Trying to figure out where how to user interrupts safely on a a MKRZERO

isr_test.ino

#include <SD.h>

// START https://mcuoneclipse.com/2014/01/26/entercritical-and-exitcritical-why-things-are-failing-badly/
#define CpuCriticalVar()  uint8_t cpuSR
 
#define CpuEnterCritical()              \
  do {                                  \
    asm (                               \
    "MRS   R0, PRIMASK\n\t"             \
    "CPSID I\n\t"                       \
    "STRB R0, %[output]"                \
    : [output] "=m" (cpuSR) :: "r0");   \
  } while(0)
 
#define CpuExitCritical()               \
  do{                                   \
    asm (                               \
    "ldrb r0, %[input]\n\t"             \
    "msr PRIMASK,r0;\n\t"               \
    ::[input] "m" (cpuSR) : "r0");      \
  } while(0)
 
#if 0 /* original, buggy version */
  #define DEFINE_CRITICAL() /* nothing */
  #define ENTER_CRITICAL()  EnterCritical()
  #define EXIT_CRITICAL()   ExitCritical()
#else /* fixed version using local variable */
  #define DEFINE_CRITICAL() CpuCriticalVar()
  #define ENTER_CRITICAL()  CpuEnterCritical()
  #define EXIT_CRITICAL()   CpuExitCritical()
#endif
// END https://mcuoneclipse.com/2014/01/26/entercritical-and-exitcritical-why-things-are-failing-badly/

volatile unsigned short volatile_ushort;
void an_isr() {
    volatile_ushort++;
}

void setup() { // attach the interrupt to an_isr
    attachInterrupt(digitalPinToInterrupt(6), an_isr, LOW);
}

void loop() {
    DEFINE_CRITICAL();
    static unsigned short local_ushort = 0;

    ENTER_CRITICAL();
    local_ushort = volatile_ushort;
    EXIT_CRITICAL();

    Serial.println(local_ushort);
    delay(500);
}