Question about Computers & Internet
IRQ, DMA, and other Computer Creatures
IRQ's (Interrupt Request) LinesIRQ's are hotlines to the main computer (CPU) that allow devices connected to the computer to signal the CPU that they need immediate attention. If you're a Batman fan, think of IRQ's as the Batphone - getting his attention immediately.
Not all devices require IRQ lines, which is good news because in modern (post IBM XT) computers, we only have 16 of them. Of those, 3 are already dedicated to the main system board itself - the system timer, keyboard, and memory parity error signal. That leaves only 13 for all the other devices connected to your computer. This is why IRQ conflicts are probably the #1 problem faced by computer users when they add hardware to their computer.
Its a general rule for ISA-type systems (the standard computer architecture used in most IBM compatible systems) that IRQ lines CANNOT by shared with multiple devices except under special circumstances. For this reason, a good understanding of what IRQ's are assigned to what devices is essential in avoiding conflicts. The table below is a general outline for standard IRQ assignments.IRQ DEVICE USED in AT, 386, 486, and Pentium Computers
0 System Timer
1 Keyboard Controller
2 Tied to IRQs 8-15
3 COM 2
4 COM 1
5 LPT2 or Sound Card
6 Floppy Diskette Controller
7 LPT 1
8 Real Time Clock
9 Substitutes for IRQ 2
10 Not Assigned
11 Not Assigned
12 PS/2 Mouse Port
13 NPU (Numerical Processing Unit)
14 Primary Hard Disk Controller
15 Secondary Hard Disk Controller
Depending on the computer's configuration, add-in devices such as SCSI controllers, sound cards, modems, cd-roms, etc. will want an IRQ line that is already used by another device, and thus we have what is commonly referred to as an IRQ Conflict.
The most common IRQ conflicts seem to be between two COM ports, generally a mouse and modem conflict that ends up freezing the mouse whenever the modem is activated. The table below explains each IRQ and the most common devices each may use.
Posted on Oct 18, 2008
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Posted on Jan 02, 2017
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