2.10 Resources are Limited
Ideal Computers
The architecture of the PC provides only a limited number of resources: IRQ's, DMA channels, I/O address, and memory regions. If there were only a limited number devices and they all used standardized bus-resources values (such as unique I/O addresses and IRQ numbers) there would be no problem of attaching device drivers to devices. Each device would have a fixed resources which would not conflict with any other device on your computer. No two devices would have the same addresses, there would be no IRQ conflicts on the ISA bus, etc. Each driver would be programmed with the unique addresses, IRQ, etc. hard-coded into the program. Life would be simple.
Another way to prevent address conflicts would be to have each card's slot number included as part of the address. Thus there could be no address conflict between two different cards (since they are in different slots). Card design would not allow address conflicts between different functions of the card. It turns out that the configuration address space (used for resource inquiry and assignment) actually does this. But it's not done for I/O addresses nor memory regions. Sharing IRQs as on the PCI bus also avoids conflicts but may cause other problems.
Real Computers
But PC architecture has conflict problems. The increase in the number of devices (including multiple devices of the same type) has tended to increase potential conflicts. At the same time, the introduction of the PCI bus, where two or more devices can share the same interrupt and the introduction of more interrupts, has tended to reduce conflicts. The overall result, due to going to PCI, has been a reduction in conflicts since the scarcest resource is IRQs. However, even on the PCI bus it's more efficient to avoid IRQ sharing. In some cases where interrupts happen in rapid succession and must be acted on fast (like audio) sharing can cause degradation in performance. So it's not good to assign all PCI devices the same IRQ, the assignment needs to be balanced. Yet some people find that all their PCI devices are on the same IRQ.
So devices need to have some flexibility so that they can be set to whatever address, IRQ, etc. is needed to avoid any conflicts and achieve balancing. But some IRQ's and addresses are pretty standard such as the ones for the clock and keyboard. These don't need such flexibility.
Besides the problem of conflicting allocation of bus-resources, there is a problem of making a mistake in telling the device driver what the bus-resources are. This is more likely to happen for the case of old-fashioned manual configuration where the user types in the resources used into a configuration file stored on the harddrive. This often worked OK when resources were set by jumpers on the cards (provided the user knew how they were set and made no mistakes in typing this data to configuration files). But with resources being set by PnP software, they may not always get set the same and this may mean trouble for any manual configuration.
The allocation of bus-resources, if done correctly, establishes non-conflicting channels of communication between physical hardware and their device drivers. For example, if a certain I/O address range (resource) is allocated to both a device driver and a piece of hardware, then this has established a one-way communication channel between them. The driver may send commands and other info to the device. It's actually more than one-way communications since the driver may get information from the device by reading its registers. But the device can't initiate any communication this way. To initiate communication the device needs an IRQ so it can send interrupts to its driver. This creates a two-way communication channel where both the driver and the physical device can initiate communication.
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