PIC32 running a program from RAM

Question

Wondering if anyone has tried running an entire program on a PIC32 out of RAM? How would one go about doing that. I understand that you can use the ramfunc keyword to move certain functions to RAM however they still require the copy to be on Flash.

What I would like to do is have a loader program on the PIC32 that can accept a hex file over the serial port etc and rather than writing it to flash have it run out of ram directly.

Ideally I was hoping that by modifying the default linker script you could target a program to run from RAM alternatively if anyone knows how we would patch up a program that was linked to run from flash to run from RAM that could work too

Thanks Mike

Answer 1

Both the longramfunc and ramfunc attributes place the function in a special area of flash that will be copied to RAM by the startup code. You don't want to use them.

For functions which will be called from flash, you want to use the longcall attribute which specifies that a 32-bit call (JALR) is needed instead of a 28-bit call (JAL), since flash and RAM are located in separate 512MB segments.

You will also need to set up a custom linker file with an executable RAM section, and include that section name on every function that is placed there.

The longramfunc macro also includes the longcall attribute, whereas the ramfunc macro does not, and is intended for calls from one RAM function to another. In your case, for functions called by other RAM functions, you do not want to use the ramfunc attribute because it sets up the copy from flash.

Since you will be copying the functions yourself (e.g. off of an SD card), you will need to include your own loader that interprets the hex code and copies the data to flash as needed.

Answer 2

I don't believe any of the pic32 support executing from external memory. I have brainstormed this question before (see this question) and it is generally a pain. There are some pics that support this, but most that I know of are 8-bit (PIC18F85J5 is one).

Generally if you need more program space, get a bigger microcontroller.

If you want to be able to update the code more frequently than you want to program the device, you can write a sort of bootloader that can copy bytes from an external source into the flash memory.

If you want to load and run a hex file it sounds like you want a bootloader. I could not find a good enough case to go in any other direction. The only real alternative I can think of is writing some kind of interpreter to run a program from ram, but that would probably introduce more overhead than you want.

Answer 3

You have three basic solutions to your problem of wanting to download programs to run after flashing the part.

1. Use an interpreted language like Lua, Basic, or Forth. Write (or, more likely, just port) an existing virtual machine that interprets text instructions rather than machine code, and download the program as text. You have to reimplement your program in the interpreted language, but that's quicker and easier (in Lua especially) than writing it in C.
2. Just get some extra storage and reflash the whole part. You haven't described what you're doing, so I can't know if this is applicable, but it's easy enough to add a serial Flash IC and/or microSD card with plenty of storage to your board and have the bootloader select a profile to run. At a Flash lifetime of 10,000 rewrites, this gives you 27 years of once-per-day reflashes.
3. Do what you originally intended. This will require some careful programming (see tcrosley's answer for some methods required) and some fancy linker scripts. Good luck.

Largely adapted from my answer to 'Compiling code to run from external RAM'; a similar problem.