7.4.5.1 Direct Evidence from Testing for Accuracy
Arguments and evidence should be available which show that the required computational precision is demonstrated under worst-case input conditions.
7.4.5.2 Direct Evidence from Analysis for Accuracy Arguments and evidence should be available which show that:
a) The sources of error for all computations associated with the software safety requirement have been identified and the worst-case errors are within the specified bounds.
b) All parameters and constants used in conjunction with the software system have been checked for correctness and internal consistency.
c) For AEL 4, the use of rigorous arguments of computational accuracy and stability have been made
d) For AEL 4 & 5, the object code is a correct translation of the source code, i.e. that as far as is reasonably practicable no additional computational inaccuracies are introduced by the translation into object code.
e) For AEL 5, there is proof that the implementation meets the software safety requirements for computational accuracy and stability.
NOTE: Typical sources of error are numerically unstable algorithms, floating-point truncation (e.g. small numbers added to large numbers), and numerical overflow. Good algorithm design can reduce the errors to tolerable levels.
7.4.5.3 Backing Evidence from Analysis of Accuracy Arguments and evidence should be available which show that: a) The error analysis is based on worst-case input values. b) Good design practice is used to minimise errors in complex algorithms.
7.4.6 Specific Requirements for Evidence of Resource Usage
It is expected that an appropriate form of direct evidence will be selected from the following table in order to demonstrate that the specified resource-usage properties have been satisfied.
Acceptable Sources of Evidence: Resource Usage (Choose 1 column only from the appropriate row)
AEL 1 TESTING FIELD SERVICE EXPERIENCE & Analysis & Testing ANALYSIS & Testing ANALYSIS & Field Service Experience & Testing
AEL 2 TESTING & Analysis ANALYSIS & Field Service Experience & Testing ANALYSIS & Testing
AEL 3 ANALYSIS & Testing ANALYSIS & Field Service Experience & Testing
AEL 4 ANALYSIS & Testing
AEL 5 ANALYSIS & Testing
7.4.6.1 Direct Evidence from Testing of Resource Usage
Arguments and evidence should be available which show that Resource usage does not exceed the specified resource constraints and has been demonstrated under worst-case conditions.
Typically these resources include: disc storage, main memory, input/output bandwidth, communications bandwidth, and processor time.
7.4.6.2 Direct Evidence From Analysis of Resource Usage Arguments and evidence should be available which show that: a) The resource usage of the software does not exceed the specified resource
constraints. NOTE: This may be based on design evidence if resources are statically assigned, or by a worst-case resource-use analysis if the resources are assigned dynamically. b) For AEL 4 & 5, use of rigorous arguments of resource usage has been made.
7.4.6.3 Backing Evidence for Analysis of Resource Usage Arguments and evidence should be available which show that: a) The resource usage analysis is based on worst-case input values. b) The worst-case conditions are credible under worst-case operational conditions.
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