Run arduino sketch from an sd card - arduino

Question

Is it possible to put a sketch (.HEX file) to an SD card and run it from there?
My objective is to utilize SD storage instead of flash memory for a program.
If yes, are there any libraries doing exactly this?
All i found was "flashing arduino from sd card", which is not what i need.
UPDATE:
the sketch's loop calling is implemented in the bootloader.
so i assume there is something like this in the bootloader:
while(true)
{
call_sketch_loop();
}
can it be changed to this? :
//signature changed from void loop() to int loop()
while(true)
{
int retval = call_sketch_loop(); //get loop call's return value
if( 0 == retval )
continue; // if 0, iterate the loop as usual
else
{
//copy 1.HEX from sd to flash and reboot
copy_hex_from_sd_to_flash( retval + ".HEX" );
reboot();
}
}
change loop singature to int loop()
put {int}.HEX files to an SD card - 1.HEX , 2.HEX , 3.HEX
the loop() call returns 0
continue with next iteration as usual
the loop() call returns 2
copy file 2.HEX from SD card into program flash memory
reboot device
with this approach, we can run flash-capacity-exceeding programs if we split them up to smaller subprograms.

Answer 1

The technical term you are looking for is "SD card bootloader".
Have you looked into this: "https://github.com/thseiler/embedded/tree/master/avr/2boots"?
As far as I understand, 2boot will first load the hex into the flash and then execute it from there. This is not exactly what you are looking for (you want to load it directly to RAM, right?).
The problem with what you are looking for is that arduino's RAM is really small. And there is liittle advantage in loading directly to RAM. Therfore such library might not exist at all.
I can sugget a poor-mans approach for doing this. First write a sketch that contains a function that have an infinite loop inside it and inside this loop, put the code of your desired "loop". In the setup of the sketch take the pointer to this function and write sufficient ammount of bytes into a binary file on the SD card.
Then upload another sketch wich has an empty buffer. This sketch will load the binary file into it and refernce to it's beginning as a pointer to a function. Viola, you can now execute your "loop".
This is ugly and unless you have very specific and isoteric need for loading directly into RAM, I suggest to try the 2boot library.