Submitted 11-Sep-00 by Alan Blundell: > I've never understood why the default IRQs for serial ports 3 and 4 are as > they are, set to the same IRQ as ports 1 and 2. In the early days of PC's serial and parallel ports were the only things attached to your machine that the designers felt you might need more than one of. The parallel port was designed to be capable of being run without an IRQ, but serial ports (an older standard) were not. With the limit of 16 IRQ's, and several of those taken up by sytem resources (drive controllers, clock, numeric coprocessor, etc.), it was decided that by default, on machines that have more than two serial ports, ports 3 and 1 share IRQ 4 and 2 and 4 share IRQ 3. The idea being that early PC designs didn't expect that you would need to access more than two serial devices simultaneously and that you would space them so that so as to avoid IRQ conflicts when using them. If you have ever run out of IRQ's trying to configure a machine, you can see why this was an essential mechanism. As an aside, there have been similar problems encountered with the onboard BIOS's of varios cards. For example, the default BIOS address of some Promise EIDE cards (ISA) conflicts with most VGA cards. Trying to boot without reconfiguring the card results in a scrambled display and no disk activity. We have moved beyond the point that the architecture is insufficient to the demands placed on it. Certainly some of this is alleviated by PCI cards capable of sharing interrupts and some newer drive controllers being able to do the same. Even serial boards have made tremendous advances in this regard, but the architecture is limited by its roots. As the ISA bus get phased out this will become less of an issue. -- Anton Graham GPG ID: 0x18F78541 <[EMAIL PROTECTED]> RSA key available upon request "When the going gets weird, the weird turn pro..." -- Hunter S. Thompson
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