Editing BASIC-PLUS (section)

===File processing===
The {{code|DIM#}} "virtual DIM" statement could map "virtual data array(s)" or "virtual array(s)" to a disk file, which allowed arrays larger than the computer's available memory (or even its address space), and allowed use of array elements to read, write, and extend disk files (persistent storage). They called this arrangement "virtual data storage" and "virtual core", but it did not use the modern approach of allocating the arrays and a [[memory-mapped file]]. Instead, a single buffer was used to store 512 bytes of data at a time, and when an entry in the virtual array was accessed, the corresponding data was read, and old data written, as required. The {{code|CLOSE}} statement caused the buffer to be written back (if necessary) before closing the file. Because no additional sectors were cached, accessing data in the "wrong" order could multiply the number of disk accesses. Additional rules were imposed on virtual arrays, such that one datum could never span a record boundary: Each data type was aligned to a multiple of its size. Virtual strings were stored as fixed-length ASCIIZ data, with sizes restricted to 2, 4, 8, 16, 32, 64, 128, 256, or 512 bytes, and were accessed using {{code|LSET}} and {{code|RSET}}.{{sfn|PLUS|1972|p=9-17}}