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To Share, Or Not To Share
There are some circumstances where you can share resources between multiple devices, but it is never a really good idea. Why? Because sharing generally relies on either the user or the software "not doing" something that would create a conflict, and it is sometimes hard to maintain the discipline to do this consistently. It also creates a confusing situation to anyone else using the PC, and makes upgrading the machine more difficult as well. Furthermore, sharing resources can sometimes cause strange problems because the system is doing things with the devices when you may not realize it. For example, many multitasking operating systems like Windows 95 can access hardware even when you aren't directly requesting its use.
There are some cases where sharing does make sense. If you have two very infrequently used serial-port devices in a PC and you are out of interrupts, you may have no choice but to set them both up on the same IRQ and just make sure you never use both at the same time. You can use an older style of printer (one that doesn't have a smart driver program) on an IRQ that another device uses, if you don't use the printer and the other device simultaneously. I still don't recommend this--just not worth the hassle. I particularly don't recommend sharing DMA channels, as conflicts on these can be very confusing and difficult to diagnose.