IRQ 2 & 9

Dubb Rich

I know IRQ 2 & 9 are cascaded to enable the use of 15 IRQ's, but can 2 & 9 be configured for use with another device, such as a video card?

Zumpel

had this link lying around:
http://www.faqs.org/faqs/windows/win95/faq/part04/section-6.html

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* 4.7.1. Help with devices that use IRQ 2 or IRQ 9

Quick background... The first PC compatibles (XTs and 8088s) made
interrupt lines 2 to 7 available for ISA cards. IRQ 2 was marked as
"reserved" but was still available for developers. This was handy to
have because if you had a floppy drive (IRQ 6), two serial ports (IRQs
3 and 4), and two printer ports (IRQs 5 and 7), you were kinda stuck
with IRQ 2. MIDI devices are the most common devices that used IRQ 2.

ATs and better added a second interrupt controller (The interrupt
controller you see in Device Manager is really two interrupt
controllers cascaded) and the second controller used IRQ 2 to indicate
an interrupt occured on a line from IRQs 8 to 15. (Remember that IBM
"reserved" IRQ 2? Now you know why.) To maintain compatibilty with
devices that used IRQ 2, ATs wired IRQ 9 in place of IRQ 2 on the bus.
Whenever you install an 8-bit card that allows you to use IRQ 2,
you're really using IRQ 9. This wasn't enough because those MIDI
programs wouldn't understand what IRQ 9 was. Every incarnation of DOS,
from 2.0 up to 6.22, would cascade IRQ 9 events to the IRQ 2 handler
so these old programs would work. Guess what? Win95 no longer does
this.

To use devices that allow IRQ 2, set that driver's setting to use IRQ
9 instead. The MPU-401 MIDI driver defaults to using IRQ 9, for
example. If you add an 8-bit internal modem to a system that has two
serial ports and a sound card, you should use IRQ 9 to avoid conflicts
with the other ports. "Basic Configuratrion 5" for a serial port lets
you select IRQ 9. Don't even try to use DOS software that attempts to
use "IRQ 2" because it simply won't work.

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