A Scheme Interpreter for ARM Microcontrollers
Documentation for Prior Versions
050 (2012) to 070 (2018)

ARMPIT SCHEME is an interpreter for the Scheme language (lexically-scoped dialect of Lisp) that runs on RISC microcontrollers with ARM core. It is based on the description in the Revised^5 Report on the Algorithmic Language Scheme (r5rs), with some extensions (for I/O) and some omissions (to fit within MCU memory). It is further designed to support multitasking and multiprocessing. Armpit Scheme is expected to be well suited to educational settings, including student projects in courses on control and instrumentation, or capstone design courses where microcontrollers are needed. It is meant to enrich the spectrum of interpreted languages available for MCUs (eg. BASIC and FORTH) and can be an alternative to MCU-based bytecode interpreters (eg. for Scheme or Java) and to compiled languages (eg. C).

The name "Armpit" was selected for this project because it includes "ARM" (as in ARM core MCU) and "pit" (as in kernel, noyau, nucleus, the core of an Operating System (OS)). Armpit Scheme, once loaded, governs the operation of the MCU, and is "Scheme to the metal" in the sense of running without any other OS. It may be thought of as turning the MCU into a rudimentary Scheme machine. The screenshot below shows the system running on a BeagleBoard XM, powered by and communicating via USB. Minicom is used to communicate with the board which reads, evaluates and prints the result of the entered expressions.

The latest development snapshot of Armpit Scheme is 070 (year 7, release 0) and the latest stable version is 050. They are distributed under the MIT License and are beta releases, with both known bugs and unknown bugs. The source code, and pre-assembled (ready-to-upload) Intel Hex and binary image files, can be downloaded from the armpit project summary page or the download site at SourceForge. Alternatively, you can view and download the source code from your web browser. The system is documented in several web pages: 

• Documentation for Armpit Scheme Snapshot 070 (December 2014 to June 2018):
  • ChangeLog
  • Known bugs and "features"
  • Common Program Examples
  • MCU-specific and Board-oriented Examples
• Documentation for Armpit Scheme Snapshot 060 (February 2013 to December 2014):
  • ChangeLog
  • Known bugs and "features"
  • Common Program Examples
  • MCU-specific and Board-oriented Examples
  • Performance
• Supplemental files for Armpit Scheme Snapshot 060:
  • A patch against snapshot 060 of Armpit Scheme that enables support for Mbed's LPC-1768 (contributed by K. Tyszko).
  • A github update of version 060 of Armpit Scheme that enables support for the ARMka board (contributed by R. Popov).
• Documentation for Armpit Scheme Version 050 (January 2012 to January 2013):
  • ChangeLog
  • Known bugs and "features"
  • Common Program Examples
  • MCU-specific and Board-oriented Examples
  • Performance
  • R5RS Conformance
  • Language Extensions
  • Implementation Reference
• Documentation for prior snapshots and versions of Armpit Scheme.
• A patch against snapshot 00.0250 of Armpit Scheme that enables support for Olimex's SAM7-P256 (contributed by R. Dinn of MicroBus, UK).
• How To: upload, communicate with and re-assemble Armpit Scheme, and some additional notes.

It is designed to run on the following boards, either from in-house development (Table 1) or contributions from developers (Table 2), as of the latest release:

(*) On-board LCDs / OLEDs are programmed from within user or library space (not in the core). The BeagleBoard onboard flash is not used in the Live-SD version option (070, 060, 050). The Blueboard and MBED 1768 have 64KB of RAM but it is not contiguous, hence only 32KB is used. The SAM9-L9261 has 64 MB of SDRAM and 16 MB of flash but Armpit Scheme uses only up to 1 MB of flash (at the top of flash) on this board and it is shadowed by 1 MB of SDRAM. There are 2 versions of the code for LPC4357-Xplorer: one (small memory) that loads by DFU and uses SPIFI (4 MB) for files, and the other (normal) that loads by ISP and uses on-chip flash (512 KB). Also, for the LPC4357-Xplorer, the LCD is on a separate baseboard.

External links of potential interest include:

• Discussions on the potential of Scheme/Lisp for Microcontrollers: A brief essay on embedded lisps and schemes by Matt Knox. A colorful project on wearable lisp and unsorted discussions or comments about the potential for Lisp/Scheme to run on MCUs (1), (2), (3), (4), (5), (6) (search for LISP or Scheme in those pages). A Scheme byte-code interpreter that runs on PIC MCUs (named BIT and PICBIT by Danny Dube and Marc Feeley of the University of Montreal. The LegoScheme project at Indiana University. XS LISP for the LEGO RCX by Taiichi Yuasa.
• Implementations of Scheme/Lisp on the metal, in assembly or small: The movitz project that targets Lisp "on the metal" (x86). Dream scheme written in PPC/x86 assembly language by David Joseph Stith. A scheme in forth by Mark Probst. TinyScheme by Dimitrios Souflis and Jonathan S. Shapiro (with versions for ARM processors). pocket scheme (for PDAs) by Ben Goetter. Hedgehog embedded LISP by Lars Wirzenius and Kenneth Oksanen. XMOS BIT Scheme, a Scheme interpreter for XMOS chips produced at the VUB in Belgium. A Scheme interpreter written in C for AT-Mega32 microprocessor, developed as ECE 476 Final Project at Cornell University.
• Other interesting functional language systems running on the metal: The Python-on-a-Chip project where the PyMite VM is implemented on microcontrollers. The eLua project where Lua is implemented on microcontrollers. The lispm project where LISP is implemented in an FPGA.
• Multiprocessing Scheme systems: Termite Scheme, a distributed scheme using Erlang-like communication (paper). Kali Scheme, a distributed implementation based on Scheme48, that is undergoing a revival. The Tube, a system for Flexible Distributed Programming using Mobile Code, in scheme.
• Web Sites Discussing Scheme in Action, Research and Education: The website of schemers.org. A bibliography of scheme research. A list of schools that use Scheme in their curricula (from Schemers Inc.).
• Online Books on Scheme: Teach Yourself Scheme in Fixnum Days by Dorai Sitaram. The Scheme Programming Language by R. Kent Dybvig. Structure and Interpretation of Computer Programs by Harold Abelson, Gerald Jay Sussman and Julie Sussman. How to Design Programs by Matthias Felleisen, Robert B. Findler, Matthew Flatt and Shriram Krishnamurthi.
• A R5RS Scheme quick-reference language card (PDF) by Aaron Lahman.

Acknowledgment and Disclaimer

Last updated December 5, 2014