[ The PC Guide | Systems and Components Reference Guide | Motherboard and System Devices | System Resources | Input / Output (I/O) Addresses ]

I/O Address Space Width

Unlike IRQs and DMA channels, which are of uniform size and normally assigned one per device--sound cards use more than one because they are really many devices wrapped into one package--I/O addresses vary in size. The reason is simple: some devices (e.g., network cards) have much more information to move around than others (e.g., keyboards).

The size of the I/O address is also in some cases dictated by the design of the card and (as usual) compatibility reasons with older devices. Most devices use an I/O address space of 4, 8 or 16 bytes; some use as few as 1 byte and others as many as 32 or more. The wide variance in the size of the I/O addresses can make it difficult to determine and resolve resource conflicts, because often I/O addresses are referred to only by the first byte of the I/O address.

For example, people may say to "put your network card at 360h", which may seem not to conflict with your LPT1 parallel port at address 378h. In fact many network cards take up 32 bytes for I/O; this means they use up 360-37Fh, which totally overlaps with the parallel port (378-37Fh). The I/O address summary map helps you to see which I/O addresses are most used, and to visualize and avoid potential conflicts.

Next:I/O Addresses, Multiple Devices and Conflicts

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