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4 Hardware Implementations of Forth
The internal architecture of Forth simulates a computer with two stacks,
a set of registers, and other well-defined features. As a result, it was
almost inevitable that someone would attempt to build a hardware representation
of the actual Forth computer.
The first such effort was made in 1973 by John Davies, of the Jodrell
Bank Radio Astronomy Observatory near Manchester, England. Davies' approach
was to re-design a Ferranti computer that had gone out of production to
optimize its instruction set for Forth.
The first actual Forth computers were bit-sliced board-level products.
The first of these was made by a California company called Standard Logic,
in 1976. By making a minor modification in the instruction set of their
board-level computer, Standard Logic's chief programmer Dean Sanderson was
able to implement the precise instruction that Forth uses in its "address
interpreter" to move from one high-level command to the next. Their
system was used widely by the U. S. Post Office.
In the early 1980's, Rockwell produced a microprocessor with Forth primitives
in on-chip ROM, the Rockwell AIM 65F11 [Dumse,
1984]. This chip has been used quite successfully in embedded microprocessor
applications. However, no attempt was made to adapt the actual architecture
of the processor (basically a 6502) for Forth support.
In 1981, Moore himself undertook to design an actual Forth chip. Working
first at FORTH, Inc. and subsequently with a start-up company called Novix,
formed to develop the chip, Moore completed the design in 1984, and the
first prototypes were produced in early 1985 [Golden,
1985]. This design was subsequently purchased and adapted by Harris
Semiconductor Corp., and formed the basis of their line of RTX processors. [note]
Starting in the early 1980's, a group at the Johns Hopkins Applied Physics
Laboratory in Maryland developed a series of experimental Forth processors
for use in space instrumentation [Hayes,
1987]. The most successful of these, marketed as the SC-32 by Silicon
Composers of Palo Alto, CA, was used to control the Hopkins Ultraviolet
Telescope which flew in the Columbia Space Shuttle in November, 1990 [Ballard_1991]. It continues to
be the basis for more space instruments under development.
Moore himself, working on his own, has continued to develop Forth-based processors
for special applications.
The various Forth processors have had an influence on Forth software
systems. In order to take full advantage of these architectures, Forth compilers
were developed by Moore, FORTH, Inc. and Laboratory Microsystems that generated
machine code optimized for the chip's internal architecture. A native looping
structure in the Novix and Harris chips called FOR … NEXT
(which counted down from a single-argument upper limit to zero) led to adoption
of this structure in other Forths as well.
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