Subject: Re: Byte offsets in Q-bus map registers?
To: John Wilson <email@example.com>
From: Anders Magnusson <firstname.lastname@example.org>
Date: 02/05/2000 21:55:59
> I have an annoying hardware question:
> Can anyone tell me whether Q-bus MicroVAXen typically support byte offsets in
> DMA map registers? E.g. on a VAX-11/780, setting bit 25 (the "byte offset"
> bit) of a map register allows a Unibus device to do word transfers to that
> page at odd physical memory addresses, even though Unibus word addresses
> are always even. Do MicroVAX machines have the same capability or do Q-bus
> word DMA transfers have to go to even addresses like on a PDP-11? Or does
> it vary from one model to another?
Due to the documentation: no. Looking at the VAX 11 series specs shows
that the BO functionality is only combined with BDP's and not valid
via direct data path.
I assume that this "trick" was "deprecated" and left to the device instead;
a device like a network card that may need to do DMA on odd addresses must
be able to handle that on a PDP11 as well.