Thursday, November 12, 2015

A Minimal Text Interpreter - Part 2

A Text Interpreter to run on a Resource Limited Microcontroller  - Part 2

In the previous post, I described the basics of a tiny text interpreter, written in C, intended for use on resource limited microcontrollers. The text interpreter would offer a natural language user interface, allowing programming and command line control of various microcontroller projects.

It will also form the basis of a wider range of self-written computing tools, including assembler and compiler, editor and file handler - all of which could be hosted, if necessary on a resource limited target board.

However, for the moment, and for ease of experimentation, the intention was to get the interpreter to run with only 2K of RAM (as per the Arduino Uno).

I envisioned the text interpreter as being a universal resident utility programme (almost akin to a bootloader) that would be initially flashed onto the microcontroller thus allowing a serial command interface and the means to exercise the hardware or develop small interactive programmes.

At work and at home, there are many instances of when I want some degree of interactive control over a small microcontroller project - even if it is just manipulating some port lines or sending and receiving a few serial responses on a terminal programme.

Some Practical Limitations

In order to keep the demands on the interpreter program reasonable it is necessary to put some limits on its capabilities.  In particular, the number of words it can recognise and create jump addresses for. For convenience I used a look up table to hold the jump addresses.  If the look up table is to remain reasonably compact - then a limit of 256 entries seems reasonable.  Restricting the word capacity will also help keep the dictionary and its headers to a manageable size in RAM. This is important when you only have 2K to play with!

As the 4 byte header is in fact a shortform, or compact coding convenience that represents the dictionary, it could be said that in very RAM limited systems that it is not actually a requirement to keep the dictionary in RAM on chip.  The only role that the dictionary performs is to allow the header entries to be expanded to the full word at times of listing.

As small micros generally have plenty Flash available, then the dictionary for all the standard words could be programmed into flash - as indeed could their headers.  If necessary, a shell hosted by a PC application could be used to host the various dictionaries and source code files needed for particular applications. However, the original aim is that this interpreter vastly increases the user-friendliness of the microcontroller target - even with just a serial terminal as the user interface.

Additionally, I have imposed a word length limit to 16 characters.  Imposing this limit means that the word length can be coded as a single hexadecimal digit - which makes it displayable in ascii and human readable. If you can't name something uniquely in 16 characters then you are probably of German extraction.


Different tasks need different tools, and as the interpreter will be used for a variety of tasks, then it seems reasonable that it can be augmented or tailored towards a particular task. This can be done very conveniently with the use of vocabularies - with a particular vocab being used for a particular task.  A vocab that contains the mnemonics of a particular processor's instruction set would be one sensible use when using the interpreter within an assembler, compiler or disassembler.  


Those of you that are familiar with Forth, will say that I am just creating a portable Forth-like environment, but rather than being coded in the native machine language of the target processor, it has been written in C for convenience and portability.

This is indeed partly true, as the utility I am creating has been inspired by the Forth language - especially in its compactness and low resource requirements.  Even in the 1960s Charles Moore was concerned how the tools provided for computing at that time hampered progress, and so set about redefining the whole man-machine interface. He compressed the previously separate editor, compiler, and interpreter programmes (none of which could be co-resident in memory at the same time) into a single compact, convenient package that did the job of all three.

When Forth was first introduced in the late 1960s, mini computers had sub-MHz clock frequencies, and very little RAM, and so benefited greatly from a moderately fast and compact language like Forth. Nowadays, typical microcontrollers have clock frequencies in the 20MHz to 200MHz range and so are not so hindered by what is essentially a virtual machine implementation of a stack processor written in C.

Virtual and Real Stack Machines

I have embarked on this journey because of my wider interest in soft-core and open core processors implemented on FPGAs. Several of these cores are based on stack machines, partly because they may be readily implemented in surprisingly few lines of VHDL or Verilog. Indeed James Bowman's J1 Forth processor is fully described in fewer than 200 lines of verilog.

Whilst a virtual stack machine might not be the easiest fit for a register based processor without performance penalties, it is a wonderful fit for a real stack machine.  A number of open-core processors including the ZPUino and James Bowman's J1 are true stack machines.  Here the instruction set of the virtual machine have a near one to one direct mapping to the the machine instructions of the stack processor.  In this case the text interpreter can be rewritten in the native assembly language of these cpus, to benefit from the vast increase in speed of running without an additional layer of virtual machine.

In order to do this an Assembler will be required  that is tailored to the instruction set of the Forth Processor, and this is one of the first tasks that the text interpreter will be used for - the assembly of machine code for a custom processor.

One of the reasons why I am concerning myself with such low level primitive tools, is the need to understand them from the ground up so that they can be implemented on a variety of non-conventional processors.

Whilst the ZPUino will execute Arduino code directly (albeit very inefficiently  - because of the C to stack machine programming conflicts), the J1 will need the tools to write it's own language from the ground up - and if you already have the mechanisms of a language in place, plus an easily customisable assembler, then it makes the job a lot easier.

In a later post, I will give an update on the text interpreter and it's application to custom code assemblers.

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