A program execution model (PXM) is a precise specification of the features of a computer system that application programs use to meet goals for users of the system. Program execution models have been studied since the late 1960s.

The PXM that is most prevalent in commercial computation had its beginnings in the operating systems built in the 1950s and 1960s and consists of the concept of a process running in an address space. The features included in the model have grown over the decades as new demands have arisen, especially for support of file access methods and databases. The result is an unplanned agglomeration of features including duplicate access methods for data and support for concurrency based on the hardware facilities easiest for engineers to provide. A valuable property not supported by the conventional PXM is the ability to freely build large programs by combining program modules; the six principles of modular programming support are broken by the conventional PXM.

Attempts to modify the conventional PXM have attracted scant attention because it is essentially impossible to construct a better PXM without invalidating the massive collection of legacy software that depends on it for correct operation. With the arrival of the multi-core era, there is a new opportunity to make a significant move to a new basis for computing – one that supports program composition, including arbitrary parallel programs.

In this article, we recount the history of program execution models and review weaknesses of the current conventional PXM. We motivate and discuss the characteristics of a better programming model for massively parallel computing – One that supports sound program structure concepts including unconstrained composition of parallel programs, and also can achieve superior performance when implemented on appropriate hardware.

Program execution models are basic to understanding significant aspects of computer systems and the structure of programs that run on them. Further research and exploration of such models is highly warranted.