Add solutions for part 1

projects/03/a/PC.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: projects/03/a/PC.hdl
+
+/**
+ * A 16-bit counter with load and reset control bits.
+ * if      (reset[t] == 1) out[t+1] = 0
+ * else if (load[t] == 1)  out[t+1] = in[t]
+ * else if (inc[t] == 1)   out[t+1] = out[t] + 1  (integer addition)
+ * else                    out[t+1] = out[t]
+ */
+
+CHIP PC {
+    IN in[16],load,inc,reset;
+    OUT out[16];
+
+    PARTS:
+    Mux16(a=s, b=false, sel=reset, out=s1);
+
+    /* Make sure we only load if we didn't reset. */
+    Not(in=reset, out=notreset);
+    And(a=notreset, b=load, out=load1);
+
+    Mux16(a=s1, b=in, sel=load1, out=s2);
+
+    /* Make sure we only inc if we didn't reset or load. */
+    Not(in=load, out=notload);
+    And(a=notreset, b=notload, out=notresetorload);
+    And(a=notresetorload, b=inc, out=inc1);
+
+    /* Calculate increment and set depending on inc flag. */
+    Inc16(in=s, out=si);
+    Mux16(a=s2, b=si, sel=inc1, out=s3);
+
+    /* s3 will have the original value if none of load/inc/reset was set. */
+    Register(in=s3, load=true, out=s);
+
+    /* Output nextstate */
+    Id(in=s, out=out);
+}