Forth (programming language)

Forth is a stack-oriented programming language and interactive integrated development environment designed by Charles H. "Chuck" Moore and first used by other programmers in 1970.

The free and open-source software Gforth implementation is actively maintained, as are several commercially supported systems.

Forth typically combines a compiler with an integrated command shell,[a] where the user interacts via subroutines called words.

For much of Forth's existence, the standard technique was to compile to threaded code, which can be interpreted faster than bytecode.

No longer constrained by space, there are modern implementations that generate optimized machine code like other language compilers.

[1] Forth is used in the Open Firmware boot loader, in spaceflight applications[2] such as the Philae spacecraft,[3][4] and in other embedded systems which involve interaction with hardware.

Moore developed a series of microprocessors for executing compiled Forth-like code directly and experimented with smaller languages based on Forth concepts, including cmForth and colorForth.

The Open Firmware boot ROMs used by Apple, IBM, Sun, and OLPC XO-1 contain a Forth environment.

[7] Electronic Arts published multiple video games in the 1980s that were written in Forth, including Worms?

This system was instrumental in creating the complex and precise camera movements required for the space battle scenes and the trench run sequences on the Death Star.

The Dykstraflex system allowed for the pre-programming of camera movements, which could then be repeated exactly for multiple takes.

Forth was chosen for its efficiency and its ability to handle real-time processing required for these intricate tasks.

[6][14] Forth was first exposed to other programmers in the early 1970s, starting with Elizabeth Rather at the United States National Radio Astronomy Observatory (NRAO).

Moore saw Forth as a successor to compile-link-go third-generation programming languages, or software for "fourth generation" hardware.

But because FORTH ran on the 1130 (which permitted only five-character identifiers), the name was shortened.FORTH, Inc.'s microFORTH was developed for the Intel 8080, Motorola 6800, Zilog Z80, and RCA 1802 microprocessors, starting in 1976.

[22][23] As of 2018, the source for the original 1130 version of FORTH has been recovered, and is now being updated to run on a restored or emulated 1130 system.

These words, being distinct modules that communicate implicitly via a stack mechanism, can be prototyped, built and tested independently.

[25] Forth relies on implicit use of a data stack and reverse Polish notation which is commonly used in calculators from Hewlett-Packard.

Extending the compiler only requires writing a new word, instead of modifying a grammar and changing the underlying implementation.

A typical reverse Polish assembler prepares the operands on the stack and the mnemonic copies the whole instruction into memory as the last step.

The compiler's words use specially named versions of fetch and store that can be redirected to a buffer area in memory.

Often the most time-consuming part of writing a remote port is constructing the initial program to implement fetch, store and execute, but many modern microprocessors have integrated debugging features (such as the Motorola CPU32) that eliminate this task.

A linked list search continues as the branch merges into the main trunk leading eventually back to the sentinel, the root.

During compilation, the data stack is used to support control structure balancing, nesting, and back-patching of branch addresses.

The snippet: would often be compiled to the following sequence inside a definition: The numbers after BRANCH represent relative jump addresses.

The execution token provides an opaque handle for the compiled semantics, similar to the function pointers of the C programming language.

The word COMPILE, (compile-comma) takes an execution token from the data stack and appends the associated semantics to the current definition.

This is also called indirect-threaded code, but direct-threaded and subroutine threaded Forths have also become popular in modern times.

Some examples include circular buffers, named bits on an I/O port, and automatically indexed arrays.

Using IMMEDIATE and POSTPONE, [CHAR] could have been defined like this: In 1987, Ron Rivest developed the RC4 cipher-system for RSA Data Security, Inc. Its description follows: We have an array of 256 bytes, all different.