Atmega328p bootloader shared function

Projects Programming
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My goal to place the "shared function" in boot sector which will be accessible for run from boot application and also from main application. Microcontroller atmega328p.

code of boot app: boot.c boot.c · GitHub

logic of boot application:

  1. If button is pressed then blink of the LED.
  2. goto main app

The shared function has name "blink_twice"

compiling:

avr-gcc -g -DF_CPU=16000000UL -Os -Wall -std=gnu99 -mmcu=atmega328p -v -c boot.c -o boot.o
avr-gcc -g -DF_CPU=16000000UL -Os -Wall -std=gnu99 -mmcu=atmega328p -v -Wl,-Ttext=0x7800 -o boot.elf boot.o

convert to asm

avr-objdump -D boot.elf > boot.asm

get for function address

grep '<blink_twice>:' boot.asm

In my case it is:
00007880 <blink_twice>:

From boot app and main app the "shared function" I can use that way:

#define shared_function_addr 0x7880 // blink_twice address
#define shared_function (*((void(*)(void))(shared_function_addr/2)))
shared_function();

The issue with this approach:

  • It is hard to guarantee that the address 0x7880 will not change on another compiler version.
  • The address 0x7880 can be changed if the boot app will have a changes.
    As consequences of that it will be required to change the address in the main app. And that I want to avoid.

One of the idea was to store "shared function" address in the constant address (0x7FFE) in flash memory.

  1. convert address to word 7880/2 -> 3c40
  2. change to big-endian 3c40 -> 403c
  3. build a "hex" line and calculate a control summ :02 7FFE 00 403c 05
  4. add the "hex" line into the hex file
    The result is here: boot.hex · GitHub

Now in the main app runtime use the pgmspace it is possible read address of "shared function" from address 0x7FFE.

But I feel the solution is a monkeypatch.
And now I do not see another way how to solve it.

I feel the solution belong to linker area but I do not have enough experience for that.
I will be really grateful for any help.

** PS
Also I tried to store "shared function" address with line:

void* const shared_function_address __attribute__((__progmem__)) = blink_twice;

It is added the address in the "hex" file but I didn't found the way how to put it on specific address in flash memory.

** PPS
Also I tried to store "shared function" address with below line:

void* const shared_function_address __attribute__((section("fnaddr"))) = blink_twice;

for compiling I used commands:

avr-gcc -g -DF_CPU=16000000UL -Os -Wall -std=gnu99 -mmcu=atmega328p -v -c boot.c -o boot.o
avr-gcc -g -DF_CPU=16000000UL -Os -Wall -std=gnu99 -mmcu=atmega328p -v -Wl,-Ttext=0x7800 -Wl,--section-start=fnaddr=0x7FFE -o boot.elf boot.o

After make asm
avr-objdump -D boot.elf > boot.asm

In boot.asm file I can see lines:

Disassembly of section fnaddr:
00007ffe <shared_function_address>:
    7ffe:	40 3c       	cpi	r20, 0xC0	; 192

But after build in "hex" file I can't find data "403c" in address "0x7ffe"

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