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previous: SpeedRacer Interrupts (IRQ) on a 8259 controll... -- 8/25/2003 6:49 AM view thread

Re: IRQ's FYI

8/25/2003 3:47 PM
Mark Lavelle
Re: IRQ's FYI
quote:
"Why they go in this order? Maybe someone older and wiser than I can answer that!"
I don't know about older & wiser, but the answer is historical in nature, so I guess I might be older... ;)  
 
The original IBM PC only had interrupts 0..7. When the AT (or maybe it was even the XT, which was the first to assume the existence of a hard disk) came along with the wider bus & lots of slots, they doubled up on the interrupt logic by hanging another 0..7 (AKA 8..15) off of int 2 on the primary interrupt interface.  
 
I'm not sure how/why 9 is aligned with 2, but 8..15 come between 1 and 3 because they are all 'sub-interrupts' of int 2 (in a sense).

 
Replies:
sr thanks! that makes perfect sense.. ... -- 9/4/2003 3:25 AM