🚀 go-pugleaf

crusoe@cs.tu-berlin.de (Andre' Zehl) writes:

>I'm working on a preparation on the HP-PA RISC Architecture.I'm desparately
>trying to figure out, how you administer the virtual to physical address
>translation. 

>Unfortunately HP's description of this translation is very vague. They
>describe, that the virtual page number is hashed, thus providing a pointer
>in a hashtable (the PDIR INdex) which provides a pointer in the physical
>page directory (to chained PDIR-Entrys), where the page attributes are
>stored (NOT the physical page number).

>The physical page number (ppn) is identical with the index from the
>hashtable. But this only makes sense, if it's not the virtual page number (vpn)
>which is hashed, but the physical page number, coming out of the
>hardware translation? IS THIS SO ? Is the PDIR Index of the hashtable vpn 
>the hashed ppn or the hashed ?

>I need help _very_ urgently, so evry little hint is appreciated _very_ much!
>Thank you very much in advance! Please e-mail reply.
>-- 
>Andre'_________________________________________________________________________
>							 crusoe@cs.tu-berlin.de

I suspect you are referencing an early PA 1.0 Architecture Reference Manual.

As I recall, those early manuals described a "suggested" VPN->PPN table
structure involving what was then called a Hash Table which pointed to
linked entries in what was then called the PDIR, or Page DIRectory.  This
historical PDIR was indeed indexed by the PPN (Physical Page Number).  Each
entry contained the Virtual Address associated with the corresponding
Physical Address given by the position within the PDIR.  Each entry also
contained a link to the next entry in the PDIR whiched "hashed" to the
same position in the Hash Table.  Hence, to go from PA to VA was a simple
PDIR table lookup.  To go from VA to PA involved applying the "hash function"
to index into the Hash Table, then follow the links into and through the
PDIR until a matching VA was found - the resultant PDIR slot yielding the PA.

All that has changed over the years...  The latter PA Ref Manuals dropped the
"suggested" section mentioned above.  In fact, PDIR is what the old Hash
Table is called now-a-days (in HP-UX).  Would anyone from HP care to comment
on the current mechanisms used in HP-UX, MPEiX, and HP-RT???

Ed "formerly of HP" McClanahan
-- 

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

  Edward McClanahan

Andre' Zehl (crusoe@cs.tu-berlin.de) wrote:
: I'm working on a preparation on the HP-PA RISC Architecture.I'm desparately
: trying to figure out, how you administer the virtual to physical address
: translation. 
... 
: The physical page number (ppn) is identical with the index from the
:hashtable. But this only makes sense, if it's not the virtual page number (vpn)
: which is hashed, but the physical page number, coming out of the
: hardware translation? IS THIS SO ? Is the PDIR Index of the hashtable vpn 
the VPN is hashed.
: the hashed ppn or the hashed ?

Each entry in the hash table points to the FIRST PPN in a linked list of all
physical pages whose VPN hash to the same value.  Thus, you walk
the linked list in the PDIR, examining each entry to see if its VPN matches
the one you are looking for (presumably, the one you got a TLB miss on).
If a match is found, you found it! (And the physical address is
the PPN (which is based on the PDIR index as you noticed) plus the original
bottom 11 or so bits (i.e.: virtual pages are aligned on physical page
boundaries).

(I emailed a little more to him)

--
Stan Sieler
sieler@allegro.com