Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Stephen Fuld <sfuld@alumni.cmu.edu.invalid>
Newsgroups: comp.arch
Subject: Re: Why I've Dropped In
Date: Sat, 14 Jun 2025 12:23:59 -0700
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On 6/14/2025 10:56 AM, EricP wrote:
> Stephen Fuld wrote:
>> On 6/14/2025 3:48 AM, Thomas Koenig wrote:
>>> quadibloc <quadibloc@gmail.com> schrieb:
>>>> On Fri, 13 Jun 2025 17:40:44 +0000, Thomas Koenig wrote:
>>>>
>>>>> Suppose you're passing an argument from a COMMON block, a common
>>>>> occurence back then (pun intended).
>>>>>
>>>>>        SUBROUTINE FOO
>>>>>        REAL A,B
>>>>>        COMMON /COM/ A,B
>>>>>        REAL C
>>>>>        CALL BAR(B)
>>>>> C ....
>>>>>        END
>>>>>
>>>>>        SUBROUTINE BAR(A)
>>>>>        REAL A
>>>>>        A = A + 1
>>>>>        END
>>>>>
>>>>> What should FOO pass to BAR?  A straight pointer to B is the
>>>>> obvious choce, but there is no base register in sight that the OS
>>>>> can know about.
>>>>
>>>> FOO passes a straight 32-bit pointer to B to BAR, using a load address
>>>> instruction to calculate the effective address.
>>>>
>>>> BAR then uses an instruction which chooses a register with that pointer
>>>> placed in it as its base register to get at the value.
>>>>
>>>> No attempt is made to pass addresses in the short base plus offset form
>>>> between routines, because they knew even then that it would never work.
>>>> At least not when subroutines are *compiled separately*, which was the
>>>> normal practice with System/360 FORTRAN.
>>>
>>> Correct.
>>>
>>> Which made nonsense the concept of making data relocatable by
>>> always using base registers.
>>
>> Forgive me, but I don't see why.  When the program is linked, the 
>> COMMON block is at some fixed displacement from the start of the 
>> program.  So the program can "compute" the real address of the data in 
>> common blocks from the address in its base register.
> 
> If the program was relocated after the call to BAR but before using
> the reference to access argument A then it reads the wrong location.

That is precisely my point.  The mechanism that IBM chose effectively 
*prevents* program relocation.  That is why I believe it was a mistake 
to choose that mechanism.




-- 
  - Stephen Fuld
(e-mail address disguised to prevent spam)