At 18:53 -0500 on 02/28/2007, (IBM Mainframe Discussion List) wrote
about Re: IBM S/360 series operating systems history:
MVT was first virtualized in early 1974 as OS/VS2 Release 1, better known as
SVS (Single Virtual Storage). A fuller version, OS/VS2 Release 2, was
available a year or so later, and it was quickly renamed MVS for
Multiple Virtual Storages. MFT evolved into VS1.
SVS was MVT with Virtual Storage (a 16Meg Machine). MVS split the
16Meg into 3 Areas - At the bottom and top of the memory space was
the Operating System and memory shared between the running Address
Spaces/Programs (the same mapping as with MVT and SVS). The big
difference from SVS was the 3rd area where the Programs ran - In MVS,
each program had its own copy of this address range so that it had
access to all the memory that was not reserved for the Operating
System (in MVT, SVS, and MFT/VS1 this area was shared between all the
running programs so there was a limit on the total combined size of
the running programs).
VS1 was similarly MFT with VS. The Program Area was divided into
Partitions (which were used to hold the running programs). When you
wanted to run a program, you scheduled it to run in a Partition that
was large enough to provide enough memory but not one that was much
larger (if possible) since once in use, you could find that all of
the large partitions were running small programs. Creating a Large
Partition by combining the memory from multiple small partitions,
required that they were adjacent.
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