[Info-vax] Alpha Fortran data alignment

[glen herrmannsfeldt]

Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
> tadamsmar wrote:
>> I was checking the locations of variables.
 
>> If I run this:
 
>>         integer*2 prmidx
>>         real*4 vlu
>>         prmidx = 1
>>         vlu = 10000000
>>         call sub(prmidx,%loc(prmidx),%loc(vlu))
>>         end
>>         subroutine sub(prmidx,lp,lv)
>>         integer*4 prmidx,lv,lp
>>         type *,prmidx,lp,lv
>>         end
 
>>           1      196608      196616
 
>> But if I change prmidx to real*4 in main and real*8 in
>> the sub, I get:
 
>>   128.000036285423           196608      196612
 
>> It appear that 8 bytes are allocated for an integer*2 and 4 bytes are
>> allocated for a real*4

There is no requirement that variables be allocated in memory
in the order that they are declared.  Sometimes the compiler even
needs temporary variables that might also be allocated.
 
>> Why is this?
 
> Because you are breaking all the rules!!!!!!!
 
> You have assigned different sizes (INTEGER*2 and INTEGER*4) for the same 
> variable in MAIN and SUB.  The subroutine will see 16 bits of data and 
> 16 bits of garbage.

That is true, but completely unrelated to the question being asked.
The question only has to do with the addresses of the variables
in the main program.  %loc() finds the address and passes that
as an integer.  The OP only was asking about the sizes and 
positions in memory of the variables.

-- glen