Once upon a time I had some work involving avionics, and discovered that (here in the US anyway) those responsible for certifying avionics (FAA here) also require that the compiler used for projects involving firmware also be certified.

The FAA wants to know that it will work exactly a certain way, and assembler provides that repeatability, as would a compiler that's run their maze.

So if you can't find a certified compiler, that pretty much leaves assembler, unless you build it solely with hardware (which happened in the case I was involved with--so sad to see all that hardware where a simple uC would have done the job).

No wonder avionics is expensive.

Dennis