Harris RTX 2000

Lore & Background

The RTX 2000 traces its lineage to Chuck Moore, inventor of the Forth language, who in 1983 implemented a Forth processor as a gate array. He formed Novix, which released the NC4000 in June 1985, later renamed NC4016, running at 7.5 MHz and up to 10 MIPS. An updated NC5016 ran at 10 MHz and 12 MIPS. A planned NC6016 appears not to have entered production as Novix shut down. Harris Semiconductor licensed the original 4016 design shortly after its introduction, modified it, and launched the RTX line in 1988. The RTX 2000 added on-chip cache memory, a multiply/divide unit, and program memory. A variant without the multiplier, the RTX 2001, was also introduced. An upgrade with input/output systems, timers, and a complete floating-point unit became the RTX 2010, produced primarily on a silicon-on-sapphire line for space applications. A similar 32-bit design, the RTX 4000, was introduced in 1989 but not put into production.

Reader's Guide

The RTX 2000's significance lies in its stack machine architecture, which contrasts with traditional register-based designs. By eliminating general-purpose registers visible to the programmer, it uses addressing modes based on stack offsets, resulting in smaller instructions—often single-byte opcodes. This makes programs much denser than typical assembler code. While accessing parameters through a stack in main memory is slower than dedicated registers, the architecture excels in context switching: saving and restoring register state is reduced to changing a single stack pointer. This yields extremely fast context switching, with subroutine calls taking one processor cycle and returns taking zero. The RTX 2000 also features a very low and consistent interrupt latency of only four processor cycles, making it well-suited for real-time applications. Multiple instruction busses allow several instructions to be dispatched simultaneously, reducing pauses while waiting on data from main memory. The RTX 2010 radiation-hardened version has been used in numerous spacecraft, including the Advanced Composition Explorer (ACE), NEAR/Shoemaker, TIMED, and Rosetta's lander Philae, demonstrating its reliability in space environments.

Did You Know?

• The RTX 2000 directly executes Forth, eliminating the need for a complete virtual machine.
• Subroutine calls take only one processor cycle and returns take zero cycles.
• The RTX 2010 radiation-hardened version has been used on the Rosetta mission's Philae lander.
• The original Novix NC4000 used only 4,000 gates constructed from 16,000 CMOS transistors.