CCSDS in Forth

This repository contains a basic implementation of the CCSDS Space Packet Protocol Primary Header. This is a simple packet header with basic information, and the code simply provides access to the fields according to the standard.

I like to see how this header looks in different programming languages, so this is my writeup of the primary header in Forth. This is a test of the language's ability to deal with binary data, to handle endianness, and to manipulate bit field definitions, which are common concerns for embedded systems programmers.

Notes

This implementation is extremely basic. It uses bit shifts and masks to get and set each field within the header. This is what I am actually interested in doing with a CCSDS header, so this code gets right to the point.

It is interesting to note that there is no structure definitions anywhere- you could very well define structures, and then accessors for the bits in their fields. This would make some of the definitions slightly simpler, but I don't think it would be by very much.

The nice thing about Forth is that I don't feel as pressured to wrap things up as nicely- there are other options for this kind of library which create abstractions or use generic bit field manipulation words to extract or lay down sections of bits, but I don't feel like it would be Forthy to do this.

Compared to other implementations, such as my Zig and Rust ones, my Forth code is smaller, denser, and with much more code per line. I like the Zig version the best, but the Forth does have a certain charm.

Conventions

I used a convention where a ">" prefix indicates that a value is being loaded to somewhere, in this case a field within the header, whlie a suffix of ">" indicates that a value is loaded from somewhere.

This leads to definitions such as ">apid" or "length>", which are possible because of Forth's unrestricted identifier names.

You would use these as in the following code which allocates a header and sets the APID and sequence flag, resulting in a valid packet with 1 data byte:

1 constant APID
3 constant UNSEGMENTED
PRILENGTH 1+ allot constant packet
packet PRILENGTH 1+ erase
APID packet >apid
UNSEGMENTED packet >seqflag

Utility Definitions

There are a handleful of utilities defined in ccsds.fs. Forth is a very free-form language, so it is both easy and natural to extend it to match your problem.

In this case I added a few definitions to increment characters while stepping through memory with c@++ and c!++, load and store words (16 bit integers) with w! and w@, and to turn a word count into a byte count like words.

The last utility is 'masked'. This is a somewhat complex word which takes a mask, a value and a new value, and ORs the two values in after clearing the bits in the given mask. The definition of 'masked' is particularly neat in my opinion: "invert and or" is the C equivalent of "a | (b & ~c)"

Tests

I liked being able to sprinkle asserts into the code, as with Zig and Rust, although in this case I'm not using any kind of test framework, not even a build in one, just asserts as test cases. This nice thing here is that unless you ask for them to be turned off, you must pass the test suite in order to even use the code.

I moved the tests to a separate file to make the SLOC count easier (27 lines), but there is no real reason to extract these- the code can just test itself every time it is loaded- Forth compiles very quickly.