FORTH is super simple, easy to understand and extend, small and quite forward. So one can simply prove the correctness of such an application. FORTH is well known in the context of micro controllers, but not limited to.
Forth on the Space Shuttle
FORTH is its own interactive operating system and programming language. It doesn?t need much memory. Some k are enough for a full development system with interpreter, compiler debugger and storage system.
From: Forth Meta Compilation:
"Forth system differ greatly in their design and implementation. There is a wide range in both size and complexity. Some are very small. A native code port of one eForth for F21 fit the code dictionary into 1K word of memory. Chuck's latest Forth needs only 1K bytesThere are a number of factors that influence the complexity of a Forth system and the meta compiler that will be needed to compile it."