Computer system resources
Quick links
System resource ABCs
What is an IRQ?
What is an I/O?
What is a DMA?
Checking resource availability
Assigning a resource setting
Changing resources
All IRQs are used, now what?
Can I add an IRQ?
IRQs not recommended
IRQ listing
DMA channels
System resources are what allocate and setup your hardware components, helping hardware to work without causing issues with other hardware within your computer. System resources are setup by one or more of the below.
- Interrupt Request (IRQ)
- Input/output (I/O)
- Direct memory access (DMA)
Short for Interrupt request, IRQ is a signal that has a direct line to the computer processor, allowing it to stop the processor momentarily and decide what to do next. Every IBM compatible computer has a maximum of 15 IRQs and are prioritized in the computer according to the importance of the device. See IRQ Listing for a list of IRQs, which may be available or are currently used.
Input Output (I/O) represents the locations in memory that are designated by use of various devices to exchange information amongst themselves and the rest of the PC. See IRQ Listing for a list of IRQs and I/O ranges.
DMA, or Direct Memory Access, are pathways provided by the hardware to allow the hardware direct access to the computer's memory. See DMA Listing for listing of DMA channels.
Checking resource availability
If you have Windows 2000 or Windows XP, follow the below steps. Click Start, Programs, Accessories, System Tools, and then System Information to display the system resource information.
If you have Windows 95, Windows 98, or Windows ME, follow the below steps.
- Open Device Manager.
- Double-click on the computer (the first icon listed in Device Manager. This will list numbers 00 - 15; any number that is not listed is an indication that the IRQ is free. If all numbers are listed once or more, this would be an indication that all IRQs are being used.
In Windows 3.x or MS-DOS, to determine the currently used resource settings, use the MSD command, located in the DOS directory.
Many devices still in use today use Jumpers to setup the Resource settings for a hardware device. If your card has Jumpers it is recommended that you set the jumpers to available settings on your computer and then install the software on the computer to help resolve issues from occurring.
If your card has no jumpers or dip switches, it is a good possibility that the card is a Windows card (Plug N Play card). This indicates that you should be able to place the card within the computer and Windows 95 or Windows 98 will setup the card for you. Many of these cards are configured through the software used to install the card or Device Manager.
Changing resourcesWithin Windows 95 and Windows 98, most PnP (Plug N Play) cards can be manually adjusted through Device Manager. If your device is encountering conflicts with another device within Device Manager, double-click the device within the device category. Such as double-clicking a 3COM NIC icon under Network Adapters. Within the properties of the device click the Resource tab. In Resources uncheck the box that says 'Use automatic settings'; once unchecked you will then be able to change the Basic Configuration. As you change the Basic configuration, each of the resource settings will change. If you only have one option for resource settings, it's a good possibility that the device may be a Legacy device or may only work with one configuration mode.
If your computer is utilizing all IRQs, unfortunately there are only a few ways around this, which can be very complicated. The first and easiest way would be to attempt to remove devices from the computer to allow IRQs to become free or substitute for external devices. The other option would be to attempt to assign the IRQ settings to IRQ settings that may already be used by another device. However, when doing this, it is recommended that you choose a device that is not going to be frequently used. Windows may report that there is a device conflicting; however, in some instances the devices can work on the same IRQ the devices but will not be able to work at the same time.
No, unfortunately with PCs they are limited to 00-15. The latest and greatest PCs will also have this limitation. However, you can add devices such as a SCSI card to the computer, daisy chain a hard drive and CD-ROM drive and other hardware devices, and because the SCSI card uses ID addresses, when the hardware devices are hooked up to the SCSI card, they will not be taking an IRQ. Therefore, you will be able to utilize up to 7 devices on one IRQ. Another recommended connection to PC computers would be the USB port, which allows up to 127 devices to be connected at once using only one IRQ.
When connecting devices it is recommended that you stay away from IRQ 9 which is a cascade port with IRQ 2. However, it is a good idea to assign devices that you wish to move and do not plan to use to IRQ 9 to allow extra IRQs for a device you may plan to use. A good example of this recommendation is moving the MPU-401 device, which is a midi device used for musical keyboards.
IRQ listingThe below lists the standard IRQ configuration found in modern PC computers. The below IRQs that have the Card Type 8/16-BIT are configurable and possibly removable. However, all others cannot be removed or shared. When listing the IRQs and encounter IRQs that are doubled, it could possibly indicate an IRQ confliction or that the IRQ is being shared. The exception to this are IRQs 14 and 15.
| IRQ | DEVICE | I/O PORT | BUS SLOT | CARD TYPE |
| 00 | System Timer | None | NO | NONE |
| 01 | Keyboard | None | NO | NONE |
| 02 | Cascade Controller 2nd PIC | None | NO | NONE |
| 03 | COM 2 and 4 | COM 2: 02F8h (02F8 or 2F8) COM 4: 02E8h (02E8 or 2E8) | YES | 8 or 16-BIT |
| 04 | COM 1 and 3 | COM 1: 03F8h (03F8 or 3F8) COM 3: 03E8h (03E8 or 3E8) | YES | 8 or 16-BIT |
| 05 | Sound Parallel Port 2 | PARALLEL PORT 2: 0278h - 0378h | YES | 8 or 16-BIT |
| 06 | Floppy | 03F0 - 03F5 | YES | 8 or 16-BIT |
| 07 | Parallel Port 1 | 0278h - 0378h | YES | 8 or 16-BIT |
| 08 | Real-time Clock | None | NO | NONE |
| 09 | Redirected IRQ 2 Open Network Available | None | YES | 8 or 16-BIT |
| 10 | Open | None | YES | 8 or 16-BIT |
| 11 | Open SCSI Video | VIDEO: 3B0-3DF | YES | 8 or 16-BIT |
| 12 | Open PS2 | None | YES | 8 or 16-BIT |
| 13 | Coprocessor | None | NO | NONE |
| 14 | Open Primary hard drive (master) Hard Drive Controller | 1ST IDE: 1F0 | YES | 8 or 16-BIT |
| 15 | Open 2nd hard drive (slave) | 2ND IDE: 170 | YES | 8 or 16-BIT |
| CHANNEL | AVAILABILITY |
| 00 | Available |
| 01 | Sound device |
| 02 | Standard floppy disk controller |
| 03 | Available |
| 04 | Direct memory access controller |
| 05 | Available |
| 06 | Available |
| 07 | Available |