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I know there are issues when reading or writing data simultaneously from/to master and slave IDE drives, since a single IDE cable is shared. I remembered I read it somewhere that newer disks can handle simultaneous access. Am I right?

Regarding DMA operations, primary and slave both share the same irq (in compatibility mode). How should I distinguish which one is expecting a interrupt? Is there a case that both of them are expecting a interrupt simultaneous?

Thanks!

Well it would be my personal advice to not try and access the master and the slave at the same time. At least at first. Then after you understand IDE and the commands a little better you can attempt to do simultaneous access to both the master and slave. On the other hand you can access drives on different controllers at the same time with no problem since they have different ports and different IRQs.