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From: mg@dsd.camb.inmet.com (Mitch Gart)
Subject: Re: Ada Portability... NOT!
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References: <199411182104.PAA04854@mail.cs.utexas.edu>
Date: Mon, 21 Nov 1994 14:59:10 GMT
Date: 1994-11-21T14:59:10+00:00
List-Id: <comp.lang.ada>

One point was implied by Robert's response but wasn't stated
explicitly.  On the HP RISC architecture, if a program 
directly references a mis-aligned piece of data (a 4 byte
quantity aligned on < 4 bytes, or an 8 byte quantity aligned
on < 8 bytes) the machine will trap.  This means that at execution 
time either 

- the program will crash 
- the program will get CONSTRAINT_ERROR
- the compiler will have to generate lots of extra instructions
  to "fiddle" as Robert says, that is, to fetch a quantity a
  few bytes at a time, and put it together into the right 
  data format, then execute some more extra instructions later
  when storing the data back into memory.

The fact that the compiler did none of the above, and instead
rejected the rep clause, seems like a reasonable thing to have
done on the HP machine.
	
	Mitch Gart