A use of fast and slow memories in list-processing languages

A scheme is described which permitting a substantial increase in memory space utilized to store list-structured data. It consists in reducing to one level a nonhomogeneous store composed of fast (core) and slow (disk or drum) memories. The space available in slow memory is divided into pages each containing a given number of machine words. The reduction to a one-level memory is performed by a program which leaves the most often called pages in the fast memory. When a new page from slow store is requested, the page in core having the longest period of inactivity is transferred back to the slow store. The complete scheme has been implemented in connection with a LISP embedding into ALGOL, using an IBM 7044 with 32K of core memory and disks. Gains in memory space were about 100-fold. As often happens in programming applications the price of the additional space is computer time. Although the disks have an access time 104 times slower than core, tests indicate that the actual slow down varied from 3 to 10, depending on the number of pages available in the fast store.