Make your own free website on

Computer Hardware

Understanding System Resources

System resources are what allocate and setup your  hardware components to help hardware to work without  causing issues with other hardware within your computer.
System resources are setup by one or more of the following:
Interrupt Request (IRQ)
Input/output (I/O)
Direct memory access (DMA)

An IRQ or Interrupt request line allows a hardware device inside of the computer a direct line to the Microprocessor and tells the Microprocessor to stop what it is doing and wait until it has further instructions. Every PC 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 and which may be available or are currently used.

An Input Output (I/O) represents the location 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.

A DMA or Direct Memory Access are pathways provided by the hardware to allow the hardware direct access to the computers memory. See DMA Listing for listing of DMA channels.

In Windows 3.x or DOS to determine the currently used resource settings run MSD, located in the DOS directory.
If you have Windows 95 or Windows 98 to display the listing of IRQs click on start / settings / control panel / System / click on the tab that says device manager / and double click on computer (first icon). 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.
To display a listing of any other resource click on one of the bubbles representing the type of resources you would like to view.

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 / 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 and or device manager. 

Within 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 which 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 its 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 and 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 however will not be able to work at the same time.

No, unfortunately with PC's they are limited to 00-15. The latest and greatest PC's will also have this limitation. However you can add devices such as a SCSI card to the computer daisy chain a hard drive / 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 generally 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.
The following lists the standard IRQ configuration found in modern PC computers. The following IRQs that have as 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 is 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                 None                            NO                          NONE
                 2nd PIC                                   
03            COM 2 / 4                                 COM 2: 02F8h            YES                        8 / 16-BIT
                                                                   COM 4: 02E8h           
04            COM 1 / 3                                 COM 1: 03F8h            YES                        8 / 16-BIT
                                                                   COM 3: 03E8h            
05            Sound                                        PARALLEL PORT   : YES                        8 / 16-BIT
                 Parallel Port 2                          0278h - 0378h
06            Floppy                                       03F0 - 03F5                YES                        8 / 16-BIT
07            Parallel Port                             10278h - 0378h           YES                        8 / 16-BIT
08            Real-time Clock                       None                            NO                           NONE
09            Redirected IRQ 2                     None                            YES                         8 / 16-BIT
                 Network Available                   
10            Open                                         None                             YES                        8 / 16-BIT
11            Open                                         VIDEO:                         YES                        8 / 16-BIT
                SCSI                                          3B0-3DF
12            Open                                         None                             YES                        8 / 16-BIT
13            Coprocessor                            None                            NO                           NONE
14            Open
                 Primary hard drive (master)   1ST IDE: 1F0              YES                         8 / 16-BIT
                 Hard Drive Controller
15            Open                                         2ND IDE: 170              YES                         8 / 16-BIT
                2nd hard drive (slave)

01Sound device
02Standard floppy disk controller
04Direct memory access controller

Back To Computer Hardware & MS-DOS Information Main Page