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ISA: Difference between revisions
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→Device Conflicts
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The ISA bus can support x86 16-bit port addresses, but many older ISA cards only support 10-bit address decode. This presents a major problem that can lead to conflict. When the processor tries to access a certain resource on the ISA bus, it will signal all ISA devices in the hopes that only one responds. If a card can only handle ten of the address bits, it can respond to IOR and IOW signals when it should not be, because it does not see the upper bits.
So if a device is located at port 100H, that 10-bit address with every possible combination for the upper 6 bits will map to the same device (e.g port 0xFF00). To make this totally impossible, one can simply restrict PnP port addresses to be within the 1024-byte range to make such conflicts impossible, though this would be a limitation.
The decode of legacy devices like the LPT, COM, etc. that are typically embedded in the mainboard are probably 16-bit on PnP boards, but
= ISA Plug-and-Play Overview =
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