[openrisc] Multiplier oddities...

[Christian Melki <christian.melki@axis.com>]

Hello ppl,

I have a few more questions about the multiplier in the OR1200.
The gmultp2 and the amultp2 are instansiated in the same manner
in the or1200.. yet when we rip the multiplier out of the design to test
it separately in synthesis and simulation we get different results from
the gmult and the amult. Most notably, a multiplication should take
2 cycles to complete, right? This is what we have learned from the
gmultp2 at least. Yet it seems like the amult completes a multiplication
in 1 cycle in simulation and the gmult completes in 2 cycles. How can this
be? There is no logic around the multiplier or clock-division that makes
such a thing work.. What am I missing here? To my knowledge the two
designs should behave in the same way? We do have some other oddities with
the asic multiplier but that has probably more to do with simulation
technical details.. sometimes it seems like results come dilutedly out of
order from the mutiplier.. lite a simulation with 2*(b=b+1 each cycle) = P
could yield a sequence with 2 4 4 2 6 8 8 6 etc.. something like that.
But im sure thats an error with the simulation and not the design.
These oddities do not occur with the generic multiplier.
Anyway, my main query is why it seems like the amult is a 1-cycle design
and i cannot see anything that should take care of this?

regards,
Christian

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