Virtual memory
Programs can run in either 70/45 mode—without virtual memory—or in 70/46 mode with virtual memory enabled. Virtual addresses are 24 bits in length. Pages can be specified to be either 2048 or 4096 bytes in length, depending on program requirements, however 2048 byte pages occupy the lower half of a page frame in memory. The system allows a maximum of 512 pages. Virtual memory is divided into "segments" of 64 pages indicated by bits 1-5 of a virtual address. Although the Instruction set architecture defines up to 32 segments, only eight are used in the 70/46. Incrementation of addresses wraps around on a segment boundary. With 4 KB pages, segments are 256 KB in length, and total virtual memory size is up to 2 MB. With 2 KB pages these numbers are halved.