Hardware Reference
In-Depth Information
onlywaytoresolvetheinterrupt-shortageproblemistoremovetheadapterboardthatyou
need the least.
16-Bit ISA, EISA, and MCA Bus Interrupts
The introduction of the AT, based on the 286 processor, was accompanied by an increase
inthenumberofexternalhardwareinterruptsthebuswouldsupport.Thenumberofinter-
ruptswasdoubledto16byusingtwoIntel8259interruptcontrollers,pipingtheinterrupts
generated bythesecondonethroughtheunusedIRQ2inthefirstcontroller.Thisarrange-
ment effectively makes only 15 IRQ assignments available, and IRQ2 effectively became
inaccessible.
By routing all the interrupts from the second IRQ controller through IRQ2 on the first, all
these new interrupts are assigned a nested priority level between IRQ1 and IRQ3. Thus,
IRQ15 ends up having a higher priority than IRQ3.
Figure 4.44
shows how the two 8259
chips were wired to create the cascade through IRQ2 on the first chip.
Figure 4.44
Interrupt controller cascade wiring.
To prevent problems with boards set to use IRQ2, the AT system designers routed one
of the new interrupts (IRQ9) to fill the slot position left open after removing IRQ2. This
means that anycard youinstall inamodern system that claims touseIRQ2isreally using
IRQ9 instead.
Table 4.58
shows the typical uses for interrupts in the 16-bit ISA and 32-bit PCI/AGP
busesandliststheminpriorityorderfromhighesttolowest.TheobsoleteEISAandMCA
buses used a similar IRQ map.
Table 4.58 16/32-Bit ISA/PCI/AGP Default Interrupt Assignments
