[jimtcl] paradigm shift to run scripts from ROM (flash)

[Steve Bennett]

> On 1 Feb 2021, at 5:57 am, Mark H <themarkitecht at gmail.com> wrote:
> 
> Hey guys,
> 
> I have a small project with Jim on an STM32F4 microcontroller.  It has about 104 KB RAM available to the app.  That actually works pretty well, and can navigate its UI on a 20x4 character LCD.  I expect to use Jim on other MCU projects as well.
> 
> Predictably, it's starting to have serious RAM-cram.  So I've been looking at ways to run scripts directly from where they reside in flash (effectively ROM).  That way the entire application script (plus Jim stdlib) doesn't get copied to RAM before it even begins interpreting.  And then each proc body copied to more RAM etc. 
> 
> So, some open questions:
> 
> 1)  Has anybody else tried something similar?  How did you approach it and how well did it work?
> 
> 2)  I've tried a few approaches so far.  Each of those has "almost" run correctly.  The basic problem is that Jim's (0.79) hashtables and certain other functions rely on null-terminated strings.  They discard or ignore the (in most cases) known length of the string.  But those nulls are not present when (for example) command or variable name tokens point to ROM instead of RAM, because I've shut off the strdup that would have made the RAM copy.  The original script in ROM has the remainder of the script where the interp is expecting the null to be.  I've tracked down and fixed those issues in a few important functions, but certainly not all.  I've gotten a few lines of script to run that way, so it probably can work for the rest.  It looks like it might be a long road to complete that approach, and would likely introduce bugs wherever i've missed a spot.  I have no obvious way to run Jim's test suite on such a small host, and no obvious way to make scripts read-only on a PC to run the tests there instead.  So...
> 
> 3) ...Instead, I'm starting to seriously consider a comprehensive solution.  Such as: throughout jim.c, replace all char* function parameters with a new structure instead, such as "Jim_Text", which carries both the char* and the known length.  That would mean basically a wholesale paradigm shift from C-style strings to known-length strings.  You might say Pascal-style strings, but I wouldn't store the length at the front of the string data like Pascal does.  That makes it too easy to overlook a piece of code that needed the rework, introducing insidious bugs.  The new design should pass all existing test suites verbatim.  And it should be able to do so on a PC, where the tests are available, proving that it's likely to also work on the MCU, where tests aren't available.
> 
> 4) Is anybody else interested in seeing that approach adopted?  There might be other benefits besides running scripts from ROM.  One benefit might be a bit faster execution, since most strlen() calls or other null-byte searches are eliminated.  Currently those are happening every time a hash key is computed, and in other hotspots.
> 
> 5)  I heard Steve mentioning recent improvements to make Jim insensitive to nulls embedded in data.  Anybody care to guess how that work interplays with this approach?
> 
> 6)  Would anybody else be interested in helping with the extensive rework?
> 
> 7)  Are there any Tcl TIP's, or Jim open issues, that relate to my goal, or to this paradigm shift either?
> 
> Thanks for any clues!

Hi Mark,

Personally, as much as I like Jim Tcl (obviously), I don't think it's the right tool for your job.
Because it is so dynamic, it is hard to have text execute in place.

I suggesting looking into lua. You can compile scripts down to bytecode that can be stored read-only in  ROM and you only
need to include the bytecode interpreter.

While lua doesn't have the system integration support of Tcl, and no utf-8, that's probably not a huge concern for you on a microcontroller.

PS, there has been some discussion of this topic on comp.lang.tcl recently

Cheers,
Steve