[Info-vax] Dave Cutler, Prism, DEC, Microsoft, etc.

[Rob Brown]

On Mon, 30 Nov 2009 at 17:00 -0500, Richard B. Gilbert wrote:

> JF Mezei wrote:
>> 
>> Out of curiosity, why do some CPUs have so much trouble fetching 4 
>> bytes from an unaligned location ? Is it the CPU and CPU 
>> architecture that is the problem, or the RAM memory subsystem that 
>> can't fetch 4 bytes starting at an uneven memory address ?
>> 
> Well VAX certainly did!  Alpha, IIRC was even more sensitive to 
> alignment issues.  If you knew that, it was not difficult to satisfy 
> the machines preferences.  The compilers took care of most of the 
> work.
>
> The VAX and Alpha architectures were by no means unique in having 
> this requirement.  The IBM System/360 and System/370 machines also 
> required proper boundary alignment.

And the PDP-11!  Try a MOV (move word) on an odd address and you trap 
through location 4.  Fatal unless you wrote your own handler.  I don't 
know of anybody who ever did.  Just align your data!

VAX allowed MOVW and MOVL on unaligned data.  It wasn't until much 
later that I learned that doing so made the program run slower.


-- 

Rob Brown                        b r o w n a t g m c l d o t c o m
G. Michaels Consulting Ltd.      (780)438-9343 (voice)
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