Harris RTX 2000. The Harris RTX 2000, now manufactured by Intersil, is a 16-bit stack machine microprocessor architecture designed for real-time computing and programmed primarily in Forth. Radiation hardened versions, the RTX 2010 series, have been used in numerous spacecraft.

A 32-bit design, the RTX 4000, was introduced in 1989, but not put into production.

Characteristics

A stack machine contrasts with traditional designs in that it lacks general purpose processor registers defined in the instruction set architecture. Instead it uses addressing modes based on offsets in the stack, which is normally stored in main memory or partially cached in registers that are not visible to the programmer.

Accessing parameters through a stack in main memory is slower than dedicated registers, and stack machines often have lower general purpose performance as a result. However, this is offset in certain uses because the data can be swapped in and out of the stack by changing a single value, the stack pointer. This makes these systems extremely fast for context switching, subroutine calls and interrupt processing, especially compared to register-heavy architectures like RISC machines where some or all of the values in the registers have to be copied out to storage and new ones read in.

The RTX 2000 is a two-stack machine, each stack 256 words deep, that supports direct execution of Forth. Subroutine calls only take one processor cycle and returns take zero. It also has a very low and consistent interrupt latency of only four processor cycles, which lends it well to realtime applications. It features multiple instruction busses that allow several instructions to be dispatched at the same time, which helps eliminate pauses while waiting on data from main memory by performing an instruction that has the data already loaded. Finally, because the instructions are directly mapped onto high-level concepts, the instruction code is very dense and programs are much smaller than normal assembler languages used with traditional designs.

History
In 1983, Chuck Moore implemented a processor for his programming language Forth as a gate array. As Forth can be considered a dual stack virtual machine, he made the processor, Novix N4000 (later renamed NC4016), as a dual-stack machine. This system used only 4,000 gates constructed from 16,000 CMOS transistors, compared to 68,000 gates for the 1979 Motorola 68000. The chip could directly execute 40 Forth primitives and 123 combinations, eliminating the need for a complete virtual machine and speeding performance of the most common instructions.

In 1988, Harris Semiconductor purchased the rights to the Novix line and began an improvement program. They added on-chip stacks to reduce the number accesses to main memory, a number of on-chip timers and counters, a dedicated interrupt controller, and a single-cycle hardware multiplier. The new version was renamed the RTX 2000 and marketed for space applications.

Example spacecraft that use the RTX2010
 Advanced Composition Explorer (ACE)
 NEAR/Shoemaker
 TIMED
 Rosetta's lander - Philae

References

External links

  Intersil's product page.

 

 

Avionics computers
Radiation-hardened microprocessors
Stack machines
16-bit microprocessors