Added the filled rectangle function

Added comments for the new functions
Finished the circle function

stage4/sign.pl

@@ -0,0 +1,19 @@
+#!/usr/bin/perl
+
+open(SIG, $ARGV[0]) || die "open $ARGV[0]: $!";
+
+$n = sysread(SIG, $buf, 1000);
+
+if($n > 510){
+  print STDERR "boot block too large: $n bytes (max 510)\n";
+  exit 1;
+}
+
+print STDERR "boot block is $n bytes (max 510)\n";
+
+$buf .= "\0" x (510-$n);
+$buf .= "\x55\xAA";
+
+open(SIG, ">$ARGV[0]") || die "open >$ARGV[0]: $!";
+print SIG $buf;
+close SIG;