Re: Why I've Dropped In

Stephen Fuld <sfuld@alumni.cmu.edu.invalid> schrieb:

On 6/12/2025 11:03 PM, Thomas Koenig wrote:

Stephen Fuld <sfuld@alumni.cmu.edu.invalid> schrieb:
 

No.  Other systems in the S/360 time frame (i.e. before virtual memory)
used a system "base register", that was hidden from the user (but was in
its context), that was set by the OS when the program was loaded, or if
it was swapped out, when it was swapped in again.  It was reloaded
whenever the program gained control of the CPU.  Besides the advantage
of not requiring a user register for that purpose, it allowed a program
to be swapped in to a different memory address than it was swapped out
from, a feature the S/360 didn't enjoy.

 
It was supposed to, but I belive that was one of the earliest
failures that they noticed, and should have realized before:
their memory to memory instructions did not have base+offset+index.
 
Also, how was storing a pointer to somewhere supposed to work
for swapping out/swapping in?

>
Since the "base" pointer was known to the OS, if the program was swapped
out and swapped back in to a different location in memory, the OS just
changed the value in the base register to reflect the new location.

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.

This didn't work in the S/360 because the OS didn't know what
register(s) the user program was using as base register(s) so it
couldn't change the values in them if the program was to be relocated.

Even with that provision, it would not have worked.