What is the relation ship between vector and irq number?

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http://www.linuxquestions.org – Hi, All: I just found in kernel-, 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", (HowTos)