project02: Comments/cleanup

02/ALU.hdl

@@ -65,16 +65,16 @@ CHIP ALU {
     Not16(in=fout, out=notfout);
     Mux16(a=fout, b=notfout, sel=no, out=out, out[0..7]=outLow, out[8..15]=outHigh, out[15]=sign);
 
-    // compute zr (zero flag)
+    // compute zr (zero flag): set if output is all zeros
     // check if any bits in low 8 are set
-    Or8Way(in=outLow, out=tmp1);
+    Or8Way(in=outLow, out=lowOnes);
     // check if any bits in high 8 are set
-    Or8Way(in=outHigh, out=tmp2);
+    Or8Way(in=outHigh, out=highOnes);
     // combine both halves (true if any bit is set)
-    Or(a=tmp1, b=tmp2, out=notzr);
-    // zr = true if no bits are set (output is zero)
+    Or(a=lowOnes, b=highOnes, out=notzr);
+    // zr set if no bits are set (output is zero)
     Not(in=notzr, out=zr);
 
-    // compute ng (negative flag)
+    // compute ng (negative flag): set if leftmost bit is 1 (two's complement)
     And(a=sign, b=true, out=ng);
 }

02/FullAdder.hdl

@@ -11,7 +11,10 @@ CHIP FullAdder {
         carry;   // Left bit of a + b + c
 
     PARTS:
+    // add first two inputs
     HalfAdder(a=a, b=b, sum=absum, carry=c0);
+    // add third input to partial sum
     HalfAdder(a=absum, b=c, sum=sum, carry=c1);
+    // combine both carry bits
     Or(a=c0, b=c1, out=carry);
 }