hazards in accordance with the SMS and, if appropriate, (2) developed and verified in  accordance with RTCA DO-278A. 6 Unaffected portions of NAS baseline software do not need to comply with the RTCA DO-278A  objectives. Unaffected portions are those that are neither changed nor affected by changes, as  determined by control flow, data flow, memory usage, or timing analysis. The safety analysis  should determine the affected and unaffected portions.  Software development and verification tools used for the baseline software may need to be  qualified in accordance with RTCA DO-278A. 6.4  Reuse of Previously Approved Software in a CNS/ATM System  RTCA DO-278A, Section 12.1.2,  Reuse of Previously Approved Software in a CNS/ATM  System , addresses CNS/ATM systems or equipment containing legacy software that has been  previously approved. The system safety assessment process evaluates the new CNS/ATM  system and determines the required AL.  The following describe the circumstances addressed in RTCA DO-278A with respect to  previously developed software and reuse:  • If the previously approved software complies with the RTCA DO-278A objectives, there  are no changes to the software, and the AL is the same for the new system, then no  additional effort is required.  • If modifications are to be made to the previously approved software, the guidance  established in RTCA DO-278A, Section 12.1.1,  Modifications to Previously Developed  Software , must be satisfied.  • If the software lifecycle data from a previous application are inadequate or do not satisfy  the objectives for the new application, the guidance in RTCA DO-278A, Section 12.1.4,  Upgrading a Development Baseline , must be satisfied.  6.5  Software Tool Qualification  Qualification of a software tool is necessary when RTCA DO-278A software processes are  being eliminated, reduced, or automated by use of the tool without the tool’s output being  verified as specified in RTCA DO-278A, Section 6,  Software Verification . RTCA DO-278A  defines a software tool as “a computer program used to help develop, test, analyze, produce, or  modify another program or its documentation.” Examples of some software tools include, but  are not limited to, the automatic source code generator, structural coverage analysis, and  software standards checkers.  RTCA DO-278A, Section 12.2,  Tool Qualification , provides guidance on applying development  assurance to software tools and on how to determine the Tool Qualification Level (TQL). In  addition, RTCA DO-330 provides objectives, activities, guidance, and lifecycle data required for  each TQL.  RTCA DO-278A, Section 12.3.4,  Service Experience , provides guidance for determining  whether equivalent safety for software can be demonstrated by the software’s product service  6.6  Service Experience  6. In rare cases, this requirement may be waived. Contact Safety and Technical Training (AJI) Policy and  Performance, AJI-3, for more information.  J-7  J_SRMGSA_202003 Originally published March 2020  Uncontrolled copy when downloaded