8259 PIC: Difference between revisions
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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. |
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. |
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Thus, in an AT, IRQ line 2 is used to signal the second chip |
Thus, in an AT, IRQ line 2 is used to signal the second chip. Because of this, IRQ 2 is not available for use by hardware devices, which got wired to IRQ 9 on the slave PIC instead. The real mode BIOS used to set up an interrupt handler for IRQ 9 that redirects to the IRQ 2 handler. This 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). |
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==How does the 8259 PIC chip work?== |
==How does the 8259 PIC chip work?== |
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