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 20:34:37 -0700
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On 6/14/2025 2:49 PM, quadibloc wrote:
> On Sat, 14 Jun 2025 21:26:10 +0000, quadibloc wrote:
> 
>> On Sat, 14 Jun 2025 19:23:59 +0000, Stephen Fuld wrote:
>>
>>> 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.
>>
>> It prevents relocation of programs currently in use that are already in
>> memory.
> 
> Actually, to be more precise, it prevents doing this _in a manner that
> is fully transparent to the programmer_.
> 
> So IBM could have created a time-sharing operating system that ran on
> models of the System/360 other than the model 67 with its Dynamic
> Address Translation hardware as follows:
> 
> Require that programs only use one set of static base registers for
> their entire run;
> 
> Require that programs describe the base registers they use in a standard
> header;
> 
> Require that programs set a flag when they have finished initializing
> those base registers (and do so very quickly after being started).
> 
> If those conditions are met, then a program in memory can indeed be
> moved to somewhere else in memory, as the operating system will know
> which base registers to adjust.
> 
> Well, sort of. Such programs would not be able to use flat addresses to
> pass pointers between routines, because they would not be valid between
> relocations. A workaround for this issue may be possible, requiring
> changes to calling conventions; for example, all routines in a program
> might need to share a common area for data values, and always use the
> same base register to point to it.
> 
> So you would have special time-sharing versions of all the compilers.

And this is better than what I proposed, and what other vendors did??????


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