🚀 go-pugleaf

In article <m0jd17b-0003uJC@rht32.pcs.com> hgw@RHT32.PCS.COM (h.-g. willers) writes:
>Endianess again!
>
>We are considering to build a VME-based computer board using a
>little endian processor. Is there any reason to swap bytes
>and shorts going from and to the (big endian) VME, or are things just more
>easy when we do not swap anything?

Interesting question ! 
Since the endian-swap depends upon the data (you would do it for
byte data but not for 32-bit integers), the swap needs to be
configurable. Also for VME there are about 12 potentially useful
different swap variations---caused by the mixture of 8,16 and 32-bit
datapaths to SLAVEs interworking with a 32-bit RISC.

What we do is put a big cross-point switch between the CPU and
the VMEbus. This can do pretty much any re-ordering of the data
and can hold up to 8 different swap functions on-line at any time.
We drive the swap function selection from a look-up RAM which 
is in turn hung off the CPU address bus. 

This gives you the potential to apply any mangling of the databus
you like, and up to 8 different kinds, configured to correspond
to various address regions in the CPU address space.

Probably overkill---but there's a Texas chip which does all this
for about $50.

David Boreham, INMOS Limited | mail(uk): davidb@inmos.co.uk or ukc!inmos!davidb
Bristol,  England            |     (us): uunet!inmos.com!davidb
+44 454 616616 ex 547        | Internet: davidb@inmos.com