Gabe Black wrote:
> nathan binkert wrote:
>
>>> From a quick read it sounds like option 2 would be better, cleaner,
>>> and more extensible. In reality other than Alpha all ISAs have some
>>> microcoded instructions. I don't see a reason not to deal with
>>> microops as first class operations.
>>>
>>>
>> I read all of both of your e-mails, and I agree with Ali on making the
>> microcode rom a first class citizen. Many ISAs will have microcode,
>> and Alpha just won't use it (though one could if they wanted to write
>> special instructions to do magic things.) Also, I'd say let's get
>> things right first, then worry about how to cache things properly. If
>> we do the full expansion of microops into StaticInst objects (which
>> sounds like the right plan to me), then we can store them in a decode
>> cache based on PC, and just encode the environment as a tag that is
>> only checked to confirm that you got the correct static inst. If the
>> tag doesn't match, then you replace the StaticInst in the cache for
>> that PC. I'd bet that in the vast majority of cases, a particular
>> microop at a particular PC is only ever seen in one environment, so it
>> would be rare for the cache to hit the PC and miss the environment.
>>
>> Were there any other questions that I should have answered? Steve
>> have any opinions?
>>
>> Nate
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>>
>>
> I think your right between those two options. Right now, the microops
> are cached as part of the macroop they're from. The macroop is cached as
> the first level instruction from the decoder, and it statically
> instantiates the microops which get stored away with it. I also agree
> that, most of the time, or at least for some period of time, the
> environment for a microop will be -similar-, but not necessarily the
> same. Some things which could change are the several register indices of
> the original macroop in the usual case (not interrupts or faults at fetch).
>
>
I should clarify that by "statically" I mean at construction rather than
on demand.
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