Virtual memory on a narrow machine for an object-oriented language

Abstract

LOOM (Large Object-Oriented Memory) is a virtual memory implemented in software that supports the Smalltalk-80(™) programming language and environment on the Xerox Dorado computer. LOOM provides 8 billion bytes of secondary memory address space and is specifically designed to run on computers with a narrow word size (16-bit wide words). All storage is viewed as objects that contain fields. Objects may have an average size as small as 10 fields. LOOM swaps objects between primary and secondary memory, and addresses each of the two memories with a different sized object pointer. When objects are cached in primary memory, they are known only by their short pointers. On a narrow word size machine, the narrow object pointers in primary memory allow a program such as the Smalltalk-80 interpreter to enjoy a substantial speed advantage. Interesting design problems and solutions arise from the mapping between the two address spaces and the temporary nature of an object's short address. The paper explains why the unusual design choices in LOOM were made, and provides an interesting example of the process of designing an integrated virtual memory and storage management system.