CASCADE_IRQ
outb(INT_CTL1, (1 << CASCADE_IRQ)); /* ICW3 tells slaves */
outb(INT_CTL1, (INT_MASK = ~(1 << CASCADE_IRQ)));
outb(INT2_CTL1, CASCADE_IRQ); /* ICW3 is slave nr */
outb(INT_CTL0, OCW2_CLEAR | CASCADE_IRQ);