As a simple example of a basic runtime system, the runtime system of the C language is a particular set of instructions inserted into the executable image by the compiler. Among other things, these instructions manage the process stack, create space for local variables, and copy function-call parameters onto the top of the stack. There are often no clear criteria for deciding which language behavior is considered inside the runtime system versus which behavior is part of the source program. For C, the setup of the stack is part of the runtime system, as opposed to part of the semantics of an individual program, because it maintains a global invariant that holds over all executions. This systematic behavior implements the execution model of the language

Notably, LLVM and JVM are by far the most prominent targets in this scenario: they’re both open-source and well-documented targets that provide a ton of firepower, and there frankly aren’t a ton of other options.

Notable early examples of runtime systems are the interpreters for BASIC and Lisp.