Charles (Chuck) Moore the inventor of Forth


Moore’s programming career began in the late 1950s at the Smithsonian Astrophysical Observatory with programs to compute ephemerides, orbital elements, satellite station positions, etc. [Moore, 1958], [Veis, 1960].

In 1961, Moore received his BA in Physics from MIT and entered graduate school at Stanford. He also took a part-time programming position at the Stanford Linear Accelerator (SLAC), writing code to optimize beam steering for the (then) pending two-mile electron accelerator, using an extension of some of his prior work with least-squares fitting. A key outgrowth of this work was a program called CURVE, coded in Algol (1964), a general-purpose non-linear differential-corrections data fitting program. To control this program, he used an enhanced version of his interpreter, extended to manage a push-down stack for parameter passing, variables (with the ability to explicitly fetch and store values), arithmetic and comparison operators, and the ability to define and interpret procedures.

In 1965, he moved to New York City to become a free-lance programmer. Working in Fortran, Algol, Jovial, PL/I and various assemblers, he continued to use his interpreter as much as possible, literally carrying around his card deck and recoding it as necessary. Minicomputers appeared in the late ’60s, and with them teletype terminals, for which Moore added operators to manage character input and output. One project involved writing a Fortran-Algol translator and file-editing utilities. This reinforced for him the value of spaces between words, which were not required in Fortran source.

Newly married and seeking a small town environment, Moore joined Mohasco Industries in Amsterdam, NY, in 1968. Here he developed computer graphics programs for an IBM 1130 minicomputer with a 2250 graphic display. This computer had a 16-bit CPU, 8k RAM, his first disk, keyboard, printer, card reader/punch (used as disk backup!), and Fortran compiler. He added a cross-assembler to his program to generate code for the 2250, as well as a primitive editor and source-management tools. This system could draw animated 3-D images, at a time when IBM’s software for that configuration drew only static 2-D images. For fun, he also wrote a version of Spacewar, an early video game, and converted his Algol Chess program into the new language, now (for the first time) called FORTH. He was impressed by how much simpler it became.

The name FORTH was intended to suggest software for the fourth (next) generation computers, which Moore saw as being characterized by distributed small computers. The operating system he used at the time restricted file names to five characters, so the “U” was discarded. FORTH was spelled in upper case until the late 1970s because of the prevalence of upper-case-only I/O devices. The usage “Forth” was generally adopted when lower case became widely available, because the word was not an acronym.

Moore found the Forth-based 1130 environment for programming the 2250 superior to the Fortran environment in which the 1130 software was developed, so he extended it into an 1130 compiler. This added looping commands, the concept of keeping source in 1024-byte blocks and tools for managing them, and most of the compiler features we recognize in Forth today.

Most important, there was now a dictionary. Procedures now had names, and the interpreter searched a linked list of names for a match. Names were compiled with a count and three characters, a practice learned from the compiler writers of Stanford and which prevailed in Forth until the 1980s. Within a dictionary entry was a “code field” containing the address of code to be executed for that routine. This was an indirect threaded code implementation (see Section 5.2) and was in use five years before Dewar’s paper on indirect threaded coded appeared in Communications of the ACM [Dewar 1975]. The use of indirect threaded code was an important innovation, since an indirect jump was the only overhead once a word had been found. Dictionary entries could consist either of pointers to other “high level” routines or of machine instructions.

Finally, in order to provide a simple mechanism for nesting routines, a second stack called the “return stack” was added. The benefit of having a stack reserved for return addresses was that the other stack could be used freely for parameter passing, without having to be “balanced” before and after calls. The first paper on Forth was written at Mohasco [Moore, 1970a]. In 1970 Mohasco assigned Moore to an ambitious project involving a new Univac 1108 handling a network of leased lines for an order-entry system. He ported Forth onto the 1108, and arranged for it to interface to COBOL modules that did the transaction processing. The 1108 Forth was coded in assembler. It buffered input and output messages and shared the CPU among tasks handling each line. It also interpreted the input and executed the appropriate COBOL modules. This version of Forth added mechanisms for defining and managing tasks, and also an efficient scheme for managing disk block buffers similar to schemes in use today. Unfortunately, an economic downturn led Mohasco to cancel the 1108 project before completion. Moore immediately gave notice, then wrote an angry poem and a book on Forth [Moore, 1970b] that was never published. It described how to develop Forth software and encouraged simplicity and innovation.

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