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

[Patrick]

On 2021-01-31 2:57 p.m., Mark H 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!
>
> --
> TheMarkitecht
> "I'm always disappointed when a liar's pants don't actually catch on 
> fire."
> https://github.com/TheMarkitecht/slim
>
>
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Hi Mark

I am sure that Steve knows best with this but just for the record, I 
have wanted to do something like this too. What OS are you running? Even 
if the microcontroller has memory, is there anything to stop us from 
using external memory? Perhaps over SPI etc ..

BTW, if you do go down the Lua road, I was part of eLua and I can tell 
you what I learned from this, just PM me.

-Pat




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