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Q&A: Liskov on Liskov

2008 ACM A.M. Turing Award winner Barbara Liskov

Credit: Jared Leeds

Barbara Liskov talks about her ground breaking work in data abstraction and distributed computing.

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CACM Administrator

The following letter was published in the Letters to the Editor in the December 2009 CACM (
--CACM Administrator

In Leah Hoffmann's interview "Liskov on Liskov" (July 2009) Barbara Liskov recalled research being conducted at the time she was beginning her career (1970). "When I started," she said, "the main way people thought about modularization was in terms of subroutines... But they didn't have any way of linking a bunch of procedures together." She also said (in Karen A. Frenkel's news item "Liskov's Creative Joy," also July 2009) "...the major challenge still is how to build large software systems."

The definition of "module" inevitably affects the building of large software systems, prompting our own recollections and connections. For example, a counterexample to the use of subroutines as the basic module existed in the form of a project that began in 1965 at Argonne National Laboratory (ANL) and was reported at the Spring Joint Computer Conference (May 1969) to develop the Argonne Reactor Computation System (ARC) from linked computational modules running on IBM System 360 computers and OS/360. The modules were FORTRAN IV main programs where the output of one program was analyzed to become the input for other programs. Reactor designers wanted three main features:

Superprograms. To link programs into superprograms directed by another FORTRAN IV main program while the original programs could still run on their own. They wanted all Assembler code and any direct communication with the manufacturer's software to be isolated for porting to other hardware. Initially, the most important Assembler code was for LINKing and LOADing modules for coordinating the I/O stream and communicating with JCL;

New algorithms. To improve the modules through new-algorithm rewrites wherever possible; and

New modules. To make it possible to build modules as required.

ARC programmers each had five to 10 years of experience (rare in 1965) and advanced degrees in mathematics, science, and engineering. They coded in Absolute and Assembler on ANL-built hardware (AVIDAC and GEORGE) and in FORTRAN on the IBM 704 and the CDC 3600. Because OS/360 was so unstable (as shipped in 1965), they needed to be able to decipher dumps. Completion of the project would have been delayed without programmers capable of working close to the hardware and operating system.

The ARC system later became the platform for ANL reactor calculations and was studied and used by other laboratories worldwide, though not before it was shown to be portable.

On the first Earth Day (Apr. 22,1970), a joint project with the Control Data Corporation aimed to port the ARC system to CDC hardware. We were aided in this effort by Richard Lee of CDC and Larry Atkins, a Northwestern University engineering student also known for writing Chess 3.6, the winner of several ACM North America Computer Chess Championships in the 1960s and 1970s. Once the modular environment was ported, the computational modules were easily ported. In fact, after one computational module was ported, the remaining work was almost automatic.

Louis C. Just
Lakewood, CO
Gary K. Leaf
Argonne, IL
Bert J. Toppel
Argonne, IL

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