Re: Why I've Dropped In

On 6/14/2025 2:26 PM, 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.

Yes.

It facilitates loading programs from object files on disk into any
desired part of memory, which is the usual meaning of "program
relocation" among System/360 programmers, perhaps because they had no
other type of it available.

OK.  But note that they did have the concept of unloading an active program from memory, called Rollout/Rollin, but you had to roll the program back in to the same memory location that it was rolled out from.

 Implementing the 360 architecture with the addition of a base and bounds
mechanism instead of full-blown virtual memory was perfectly possible.
However, the System/360 was originally conceived as a computer for use
in batch processing.

But batch processing didn't mean only one program at a time loaded into memory.  If only one program was loaded in memory at a time, you wouldn't need any base registers, as all addresses could be zero relative.  Even IBM's two major operating systems, DOS, and OS supported multi-programming.

Hence, TSS/360 was a kludge and ran slowly, and it
took the 360/67 with special hardware to facilitate timesharing for IBM
to have something that addressed that function effectively.

True, but irrelevant.
--
  - Stephen Fuld
(e-mail address disguised to prevent spam)