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8259 PIC: Difference between revisions

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m ICW1_INIT and ICW1_ICW4 are actually flags, so OR them not ADD
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The IBM PC/AT extended the PC architecture by adding a second 8259 PIC chip. This was possible due to the 8259A's ability to cascade interrupts, that is, have them flow through one chip and into another. This gives a total of 15 interrupts. Why 15 and not 16? That's because when you cascade chips, the PIC needs to use one of the interrupt lines to signal the other chip.
 
Thus, in an AT, IRQ line 2 is used to signal the second chip... ButBecause toof confuse things morethis, IRQ 92 is redirectednot available for use by hardware devices, which got wired to IRQ 29 on the slave PIC instead. SoThe whenreal youmode getBIOS used to set up an interrupt handler for IRQ 9, that redirects to the signalIRQ is2 redirectedhandler. toThis way DOS drivers who used IRQ 2 continued to work. This two-chip architecture is still used and available in modern systems, and hasn't changed (except for the advent of the above-mentioned APIC architecture).
 
==How does the 8259 PIC chip work?==
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