[BBC-Micro] Reversing the Tube ULA (destructively)

Michael Firth mfirth at firths.org
Wed Oct 27 22:41:52 BST 2010

  On 27/10/2010 22:15, John Kortink wrote:
> No, it's the DRQ pin of the Tube ULA that is connected (to
> a DMA request pin of the 80186). In practice, though, DRQ
> is the inverse of nNMI. But DRQ cannot be disabled, while
> NMI can (with the M config bit).
> Rather unexpectedly, the 80186 does not use DRQ's partner
> nDACK (the 80186 simply generates an I/O port access, as
> usual). It remains unclear why nDACK is even there (and
> its sense of read and write is swapped). There seems to
> be no coprocessor that uses it.
Is it possible that, as the Tube ULA was designed several years before 
the 80186 Co-Pro, that Acorn had intended the DRQ and nDACK signals to 
be available as a partner pair for future designs, but that (for 
whatever reason) when it came to implementing the feature in the 80186 
the nDACK signal didn't work as needed?

Is there a list somewhere of what Tube features all the different 
Co-Pros that exist use or don't use?



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