ANSWERS: 1
  • IRQ The old XT BIOS provided for eight IRQ lines (0-7). The AT BIOS Adds Eight more IRQ's (8-15). Instructions for controllers designed to work on an AT motherboard usually are located at IRQ 9. However these Instructions are redirected (or cascaded) to IRQ 2 just as if it were on an XT motherboard to prevent conflicts on IRQ 9. Just remember though. If IRQ 2 is being used by the BIOS instructions that IRQ 9 is also being used simultaneously. A lot of times IRQ 9 will be used for video. Except in the case of Diamond Video Cards (IE. Viper II) which like to use IRQ 11, and thus cause coonflicts with a lot of NICS and Modems that also like to use IRQ 11. It looks like IRQ 9 was just a random pick. They actually could have used any IRQ between 8 and 15 though. They just decided to used 9 for some reason. I hope this helps you in your quest for the http://en.wikipedia.org/wiki/Interrupt_request Default ISA IRQ Assignments IRQ 0 System Timer IRQ 1 Keyboard IRQ 2 Cascaded with IRQ 9 IRQ 3 Default COM2 and COM4 IRQ 4 Default COM1 and COM3 IRQ 5 LPT2 IRQ 6 Floppy Drive Controller IRQ 7 LPT1 IRQ 8 Real Time Clock IRQ 9 SEE 2 IRQ 10 Open IRQ 11 Open IRQ 12 PS/2 mouse or Open IRQ 13 Math Coprocessor IRQ 14 Primary Hard Drive controller IRQ 15 Secondary Hard Drive controller http://www.helpwithpcs.com/upgrading/change-irq-settings.htm IRQ 2 - Cascaded signals from IRQs 8-15. A device configured to use IRQ 2 will actually be using IRQ 9 IRQ 9 - Free / Open interrupt / Available / SCSI. Any devices configured to use IRQ 2 will actually be using IRQ 9.

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