Technical Upgrading A Computer Resolving Conflicts

Resolving Hardware Conflicts

Until Plug and Play (PnP) does deliver its promise you will have to resolve any hardware conflicts. A conflict is caused when two devices take the same hardware resource and neither device is designed to share the resource with the other device. As a result of a resource conflict you usually end up with a non-functioning device, peripheral or worse a personal computer that has crashed. The hardware resources in a personal computer are strictly limited and it can often take a bit of juggling around to get all of your expansion cards and devices to coexist with each other. So, just what are those hardware resources ? The hardware resources are divided into four categories.

1. Direct Memory Access (DMA) is a method whereby a device can access memory by itself without using the processor. There are only eight DMA channels on a 286 or better processor, but usually as few as five of those will be free. Normally you will be able to DMA channels 1,3,5,6, or 7 for your expansion cards.
2. Input/Output Port (I/O) is an address in memory which acts as a gateway, permitting a device to communicate with the processor. There are several hundred of these and should not be in short supply unless your card refuses use bit only a few of them.
3. Interrupt Request (IRQ) is a hardware link between a device and the CPU. Personal computers use two Programmable Interrupt Controller (PIC) chips and each controls eight IRQs. The second slave PIC is co-ordinated through its IRQ 9 and the master PICs IRQ 2. As a result IRQs 2 and 9 become one and the same. Because high IRQs receive greater priority than low ones, if you have a device that makes intensive use of interrupts, it would make much more sense to allocate it a high IRQ number instead of a low one.
   You can't normally share IRQ lines but that is now changing. It is common to share sound cards with the IRQ 7, normally assigned to the printer port. The printer port is hardly likely to be used at the same time. For every IRQ line there is a corresponding I/O port address.
4. A ROM Address is needed by some expansion cards that incorporate a ROM or ROM BIOS and this has to sit in memory alongside other ROMs in the system, for instance the video BIOS and boot ROM. All ROMs must have a unique address and cannot share address ranges with anything.

Not all expansion cards use all of these resources. For example a Creative Labs Sound Blaster 16's default settings are DMA channels 1 and 5, I/O port 220 and IRQ 5.

Troubleshooting

You will know if you have a hardware conflict when something stops working. A typical example is putting a modem on COM 2 which shares IRQ 3 with COM 4 and a network card at its default settings, also IRQ 3. The network card works just fine until you launch a comms program, when everything may stop only to automatically reappear if you log out.

As I have explained, hardware resources are scarce and that some cards may try to make use of resources already used. So, how do you prevent these conflicts from occurring ?

There is no substitute for an audit of your personal computer hardware resources, taking note which device uses what resources. When you add a new card you should also know exactly which hardware resources are free and what are already used and therefore be able to make any necessary changes to the settings of the new card or possibly an existing card.
You might have to remove every card to find out its precise settings but some respond to software like WinCheckit or AmiDiag and will take you what you need to know.
Windows 9x is not bad at resolving conflicts, even when you are using legacy (non plug and play) cards. Device Manager will flag all hardware conflicts and tell you which device it is conflicting with. You know precisely where the problem is, which is half of the battle.
You might have to remove every card in the system until all you have is graphics, hard drive and keyboard, then add cards one at a time, until your computer complains of a conflict. You are then in a position to think logically about how to reconfigure the settings.

Modern PCI cards can simply be reassigned another resource by moving the card to another free slot or perhaps swapping the card over with another in a different slot. Some CMOS setup utilities allow you to do this without needing to open the cover. Look for a field that changes the 'auto-assign IRQ to PCI' option.

Legacy Expansion Cards

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