Ada and RealTime Systems

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Ada and Real-Time Systems
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What is Ada?

• Modern programming language.
• Initially developed by the US DOD.
• Named after Ada Byron (1815-1852).
• Initial standard released in 1983.
• Updated in 1995.

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Where is it Used?

• Financial Systems Swiss Bank Electronic Funds
  Transfer
• Air Traffic Control Systems Lockheed Martin ATM
• Commercial Aviation Boeing 777
• Scientific Space Vehicles QuickBird
• Electric Rail Systems Paris Metro
• Military Aircraft USAF B1- Bomber
• and many other areas

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Whats Needed?

• Readability.
• Flexibility.
• Control.
• Accuracy.
• Determinable Execution.

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General Readability

• Should favour reader rather than writer
• Ada
• No length limit on identifiers.
• No need to abbreviate.
• Use of underscore as separator
• Assists with human reading.

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Safer Syntax

• Common typing errors should not produce valid
  code.
• Better if compiler can find problems early in
  development.

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Safer Syntax (cont)

• Example Fortran
• The line
• DO 20 I 1,100
• When mistyped as
• DO 20 I 1.100
• Declares a new Real instead of a loop.
• A mistake like this reputedly resulted in an
  American Viking Venus probe being lost!

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Variable Declaration

• Variables should be defined before use
• Helps compiler enforce proper usage
• Helps developers know where to look for
  definitions
• Ada
• Variables defined at the top of a code block.

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Exception Handling

• Exception handling allows application to handle
  unexpected situations.
• Ceasing execution is not always safe.
• Ada
• Has built in exception handling.

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Interrupt Handling

• Real time systems need low-level access.
• Frequently used to interface hardware.
• Ada
• Built in interrupt handling

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Code Reuse

• Code reuse encourages use of tested code.
• Reduces development costs.
• Encourages design of small units.
• Ada
• Supports packaging code.
• Object-oriented development.

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Dynamic Memory Allocation

• Allows dynamic data structures.
• Ada
• Allows dynamically assigning more memory using
  new keyword.
• Data must be free manually after use.
• Override heap algorithms.

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Thread Priorities

• Wide range of thread priorities.
• Dynamic changing of priorities.
• Good synchronisation primitives.
• Ada
• Wide range of thread priorities
• Dynamic changing of priorities.
• POSIX style synchronisation primitives.
• Optional restricted tasking model

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Program Control Transfer

• Transfer based on time.
• Transfer based on asynchronous event.
• Ada
• Can do both

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Timing Precision

• High accuracy
• Must be higher than the task requires.
• Ada
• Nanosecond precision
• Monotonic Time

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Weaknesses of Ada

• Manual Memory Management
• Cause memory leaks.
• When memory freed no checking of whether variable
  is in scope.
• Language defined in English Prose
• Difficult to develop compilers.
• Test suite helps to check conformance.

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Weaknesses of Ada (Cont)

• Verbosity of Language
• By appealing to reader, hinders developer.
• Negative Image
• Compared to Java/C
• Smaller developer community

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Summary

• Ada supports many modern programming language
  concepts.
• Gives developers flexibility and control.
• Assists developers produce reliable code.
• Generally very suitable for Real-Time Systems.