Refactor Cycle to CpuCycle: to store N and S separately for code and data access.
Refactor magic number to constants and add a helper function for converting int to long with its unsigned value.
Refactor in simplifying the determination of overflow.
Refactor the test in OpcodeTest to use a inner class instead of parallel arrays.
Implemented execute function for opcode SUB_REG_REG (Format 2).
Implemented the remaining execute function for Format 2 opcodes.