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diff --git a/docs/introduction.rst b/docs/introduction.rst new file mode 100644 index 0000000..e4d57be --- /dev/null +++ b/docs/introduction.rst @@ -0,0 +1,69 @@ +Introduction +============ + +Modern `microcontrollers <https://en.wikipedia.org/wiki/Microcontroller>`_ +provide powerful computational platforms in form of +affordable and widely usable integrated circuit packages. Although, execution +performance is often more than enough for execution of complex algorithms, +some of constraints can represent challenge in implementing certain kind of +applications. In case of interpreters for high level languages, amount of +available RAM is significant factor which should be taken into account during +design of interpreter and interpreted applications. This project explores +these and similar impacts by implementing Lisp interpreter capable of +interactive execution on 8bit/32bit microcontrollers. + +`Lisp <https://en.wikipedia.org/wiki/Lisp_(programming_language)>`_ is family +of high level programming languages/dialects characterized with +concise `homoiconic <https://en.wikipedia.org/wiki/Homoiconicity>`_ syntax. +`Scheme <https://en.wikipedia.org/wiki/Scheme_(programming_language)>`_, as one +of Lisp dialects, provides few powerful core functionalities which can be used +to extend language itself with wide variety of derived constructs applicable to +different programming paradigms and domains. Therefor, reduced core of Scheme, +with only few data types and builtin functions/syntaxes, can provide good base +system for execution of high level applications on embedded/constrain +platforms. + +One of characteristics of most Lisp implementations is support for interactive +programming based on +`REPL <https://en.wikipedia.org/wiki/Read%E2%80%93eval%E2%80%93print_loop>`_. +If we take into account that microcontrollers usually provide wide range of +different I/O peripherals, REPL executing instructions on microcontroller can +represent exploratory environment for testing interaction with low level +peripheries not usually available on more powerful general purpose computing +platforms. Although, this kind of environment can be provided by splitting +functionality between microcontroller and more powerful general purpose +computer which communicates with microcontroller, that approach relies on +availability of general purpose computer. By executing interpreter as a whole +on microcontroller, more self sufficient interactive platform is available in +form of a single microcontroller. Interaction with interpreter running on +microcontroller can be based on full duplex +`UART <https://en.wikipedia.org/wiki/Universal_asynchronous_receiver-transmitter>`_ +communication. In this way, wide range of simple terminals can be used (even +those utilizing other microcontrollers). + +Because implementation of Lisp interpreter in C programming language doesn't +require a lot of dependencies specific to single target platform, +implementation can be based on thin abstraction layer that will provide +necessary interaction with host platform. This enables running implementation +based on same source code on different platforms, including more powerful +general purpose computers. Implementation targeting POSIX systems enables +easier testing and provides more developer friendly environment for exploring +interpreter characteristics. Usability of single code base for different +execution environments impacts some of API design decisions. Most notably, +interpreter uses memory locations referenced by additional pointer indirection +instead of statically allocated locations. This approach induces some penalties +in form of execution speed but at the same time provides API more suitable +for interaction with +`foreign function interface <https://en.wikipedia.org/wiki/Foreign_function_interface>`_ +and usage of multiple independent interpreter instances as part of single POSIX +process. + +Existence of this project is mostly motivated with educational and research +reasons. Therefore, significant emphasis is given on this documentation as +integrated part of project. Rest of documentation tries to explain and document +implementation in gradual bottom-up approach. It is advised to read this +documentation sequential in order because each chapter depends on explanations +available in previous chapters. Source code is organized to closely follow +documentation structure and its listing is available as part of each associated +chapter. Reader's knowledge of C programming language and related +platform/memory model is assumed. |