The Users' Virtual Memory Space

The Time-sharing and Batch users running under the SINTRAN III system normally have a logical (or virtual) address space for a program of 128 Kbytes, all one one segment.

The transformation of a logical (virtual) address to a physical address is performed by the paging system by means of the hardware page index tables.

The virtual address space is like a contiguous area, but the corresponding physical pages may be scattered throughout the physical memory. When a part of the program which is not in memory is addressed, the SINTRAN III system will make it available in pages of 2 Kbytes. If there are no free pages in memory, the SINTRAN III system will make space available by writing one of the least recently used pages back on the intermediate store. Thus, the programs in in Time-sharing / Batch mode execute in a demand-page mode, and may utilize less physical memory than the logical address space should require, however, the turnaround time will be longer due to additional reading and writing of intermediate pages.

In addition the SINTRAN III system usees a 4 page system segment for each background process.