Question

I’m writing a real-time operating system for a course in school. I’m having a lot of problems getting the initial process’ context switch (the handover from OS to process) so I’ve simplified the problem to try and find out where I’m making a mistake.

In the following code, what I’m trying to accomplish is placing the test_func address on the stack pointer and then returning, thus loading that function’s address into the program counter. What’s happening is that the board is re-invoking the Arduino setup() method, which repeats the whole process.

void test_func(void)
{
    while(1 == 1){
    digitalWrite(13,HIGH);// to let me know everything has worked
    DEBUG("test_func");
    }
} 
//...
volatile void *programcounter;
//...
programcounter = (void *)(&test_func);
asm volatile(
"lds  r26, programcounter      \n\t" \
"lds  r27, programcounter+1    \n\t" \
"ld   r28, x+                  \n\t" \
"out  __SP_L__, r28            \n\t" \
"ld   r29, x+                  \n\t" \
"out  __SP_H__, r29            \n\t" \
"ret");

This was done following the examples from the FreeRTOS project -guide located here (pdf). I’m writing this in the Arduino IDE as a C file. Thank so much for any input you have.

EDIT:

Trying another, simpler example. I’m looking at the Bertos implementation: http://dev.bertos.org/wiki/ArduinoHowto . I’m starting out with a fresh sketch:

    #include <avr/interrupt.h>
    #include <avr/io.h>

    void test_func()
    {
      while(true){
            Serial.write("Testing ...");
            digitalWrite(13, HIGH);
      }
    }

    volatile void (*tempcounter)(void);
    volatile void (**programcounter)(void);

    void setup() {
      // put your setup code here, to run once:
      Serial.begin(9600);
      pinMode(13, OUTPUT);
      digitalWrite(13, LOW);

      tempcounter = (volatile void (*)())(&test_func);
      programcounter = &tempcounter;

      Serial.println("Setting up ...");

      asm volatile (
      "cli\n\t"\
    "lds  r24, programcounter      \n\t" \
    "lds  r25, programcounter+1    \n\t" \
    "movw  r26,r24\n\t"\
    "ld  r18,X+\n\t"\
    "ld  r19,X\n\t"\
    "out  __SP_L__,r18\n\t"\
    "out  __SP_H__,r19\n\t"\
    "sei\n\t"\
    "ret");
      Serial.println("This should never appear");
    }

    void loop() {
      // put your main code here, to run repeatedly: 
      Serial.println("looping");
    }

Answer 1

According to clawson on the AVR Freaks Forum, the issue is a misunderstanding of how the ret works. I’ve got it working now; his implementation looks like:

asm volatile ( 
"cli\n\t"\ 
"lds  r24, programcounter      \n\t" \ 
"lds  r25, programcounter+1    \n\t" \ 
"ldi  r18, 0x00\n\t" \ 
"ldi  r19, 0x03\n\t" \ 
"out  __SP_L__,r18\n\t"\ 
"out  __SP_H__,r19\n\t"\ 
"push r24\n\t"\ 
"push r25\n\t"\ 
"sei\n\t"\ 
"ret\n\t");