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[jfox@xxxxxxxxxx (Jeff Fox)]

>Mats:
>Jeff, would be possible for you to explain Chuck's motive for using
>complementary polarity in a way such that we all, when shifting
>mental viewpoints, can be helped by thinking: Every other bit uses
>a different polarity because <some logical reasoning>.
>
>/Mats Eriksson  (Mats.Eriksson@eua.ericsson.se)

The complementary polarity on the bus of MuP21 evolved when the chip
went from MuP20 to MuP21.  It was done entirely for hardware reasons.
Namely, it allows Chuck to greatly reduce the transistor count, and
to get the slowest operations to happen faster.
Think of a shift register.  To shift a word one bit left or right you
have to move all the bits left or right.  A single transistor is the
fastest, but transistors invert.  SO if you use only one transistor
to shift a bit it will invert the polarity of the bit.  So you can
make it twice as fast with half as many transistors (on certain
critical circuits) by using complementary polarity.  In a conventional
design you would need two transistors to invert each bit twice to
keep the polarity the same.  However this introduces two delays
instead of one, and it takes twice as many transistors.
You didn't ask about instruction bit interleaving, but it is also
done for hardware reasons.  It helps balance the current flow and
makes decoding of the instructions work better.  Bit interleaving
will go away on F21, but the complementary polarity will remain.

Jeff Fox