>If someone knows how it works - I suppose the alternate register status
>doesn't exist on ST506 drives/controller (it has the property
>to not ack the interrupt on IDE systems, where the status register does) ?

You are not looking for the ST506 spec.  That one deals with the protocol
between drive and controller.  You want to look at the documentation for
the WD1003 controller (and the interface to the AT bus).

-- 
Christoph Badura

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