[Info-vax] nice for VMS

[Richard B. Gilbert]

Arne Vajhøj wrote:
> Richard B. Gilbert wrote:
>> JF Mezei wrote:
>>> Bob Koehler wrote:
>>>
>>>>    There are a variety of states related to memory management, 
>>>> including
>>>>    PFW, COLPG, and others I could dig up somewhere.
>>>
>>> But virtual memory management is under the responsability of the OS. But
>>> waiting for 8 bytes of physical memory to be loaded into a the register
>>> is very much hardware based and would the OS be aware of this operation
>>> taking a long enough time to warrant a switch processing to another
>>> process ?
>>
>> If your memory is aligned properly; e.g. the three low order bits of 
>> the address are 0, a load of 8 bytes should go quickly.  If they are 
>> not 0, the O/S will do its best which, admittedly, is not very good.  
>> ISTR that it must move the eight bytes to properly aligned temporary 
>> storage and then load them into a register!  This takes many cycles 
>> longer than a properly aligned quadword load.  The compilers know how 
>> to do it right but if you use Macro and fail to align things properly, 
>> your code may run like a dog!
> 
> Most HLL compiler allow you to specify that data should not be
> natural aligned. And take the performance hit, which even at older
> Alphas could be >x10.
> 
> Arne

I wonder if anyone ever uses that "feature".  I'd also wonder why?