DLL support

This is an idea I had and wanted to test with a game, but my hands are full with ProjectABE and the Bitsy port so I’ll document it here, in case somebody else wants to give this a shot.

According to the M0+ TRM, the following regions of the address map are marked as executable:

0x60000000 - 0x9FFFFFFF  // nothing?
0x20000000 - 0x3FFFFFFF  // 2kb at SRAM1 + 2kb at USB SRAM
0x00000000 - 0x1FFFFFFF  // flash + 32kb at SRAM0

In theory, one can simply write thumb2 code into SRAM0 and call it with a function pointer. The problem is that SRAM0 is where the heap, framebuffer and the stack are, so there’s not much space for code there.

But what about SRAM1 and USB SRAM? As far as I can tell, they are both disabled by the PokittoLib in begin(), but can be enabled easily enough.

What if we were to compile small modules for things like AI and swap them as-needed into SRAM1 and USB SRAM? By patching the mbed linker script, I think it’d be possible to make GCC emit code that would run in those regions, and it could share the main program’s stack and heap. To avoid code duplication, we’d pass the module a struct with function pointers to things provided by the game’s engine and malloc/free/etc.

For an RPG, the main engine would go in flash and AI for each NPC could go in a separate DLL in SRAM1. NPC states could be stored in USB SRAM. That leaves all of SRAM0 to the game engine. If necessary, this can be broken down further: each state of a particular NPC can be put into a separate DLL, allowing for indefinite amounts of NPCs with arbitrarily complex behaviors. The engine would have support functions (like, say, A*) and the NPC’s AI would only need to be loaded/executed when a decision needs to be made (done walking, what now?) On average, swapping would occur less than once per frame, even with multiple NPCs on-screen.

Compared to having an interpreter, this approach has a few advantages: loading up a DLL is simply a matter of reading a file to memory, and module performance might actually be faster than the code running in flash (not sure, I vaguely recollect an article saying that executing from flash is slower than SRAM).


Intesting idea! Did you receive your Pokitro yer?

AFAIK i remember reading that running code from RAM is not that much faster / not at all faster on ARM Cortex MCUs


No, not yet… the Brazilian mail system is infamously slow on its good days, and they were on strike recently. -_-

When it arrives I’ll try to implement this, unless somebody beats me to it or can spot some reason why it wouldn’t work.

1 Like

I’ve been dealing with USPS lately. You would not believe the amount of packages I have had to reship.

It’s not so good in Finland either, the system is breaking under the weight of China mail. Another enterpreneur joked two days ago that the best way to get rid of a body is to send it by post…


Did you happen to get a tracking number? It’s good to keep an eye on them. Once I had to run to the post office because the mailman was too lazy to actually deliver a package and marked it for return. :rage:

Oh well, in the worst case, it’s going to be just as fast as normal code… still much smaller/faster than having a VM running bytecode.

1 Like