two's complement converting function

system.cpp

@@ -37,6 +37,25 @@ uint16_t revlendianbytes(uint8_t lsig, uint8_t msig)
     return offset;
 }
 
+/**
+ * Convert a two's comp value to regular signed number
+ * @param in
+ * @return
+ */
+int8_t decode_twos_comp(uint8_t in)
+{
+    // if not two's comp, return the original number
+    if((in & 0b10000000) == 0b0)
+    {
+        return in;
+    }
+    else
+    {
+        uint8_t minusone = in - 0b1;
+        return -(~minusone);
+    }
+}
+
 cpustate * initcpu()
 {
     // initialize cpu state

tests/systemtests.h

@@ -127,5 +127,17 @@ class TestDecoder : public CppUnit::TestFixture
 
 CPPUNIT_TEST_SUITE_REGISTRATION( TestDecoder );
 
+class TestUtilFuncs : public CppUnit::TestFixture
+{
+    CPPUNIT_TEST_SUITE( TestUtilFuncs );
+    CPPUNIT_TEST( test_decode_twos_comp );
+    CPPUNIT_TEST_SUITE_END();
+
+    public:
+        void test_decode_twos_comp();
+};
+
+CPPUNIT_TEST_SUITE_REGISTRATION( TestUtilFuncs );
+
 #endif //OPENNES_SYSTEMTESTS_H
 

tests/testutilfuncs.cpp

@@ -0,0 +1,13 @@
+#include "systemtests.h"
+#include <bitset>
+using namespace CppUnit;
+
+
+void TestUtilFuncs::test_decode_twos_comp()
+{
+    // todo: assert fails
+    //CPPUNIT_ASSERT( 0b10101011 == -85 );
+    //std::cout << std::bitset<8>(decode_twos_comp(0b10101011)) << std::endl;
+    CPPUNIT_ASSERT(decode_twos_comp(0b10101011) == -85);
+    CPPUNIT_ASSERT(decode_twos_comp(0b01010101) == 85);
+}