Re: [f-cpu] Status quo

[whygee@xxxxxxxxx]

Le 2015-04-01 12:36, Nikolay Dimitrov a ÃcritÂ:
> Hi Yann,
Hi !

> Well, I personally don't like the idea of having such CPU instructions
> in the first place,
Why ?

> so didn't intended to put this LUT on the CPU data path at all.
The CPU datapath is where data are treated, modified, etc.


> I think it can be implemented like MMU extension module, to handle the
> address-conversion only for the tiled pages (for graphics purposes,
> if/when needed).
There is no "magic box" called MMU, there's the LSU, the TLBs,
some buffers here and there, cache memory...
Adding a LUT "when requested" in the address path would not just 
increase the memory latency
but also make it more complex because "certain cycles" will have one 
more
cycle of latency.
It adds a new "visible state" to the CPU architecture : in a 
multithreaded,
one thread would want one conversion, another wants a different one,
so the thread switch is ridiculously slow.
And you still haven't specified how you'll tell the LUT to be active or 
not.

OTOH, a LUT instruction is feasible. some address bits
may be used to select one thread's contents and it would be
useful for many many applications (gamma conversions, crypto, hash...)
unlike reordering address bits which is useful only for one specific 
use.

> Regards,
> Nikolay
yg
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