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

John Kortink kortink at inter.nl.net
Wed Oct 27 22:15:00 BST 2010


On Wed, 27 Oct 2010 21:55:31 +0100, "J.G.Harston" <jgh at mdfs.net>
wrote:

>Michael Firth wrote:
>> needed for some specific re-implementations (e.g. the DMA functions used
>> on the 80186 Co-Pro)
>
>The 80186 doesn't use the DMA functions. The NMI from the Tube is
>connected to the *80186's* DMA input pin instead of the 80186's NMI
>pin.

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.


John Kortink

-- 

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