ECE 422 day12

/*
   ECE 422
   Calling assembly functions from C(++)
*/


/*
   Global variables placed in SRAM
*/
volatile char a, b, c, d;
volatile char out;  // holds function return values


#ifdef __cplusplus
extern "C" {
#endif

char asmOne(char);

#ifdef __cplusplus
}
#endif



/*
   The "__attribute__ ((noinline))" business is GCC-specific syntax
   saying to NOT optimize by inlining the code at the point of use.
   We *want* to see the rcall / ret happening!

   https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html
*/
char __attribute__ ((noinline)) fOne(char first) {
  return first + 1;
}


char __attribute__ ((noinline)) fTwo(char first, char second) {
  return first + second + 1;
}


char __attribute__ ((noinline)) fThree(char first, char second, char third) {
  return first + second + third + 1;
}


char __attribute__ ((noinline)) fFour(char first, char second, char third, char fourth) {
  return first + second + third + fourth + 1;
}




void setup() {
  pinMode(LED_BUILTIN, OUTPUT);

  a = 2;
  b = 4;
  c = 16;
  d = 64;
}

// the loop function runs over and over again forever
void loop() {
  digitalWrite(LED_BUILTIN, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(1000);                       // wait for a second
  digitalWrite(LED_BUILTIN, LOW);    // turn the LED off by making the voltage LOW
  delay(1000);                       // wait for a second

  // call function with one 8b argument
  out = fOne(a);

  // assembly version with one 8b argument
  out = asmOne(a);

  // call function with two 8b arguments
  out = fTwo(a, b);

  //

  // call function with three 8b arguments
  out = fThree(a, b, c);

  // call function with four 8b arguments
  out = fFour(a, b, c, d);
}

/*
 * Pre-defined symbols for our platform
 */
#include <avr/io.h>


/*
 * Place this in the section for program code
 */
.section .text


/*
 * make label global so that the symbol/label is visible outside of this file
 */
.global asmOne
asmOne:
  subi r24, 0xff
  ret

int foo = 1;