Page "MVS" Paragraph 5

In the early 1970s IBM sought to mitigate these difficulties by introducing virtual memory ( which IBM called " virtual storage "), which allowed programs to request address spaces larger than physical memory.

The original implementations had a single virtual address space, shared by all jobs.

OS / VS1 was OS / 360 MFT within a single virtual address space ; OS / VS2 SVS was OS / 360 MVT within a single virtual address space.

So OS / VS1 and SVS in principle had the same disadvantages as MFT and MVT, but the impacts were less severe because jobs could request much larger address spaces and the requests came out of a 16 MiB pool even if physical storage was smaller.