According to text, systems developers.

 Systems developers maintain, audit and improve organisational support systems by working on the internal operations of computers, using existing systems or incorporating new technologies to meet particular needs, often as advised by a systems analyst or architect. They test both hard and software systems, and diagnose and resolve system faults.
   Their role also covers writing diagnostic programs, designing, and writing code for operating systems and software to ensure efficiency. When required, they make recommendations for future developments. Depending on the type of organisation, developers can become either systems or applications specialists.
   The work undertaken by systems developers is generally of a highly complex and technical nature, and involves the application of computer science and mathematics in an environment which is constantly evolving due to technological advances and the strategic direction of their organisation.

  Internet: (adapted).