Need help to do function pointer in PIC assembler

[glias]

Hello,
I would want to do a function pointer in PIC assembler, does anyone know how to do it ? (I use a PIC18F4620 with MPASM)

thanks

 

[alexxx]

I don't use pic, but I use function pointers.

void function1 (void) //declare two functions: function 1
{
  //function 1 code
}
void function2 (void)  //and 2
{
  //function 2 code
}

void (*fp) (void);  //declare function pointer. The pointed function must 
//be void on both return value and arguments fields, like function1 and function2

void main (void)
{
  uint8 status;
  while(1)
  {
    status = pin_read(P0);
    if (status)  //depending on pin status
    {
      fp = function1;  //fp points to function1
    }
    else
    {
      fp = function2;  //or to function2
    }
    fp();  //run function (function1 or 2, depending on the pin status)
  }
}

Now you could have a function pointer like:

uint16 (*fp) (uint8, uint8); 
uint16 function3 (uint8 x, uint8 y);  
fp = function3; 
uint16 z = fp(219,5);

Hope that helped.

Sorry, I just saw you asked for assembler, anyway I leave the code for someone who will need it on C.

 

[bigdogguru]

What your attempting to implement is a function pointer table or branch table:



The link above shows various examples of a function pointer table.

Another example:

Branch table - Wiki

Hope the links help.

 

[alemaxx]

Since I was facing the same problem, here is my solution:

	; setup function pointer
	movlw	HIGH(func_a)
	movwf	FPTR_H  	  	; high address of function
	movlw	LOW(func_a)
	movwf	FPTR_L  	  	; low addr ...

; ------------------------------------------------------------------------

	; call function pointer

	; 1. push return value onto the call stack
	push	   	; make room on stack
	movlw	HIGH(return_from_function_call)
	movwf	TOSH
	movlw	LOW(return_from_function_call)
	movwf	TOSL
	clrf	TOSU

	; do the call / load function pointer into PC (PCL last, executes PC update)
	clrf		PCLATU
	movf	FPTR_H, W, ACCESS
	movwf	PCLATH
	movf	FPTR_L, W, ACCESS
	movwf	PCL			; call function

return_from_function_call

so since my device has only 16k of flash I only take care of the lower 16bits if your function may be located at >64k you should update the "UPPER" registers to.

Hope this helps

 

[glias]

Hello all,
Thanks a lot for your help, I'm going to take a look at your example alemaxx, it's seem to be exactly the same as I want to do

I hope that I will understand your code and use it in my application context

regards