ppc LE questions (seeking help hand info pointers)
rcblach at raleigh.ibm.com
Sat Sep 22 03:01:49 EST 2001
I just wanted to clear up some inaccuracies in this previous note.
Dan Malek wrote:
> "Timothy A. Seufert" wrote:
> > Finally, Book E has been mentioned.
> Too bad.....
> > ... a TLB entry bit (bit 'E' for endian) which marks a page as
> > little endian.
> This has been discussed in the past and suffers from the same
> problem as trying to byte swap in a bridge. Although you can use
> any load/store in this space, unless you are performing the access
> on the natural size of the object it won't have the proper effect.
> Since you must require that knowledge, it is just as easy to choose
> the proper byte swapping variant of the instruction.
This is NOT true on the 405 core processors. They support TRUE little
> > Unfortunately, Book E does not guarantee that the 'E' TLB entry bit
> > is really supported in hardware:
> For all practical purposes, the engineering world is already used
> to working with the advantages of a big endian processor and dealing
> with the issues of accomodating little endian when necessary. Adding
> a feature like this only complicates the use of legacy software and
> creates significant discussion for a feature trying to find a
> problem to solve. This feature isn't a performance enhancement,
> only a detriment because it requires additional software to manage
> something not natural to use. I suspect it was just easier to write
> this into the specification (i.e. it's there but not required) than
> spending time discussing it's technical merit.
The book E specification requires the little endian bit. It must be
there for the processor to be a Book E.
> > But so far as I can tell, the 750, 7400, 7450, etc. are not Book E
> > Too bad this part of Book E wasn't in the architecture from the start...
> You are one of few to feel that way. Those of us that have worked
> with PowerPC from the start, and were part of the original design
> discussions, view Book E as a totally new processor that may execute
> similar instructions to traditional PowerPC. I have spoken with several
> embedded product companies, a couple very large, that have huge
> investments in PowerPC software that are now trying to determine
> what to do. They are quite upset that they now have to invest in
> a new processor design, and it makes them likely to choose something
> else. Motorola has proven you can build some very powerful embedded
> processors with the traditional PowerPC core and memory mapped I/O
> peripherals, a very nice and efficient programming environment. The
> Book E appears to be a marketing vehicle put together by people that
> didn't understand or appreciate the previous PowerPC architecture
> specifications. We can hope the where Book E allows variation and
> extension, designers will borrow from traditional PowerPC and not
> go in some other "creative" direction :-).
I know the discussions on Book E are incorrect. I have had the
privilege here in
IBM to work on the 440gp processor. Although my knowledge of powerpc is
far from complete, I can tell everybody that from my experience that the
Book E architecture is superior.
There are two things that make it superior.
1)Not turning off translation when an interrupt is taken
2)The IVOR registers which allow the system interrupts to be place
If one stops to think about it, running Linux out of Ram on a PCI card
becomes very easy. This is because the processor does NOT have to fetch
all interrupts from
physical location 0. And now the interrupt routines run with protection
ON, so protection of task by bad operations in the interrupt handler is
This will lead to a more secure OS.
I believe that Book E processor which have been announced by both
Motorola and IBM
will be VERY successful.
> I'm not really saying anything new here, this has been discussed
> among all of us that work on PowerPC ever since the Book E announcement
> long ago.
> I guess I better go looking for those m68k projects now :-).
Too bad you feel this way. Book E is definitely better and I am sure,
that once you
got one, you would have 100 great ideas on how linux could use its
I am personally excited by the future of book E. The Engineers here
working on delivering it are simply the best with whom I have every
worked. ( I have been at IBM for over 20 years).
> -- Dan
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