 if (out == 0), 0 otherwise */
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ng; /* 1 if (out < 0), 0 otherwise */
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PARTS:
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/* zero the x input */
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And16(a=x, b=false, out=zerox);
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Mux16(a=x, b=zerox, sel=zx, out=x1);
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/* negate the x input? */
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Not16(in=x1, out=negx);
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Mux16(a=x1, b=negx, sel=nx, out=x2);
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/* zero the y input */
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And16(a=y, b=false, out=zeroy);
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Mux16(a=y, b=zeroy, sel=zy, out=y1);
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/* negate the x input? */
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Not16(in=y1, out=negy);
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Mux16(a=y1, b=negy, sel=ny, out=y2);
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/* compute out = x + y (if 1) or x & y (if 0) */
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Add16(a=x2, b=y2, out=sum);
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And16(a=x2, b=y2, out=and);
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Mux16(a=and, b=sum, sel=f, out=result);
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/* negate the out output? */
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Not16(in=result, out=negresult);
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Mux16(a=result, b=negresult, sel=no, out=out);
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Mux16(a=result, b=negresult, sel=no, out=resultout);
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/* if the ALU output == 0, zr is set to 1; otherwise zr is set to 0; */
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Or16Way(in=resultout, out=notzr);
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Not(in=notzr, out=zr);
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/* if the ALU output < 0, ng is set to 1; otherwise ng is set to 0. */
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IsNeg(in=resultout, out=ng);
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}
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