Retro Replay Retro Gaming History, Reviews & Emulation
Larry M. Moss occupies a small but enormously important place in the history of computing. Decades before Nintendo, Sega, Sony or arcade machines became the targets of software emulators, Moss was helping IBM solve essentially the same problem: how could software written for one computer continue running after the original hardware had been replaced?
The answer developed during IBM’s System/360 project helped establish what became known as computer emulation. Moss’s contribution belongs to a world of corporate mainframes rather than video games, but the principle behind his work would eventually become fundamental to the preservation and recreation of obsolete computer and gaming hardware.
IBM’s Compatibility Problem
In the early 1960s, IBM was preparing one of the most ambitious computer projects in its history. The company had built several successful computer families, but those machines were frequently incompatible with one another. Customers that upgraded hardware could face the expensive prospect of converting or rewriting software that had taken years to develop.
The IBM System/360, announced in 1964, was designed to change that model by creating a family of computers around a common architecture. Software written for one System/360 model could move much more easily to another member of the family.
That solved IBM’s compatibility problem for the future, but it did not automatically rescue software written for the company’s older machines. Customers still depended on programs created for systems such as the IBM 1401 and members of the IBM 7000 series.
Larry Moss and the Birth of Emulation
IBM engineers investigated ways to prevent those existing software investments from becoming obsolete. Historical accounts identify Larry M. Moss and Stuart G. Tucker as leaders in an effort that combined hardware, software and microprogrammed control to make System/360 machines execute software created for earlier IBM computers.
Computer historian Mark Smotherman’s history of microprogramming describes how the compatibility project combined portions of earlier software-simulation techniques with microprogrammed control. Instead of reproducing an older computer entirely through conventional software, portions of its behavior could be implemented much closer to the hardware.
This mattered because performance was crucial. It was already understood that one general-purpose computer could theoretically imitate another, but a purely software-driven simulation could impose such a severe performance penalty that the solution became impractical. IBM needed something its customers could actually use.
Moss reportedly believed the new technique deserved a stronger word than simulation or imitation. Because the replacement machine could equal and in some circumstances outperform the older hardware while executing its software, he used the term emulation.
The IBM 7070 Connection
One important target for this work was the IBM 7070 family. The 7070 belonged to an earlier generation of IBM commercial computers and used an architecture fundamentally different from System/360. Customers had nevertheless accumulated valuable programs for it.
System/360’s microprogrammed control made it possible for suitably configured models to reproduce the behavior expected by software written for machines such as the 7070. IBM’s new hardware could therefore replace an older machine without immediately forcing the customer to abandon all of the software associated with it.
An extensive Computer History Museum copy of IBM’s System/360 retrospective specifically names Moss and Tucker while describing the breakthrough that allowed System/360 to support emulators for older IBM systems.
Why Moss Matters to Video Game Emulation
Larry Moss never created an NES emulator, arcade emulator or PlayStation emulator. His work took place decades before the hobbyist emulation scene existed. Yet the problem he helped solve is immediately recognizable to anyone familiar with modern emulation.
Software expects a particular machine. The original machine is unavailable, obsolete or undesirable to use. Another system reproduces enough of the original machine’s behavior that the software continues operating.
That idea eventually allowed software to survive far beyond the commercial life of its original hardware. Mainframe programs could outlive the computers they were written for; decades later, video games would do the same.
Moss’s name is therefore important not because he created video game emulation itself, but because his work belongs to the technological lineage that made video game emulation possible.
Larry Moss in the History of Video Game Emulation
1960–1989: Before Console Emulation — How the Idea Was Born
Moss appears at the beginning of the Retro Replay timeline as one of the IBM engineers whose System/360 compatibility work helped establish computer emulation decades before game-console emulators appeared.
Related People
Stuart G. Tucker — IBM engineer who worked alongside Moss on System/360 emulation and later published important technical papers explaining emulation and microprogrammed control.