Q99357: Hardware Interrupt Priorities–Background and Usage

Article: Q99357
Product(s): Microsoft LAN Manager
Version(s): 
Operating System(s): 
Keyword(s): 
Last Modified: 30-JUL-2001

SUMMARY
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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
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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

Additional query words: 2.20 2.2

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