Hi, Richard,
With any virtual memory system, more real "backing store" main memory is always "better" as less actual paging in and out (from disk or drum) will be required.
In the 1970s, customers almost never had 16 MB of "real" memory on any models of IBM 370, so for example on a 158-3 we had 2 MB of real memory; later after upgrading with 3rd party (Memorex) memory, it was increased to 6MB of real memory. I think the largest amount of memory I ever heard of on a model 168 was 8 MB. By the late 1970s, with Amdahl's 470 series and IBM's 3033, we start to see the possibility of machines with 16 MB or more of real memory. And of course special hardware or microcode was required to allow for > 16 MB of real memory. The virtual address space remained at a maximum of 16 MB (24 bits) until the arrival of 370 Extended Architecture (XA), with the possibility of 31-bits of virtual address space was introduced, in the early 1980s.
I think the Functional Characteristics manuals for the various models describe the maximum configurations.
Mark S. Waterbury
