Date   : Wed, 27 Oct 2010 22:26:20 +0100
From   : jgh@... (J.G.Harston)
Subject: Reversing the Tube ULA (destructively)

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@...