[Info-vax] nice for VMS

[Arne Vajhøj]

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