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 09:45:23 -0700
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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.


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