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

J.G.Harston jgh at mdfs.net
Wed Oct 27 22:26:20 BST 2010

John Kortink wrote:
> No, it's the DRQ pin of the Tube ULA that is connected (to
> a DMA request pin of the 80186).

Hold on, let me get that Technical Manual back out the box I
put it in yesterday. Yes, you're right. :( However...

> In practice, though, DRQ is the inverse of nNMI.
> But DRQ cannot be disabled, while NMI can (with the M config bit).

...so leaving it off a reimplementation is only saving a
couple of gates or so. (one latch, one AND?)

> 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.

Propably put into the design as a good idea along with DRQ,
but never used in practice, like HostIRQ.

J.G.Harston - jgh at mdfs.net

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