Hi, All:

I just found in kernel-2.6.35.3, in file arch/x86/kernel/irq.c:

unsigned int __irq_entry do_IRQ(struct pt_regs *regs)
{
struct pt_regs *old_regs = set_irq_regs(regs);

/* high bit used in ret_from_ code */
unsigned vector = ~regs->orig_ax;
unsigned irq;

exit_idle();
irq_enter();

irq = __get_cpu_var(vector_irq)[vector];

if (!handle_irq(irq, regs)) {
ack_APIC_irq();

if (printk_ratelimit())
pr_emerg("%s: %d.%d No irq handler for vector (irq %d)\n",
__func__, smp_processor_id(), vector, irq);
}

irq_exit();

set_irq_regs(old_regs);
return 1;
}

Does any body know the relationship between the vector number and irq number?

If you google "interrupt vector table" you will find full information. The essential point is that the IRQ value is used by the CPU to calculate the memory address of the interrupt vector so that it can perform a jump to an interrupt handler pointed to by the interrupt vector.
HTH and welcome to LQ!