Runtime system

In

Most scholarly papers on runtime systems focus on the implementation details of parallel runtime systems. A notable example of a parallel runtime system is Cilk, a popular parallel programming model.^[2] The proto-runtime toolkit was created to simplify the creation of parallel runtime systems.^[3]

In addition to execution model behavior, a runtime system may also perform support services such as

The runtime system is also the gateway through which a running program interacts with the runtime environment. The runtime environment includes not only accessible state values, but also active entities with which the program can interact during execution. For example, environment variables are features of many operating systems, and are part of the runtime environment; a running program can access them via the runtime system. Likewise, hardware devices such as disks or DVD drives are active entities that a program can interact with via a runtime system.

One unique application of a runtime environment is its use within an operating system that only allows it to run. In other words, from boot until power-down, the entire OS is dedicated to only the application(s) running within that runtime environment. Any other code that tries to run, or any failures in the application(s), will break the runtime environment. Breaking the runtime environment in turn breaks the OS, stopping all processing and requiring a reboot. If the boot is from read-only memory, an extremely secure, simple, single-mission system is created.

Examples of such directly bundled runtime systems include:

• Between 1983 and 1984,
  SpeedStart CP/M-86, a reduced version of CP/M-86 as runtime environment.^{[5][6][7][8][9]}
• Some stand-alone versions of
  Artline (1988–1991),^[10] Timeworks Publisher (1988–1991) and ViewMAX (1990–1992)^[11][12][13] contained special runtime versions of Digital Research's GEM as their runtime environment.^[10]
• In the late 1990s,
  batch jobs in order to create unmodifiable executables from batch scripts and run them on systems without 4DOS installed.^[14]

The runtime system of the C language is a particular set of instructions inserted by the compiler into the executable image. 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 determining which language behaviors are part of the runtime system itself and which can be determined by any particular source program. For example, in C, the setup of the stack is part of the runtime system. It is not determined by the semantics of an individual program because the behavior is globally invariant: it holds over all executions. This systematic behavior implements the execution model of the language, as opposed to implementing semantics of the particular program (in which text is directly translated into code that computes results).

This separation between the semantics of a particular program and the runtime environment is reflected by the different ways of compiling a program: compiling source code to an

As an extreme example, the physical CPU itself can be viewed as an implementation of the runtime system of a specific assembly language. In this view, the execution model is implemented by the physical CPU and memory systems. As an analogy, runtime systems for higher-level languages are themselves implemented using some other languages. This creates a hierarchy of runtime systems, with the CPU itself—or actually its logic at the microcode layer or below—acting as the lowest-level runtime system.

Advanced features

Some compiled or interpreted languages provide an interface that allows application code to interact directly with the runtime system. An example is the Thread class in the

Higher-level behaviors implemented by a runtime system may include tasks such as drawing text on the screen or making an Internet connection. It is often the case that operating systems provide these kinds of behaviors as well, and when available, the runtime system is implemented as an abstraction layer that translates the invocation of the runtime system into an invocation of the operating system. This hides the complexity or variations in the services offered by different operating systems. This also implies that the OS kernel can itself be viewed as a runtime system, and that the set of OS calls that invoke OS behaviors may be viewed as interactions with a runtime system.

In the limit, the runtime system may provide services such as a

A modern aspect of runtime systems is parallel execution behaviors, such as the behaviors exhibited by mutex constructs in Pthreads and parallel section constructs in OpenMP. A runtime system with such parallel execution behaviors may be modularized according to the proto-runtime approach.

History

Notable early examples of runtime systems are the interpreters for BASIC and Lisp. These environments also included a garbage collector. Forth is an early example of a language designed to be compiled into intermediate representation code; its runtime system was a virtual machine that interpreted that code. Another popular, if theoretical, example is Donald Knuth's MIX computer.

In C and later languages that supported dynamic memory allocation, the runtime system also included a library that managed the program's memory pool.

In the

See also

• Execution model
• Programming model
• Runtime (program lifecycle phase)
• Self-booter
• Static build

References