Sim-Ex™ Tutorial for A+ : Hardware

IRQ and I/O addresses

IRQ ( Short for Interrupt ReQuest):

An IRQ allows a device to request certain system resources on priority. Normally, the system allowcates the different non-competing IRQ numbers to different devices. The system allocated IRQs may some times be manually changed. However, it may lead to serious IRQ conflicts resulting in a hung operating system.

The table below gives normally assigned IRQ numbers in Pentium compatible systems

IRQ	Usage	Comments
0	System Timer	This IRQ is used within the system board for system timing.
1	Keyboard Controller	This IRQ is assigned to the keyboard. Not available to any other add-in cards.
2	Tied to IRQs 8-15	This IRQ was assigned to older EGA video cards.
3	COM2	This IRQ is assigned to the serial ports COM 2, and COM 4. Avoid setting this IRQ to other devices.
4	COM1	This IRQ is assigned to the serial ports COM 1, and COM 3. Avoid setting this IRQ to other devices.
5	LPT2 or Sound Card	This IRQ is assigned to a secondary printer port LPT2. If LPT2 is not available, it is used for sound card.
6	Floppy Diskette Controller	This IRQ is assigned to the Floppy Diskette Controller. Since most systems use FDC, do not use this IRQ for other requirements.
7	LPT1	This IRQ is assigned to the parallel port LPT1. It can also be made available to other add-in cards, but avoid using it for any other cards. It may lead to conflicts at a future time, when you least anticipate it.
8	Real Time Clock	This IRQ is reserved for the internal real-time clock. This line is NOT available to other add-on cards.
9	Substitutes for IRQ 2	This IRQ uses IRQ 2 to talk to the CPU. Normally used for network cards.
10	Not Assigned	Can be used for network card, or others
11	Not Assigned	Left open for use with any other device
12	PS/2 Mouse Port	This IRQ is used for the PS/2 style mouse port normally available on many motherboards. If you are using PS/2 Mouse port on system, do not use this IRQ for anything else.
13	Numerical Processing Unit (NPU)	Used for math co-processor, and not available for other devices.
14	Primary Hard Disk Controller	This IRQ is assigned to the primary hard drive interface
15	Secondary Hard Disk Controller	This IRQ is assigned to the secondary hard drive interface.

Memory Mapped Input/Output Addresses:

Input/output addresses ( I/O addresses for short) are resources used by almpst every device in the computer. The I/O addresses represent the location in memory for a given device to exchange information between itself, and the rest of the system. These address spaces are permanently allocated to the system devices. If an address space allocated to one device (say, LPT) is assigned to another device (say, IDE controller) than I/O address conflict will arise, and the system may hang or not function normally.

Given below is the list of commonly used I/O address spaces allocated to various system devices:

I/O Address	Common Device using the Address Space
060h and 064h	Used by keyboard controller
130 and 140	Used by SCSI host adapters
170h	Secondary IDE Interface
1F0h	Primary IDE Interface
220	Default address for Sound cards
240	Optional address for sound cards and network cards
260 and 270	Optional address for sound cards and network cards.
278h	Assigned to LPT2 or LPT3
280	Optional address for sound cards and network cards
300	Default for many network cards
2E8h	Assigned to COM 4 and used with IRQ 3
2F8h	Assigned to COM 2 and used with IRQ 3
300h	nother Network Interface Card choice
320h	Used for a SCSI host adapter or MIDI device. You can use this for a Network card when not assigned to any SCSI or MIDI device.
330h	Used for the SCSI host adapters
340h	Could be used for a SCSI host adapter
360h	Could be used for a Network card, provided there are no conflicks.
378h	This is assigned to the first parallel printer port (LPT 1) in color systems. Normally used with IRQ 7.
3BCh	This is assigned to the first parallel printer port (LPT1) in monochrome systems.
3E8h	Assigned to COM 3 and used with IRQ 4
3F8h	Assigned to COM 1 and used with IRQ 4

A rule of thumb for IEQ, and IO address conflicts is that the PC is unable to boot or giving error codes during boot process. If you have installed any device recently, remove or disable it and see if the PC boots properly.

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