m68k bsp question

[Stan]

hello,

Does anyone can explain why ' format_id = slot << 2' ?

************************************************************
void _CPU_Initialize(
  rtems_cpu_table  *cpu_table,
  void      (*thread_dispatch)      /* ignored on this CPU */
)
{
#if ( M68K_HAS_VBR == 0 )
  /* fill the isr redirect table with the code to place the format/id
     onto the stack */

  unsigned32 slot;

  for (slot = 0; slot < CPU_INTERRUPT_NUMBER_OF_VECTORS; slot++)
  {
    _CPU_ISR_jump_table[slot].move_a7 = M68K_MOVE_A7;
    _CPU_ISR_jump_table[slot].format_id = slot << 2;
    _CPU_ISR_jump_table[slot].jmp = M68K_JMP;
    _CPU_ISR_jump_table[slot].isr_handler = (unsigned32) 0xDEADDEAD;
  }
#endif /* M68K_HAS_VBR */

  _CPU_Table = *cpu_table;
}



************************************************************


thanks a lot.


Stan
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