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TACKtech Corp. > Articles > Hardware > Miscellaneous

What are IRQ's (TTID #8)

Author: Kevin   Views: 15,331 /  Created: January 1, 2001
I have a Voodoo3 3000 AGP, SoundBlaster Live!, Adaptec 29160, Intel Pro100 NIC, and a Zoom 56K external modem. Let me tell you I had a heck of a time freeing up enough IRQ's on my system to get all this stuff in. The problem is single-processor mainboards only have 16 IRQs. Yes, only sixteen. This can be very irritating to power users who add all sorts of internal gadgets to their PC's. The second interupt controller rides piggy-back on the first controller. IRQ 2 and 9 act like a bridge between them. My main board has 1 AGP slot, 6 PCI slots and 2 ISA slots. I'm only using 1 AGP, and 3 PCI's and I am currently out of IRQ's. What's the stupid point of putting so many slots onto a main board when you can only use 4 cards on it.

Typical IRQ Layout.
IRQ# Primary Uses Related Uses
00 System Timer  
01 Keyboard  
02 I/O channel (Cascade Bridge)  
03 COM2/COM4(serial port) Modem
04 COM1/COM3 (serial port)  
05 Open Sound card, LPT2
06 diskette controller  
07 LPT1 (Parallel Port 1) Printer, ext. Zip Drive
08 Real-Time Clock  
09 Software Redirected to IRQ2 (Bridge)  
10 Open Network Card
11 Open Video Adapter
12 PS/2 Mouse  
13 Math Coprocessor/FPU  
14 Primary IDE controller  
15 Secondary IDE Controller  

The IBM based PC's use interrupt-driven processors. Several of the interrupts are more or less user-definable, and given special designations as "IRQ" numbers. This is an abbreviation for (I)nterrupt (R)e(Q)uest. The original PC/XT had only IRQ0-IRQ7; 286's and beyond have IRQ0-IRQ15. Even the Intel Pentium III and AMD Athlon's still has only IRQ0-IRQ15. Devices can share IRQs but only when they are NOT active at the same time. Microsoft Windows 98 introduced the ablity to share IRQ's. This does slow your system down. Dual mainboards have more IRQ's but not 32 like you may hope. Now don't go run out for a dual processor mainboard unless you have an operating system which supports then such as Microsoft Windows NT.

There are some things you can do to free up some IRQ's.

If your modem is internal and you have a PS/2 mouse, maybe you don't need either of your external serial ports. disable them in the BIOS, and you should end up with an extra IRQ or two. (Note: when you disable com ports on some computers with Pheonix Bios's they have a tendency still allow Windows to detect them.  If you don't have a parallel printer, or any other device, connected to LPT1 then disable it as well.

Also, I notice that you have a SBLive!, it takes 2 IRQ's if you use DOS emulation. Allowing the DOS emulation to share with your LPT port will free this one up.

Other Resources:
Microsoft Knowledge Base Article - 182604

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