When the IVSEL bit is cleared (zero), the Interrupt Vectors are placed at the start of the Flash memory. When this bit is set (one), the Interrupt Vectors are moved to the beginning of the Boot Loader section of the Flash. The actual address of the start of the boot Flash section is determined by the BOOTSZ Fuses. Refer to the section Boot Loader Support – Read-While-Write Self-Programming for details. To avoid unintentional changes of Interrupt Vector tables, a special write procedure must be followed to change the IVSEL bit:

1. 1.Write the Interrupt Vector Change Enable (IVCE) bit to one.
2. 2.Within four cycles, write the desired value to IVSEL while writing a zero to IVCE.

Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled in the cycle IVCE is set, and they remain disabled until after the instruction following the write to IVSEL. If IVSEL is not written, interrupts remain disabled for four cycles. The I-bit in the Status Register is unaffected by the automatic disabling.

The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is cleared by hardware four cycles after it is written or when IVSEL is written. Setting the IVCE bit will disable interrupts, as explained in the IVSEL description above. See Code Example below.

Move_interrupts: 
   ; Enable change of Interrupt Vectors 
   ldi r16, (1<<IVCE) 
   out GICR, r16 
   ; Move interrupts to boot Flash section 
   ldi r16, (1<<IVSEL) 
   out GICR, r16 
   ret

void Move_interrupts(void)
{ 
   /* Enable change of Interrupt Vectors */ 
   GICR = (1<<IVCE); 
   /* Move interrupts to boot Flash section */ 
   GICR = (1<<IVSEL);
}