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From: John Levine <johnl@taugh.com>
Newsgroups: comp.arch
Subject: Re: ancient OS history, ARM is sort of channeling the IBM 360
Date: Mon, 24 Jun 2024 02:52:05 -0000 (UTC)
Organization: Taughannock Networks
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According to Lawrence D'Oliveiro  <ldo@nz.invalid>:
>The difference was, with MFT, a program had to declare its memory 
>requirement before it could be started, and the only way to change that 
>was to stop the program and start it again. Whereas MVT allowed a program 
>to change its memory requirements while it was executing. (Whoah! Program 
>relocation requirement styleee!)

Nope. MFT partitioned memory into fixed sized areas when the system
started, MVT assigned each program as much memory as it said it
needed, and the areas could be reallocated between job steps. In every
case the JCL had to say how big a partition each job step needed. 

MFT II made it possible to change the partition sizes, but it was
still manual, as opposed to MVT which allocated partitions as needed.

Regardless of which flavor of OS you used, there was no way to
relocate a program once it had been loaded into memory.
Roll-out/roll-in was a primitive kind of swapping, but it swapped a
program out and later back into the same place.

>Eventually, the MFT→OS/VS1 line died a long-overdue death, and OS/VS2 
>became “MVS”. Not sure what other name changes happened along the way, but 
>nowadays this is known as “z/OS”.
>
>Does that make sense?

Not really. VS1 was basically MFT running in a single virtual address
space. The early versions of VS2 were SVS, MVT running in a single
virtual address space, and then MVS, where each job got its own
address space. As Lynn has often explained, OS chewed up so much of
the address space that they needed MVS to make enough room for
programs to keep doing useful work.

-- 
Regards,
John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. https://jl.ly