Hardware Interrupt Priorities--Background and Usage (99357)

SUMMARY

IBM PC-compatible computers (EISA-based or on the AT bus standard) use the equivalent of two Intel-8259 style interrupt controllers. Each 8259 has eight interrupt request (IRQ) inputs and a consolidated interrupt output. The two 8259s are arranged in a master/slave configuration. The interrupt output of the slave 8259 is cascaded through master controller IRQ input 2, leaving a total of 15 interrupts available.

Controller priority works this way: if two simultaneous interrupts come in on different interrupt lines, the interrupt request with the LOWER number has HIGHER priority and is serviced first.

The master/slave configuration changes interrupt priority so that any interrupt coming in on the slave interrupt controller has higher priority then any interrupts on the master interrupt controller that are downstream of IRQ 2.

MORE INFORMATION

The original IBM PC architecture used only one 8259 interrupt controller with eight interrupt inputs--six available on the PC-bus, and the other two used for keyboard and interval timers (on the system board). The AT and EISA architectures added a second 8259, using one of the interrupts for use as the cascade from the slave to the master interrupt controller. To maintain compatibility with old PC cards, the pin that used to be called IRQ 2 on the bus was changed to IRQ 9 and reserved for use by the slave interrupt controller. Older drivers and documentation still refer to this interrupt as IRQ 2. You can use the terms interchangeably, because they refer to the same pin on the ISA/EISA bus.

In AT and EISA style machines, five interrupts are unavailable--IRQ2/9 (for the slave controller) plus four others used for dedicated system purposes:

IRQ 0--for the tick counter
IRQ 1--for the keyboard controller
IRQ 8--as the interrupt for the real time clock
IRQ 13--to signal coprocessor error

Here is a list of the interrupts on the bus showing priority (highest is at the top) and common usage.

   IRQ 9  (IRQ 2)
   IRQ 10
   IRQ 11
   IRQ 12
   IRQ 14     fixed disk     (non-SCSI-style)
   IRQ 15
   IRQ 3          COM2
   IRQ 4          COM1
   IRQ 5          LPT2
   IRQ 6          diskette drives
   IRQ 7          LPT1, sound card