Can't access variable in IRQ Handler

Question

Hi,

I'm getting familiar with my nRF52 DK (PCA 10040).

I stumbled on something when I was doing an application that toggles LED1 on every TIMER0 event (200ms). Here is my code

#include "stdint.h"
#include "nrf52.h"

//volatile uint32_t varA;	<POS A>

void initSystem (void) {	
    NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
    NRF_CLOCK->TASKS_HFCLKSTART = 1;	
    while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0) { }

    NRF_TIMER0->MODE		= 0;
    NRF_TIMER0->BITMODE		= 3;
    NRF_TIMER0->PRESCALER	= 5;
    NRF_TIMER0->CC[0]		= 100000;		// TIMER0_IRQn event each 200ms

    NRF_TIMER0->INTENSET	= (1<<16);

    NRF_TIMER0->TASKS_CLEAR = 1;
    NRF_TIMER0->SHORTS		= 1;
    NRF_TIMER0->TASKS_START = 1;

    NVIC_SetPriority(TIMER0_IRQn, 7);
    NVIC_ClearPendingIRQ(TIMER0_IRQn);
    NVIC_EnableIRQ(TIMER0_IRQn);
}

int main (void) {		
    NRF_P0->DIRSET = (1<<17);
    NRF_P0->OUTSET = (1<<17);

    initSystem();

    while (1) { }
}

void TIMER0_IRQHandler (void) {	
    volatile uint32_t varA;		// <POS B>

    varA = NRF_P0->OUTSET & (1<<17);

    if (varA)
	    NRF_P0->OUTCLR = (1<<17);
    else
	    NRF_P0->OUTSET = (1<<17);	

    NRF_TIMER0->EVENTS_COMPARE[0] = 0;
}

Everything is great. But if I move the declaration of the varA to the top (i.e. in my opinion declaring it as a global variable) it does not work. I don't get a compile or link error. The LED just won't toggle.

Why can't I access global variables inside my IRQ Handler??

NOTE: I'm only using CMSIS: Core and Device startup