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tools/builtInChips/ALU.hdl

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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;
+}