> > By some reasom MAXPHYS has been 63k on VAX since day one (I think it
> > comes from b_bcount being a short, but I really don't know).
> 
> Fuzzy memory, which I think came from reading 4.3 kernel code, says
> that it comes from 16-bit transfer counts for unibus devices (which
> impose a limit of 64k, or 64k-1, depending on what the device does when
> given a transfer count of zero) cropped back a little bit to catch
> small negative values.  Perhaps someone with 4.3 source online can
> check this?  IIRC it was commented near the relevant #define, or the
> code for minphys(), or some suchlike related place.
> 
Yeah, I thought of something like that also, but I haven't been able
to find any device with that problem. The minphys()/MAXPHYS definitions
don't have any comments.

Another reason I can think of is that the map registers were handled in
a resource map, that couldn't use map 0. With 1k pages that would be
63k MAXPHYS. But that really do not match how things were implemented.

-- Ragge