Re: Why I've Dropped In

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)