Correct 5-bit sign extension on the 6809 processor. Allows CLR 5-bit offset indexed disassembly to work correctly.

Signed-off-by: Adrian Conlon <Adrian.conlon@gmail.com>

src/Processor.cpp

@@ -31,3 +31,11 @@ int EightBit::Processor::run(const int limit) {
 		current += step();
 	return current;
 }
+
+// http://graphics.stanford.edu/~seander/bithacks.html#FixedSignExtend
+int8_t EightBit::Processor::signExtend(int b, uint8_t x) {
+	const uint8_t m = 1 << (b - 1); // mask can be pre-computed if b is fixed
+	x = x & ((1 << b) - 1);  // (Skip this if bits in x above position b are already zero.)
+	const auto result = (x ^ m) - m;
+	return result;
+}