> 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.

/~\ The ASCII				der Mouse
\ / Ribbon Campaign
 X  Against HTML	       mouse@rodents.montreal.qc.ca
/ \ Email!	     7D C8 61 52 5D E7 2D 39  4E F1 31 3E E8 B3 27 4B