Greg was the super coder at CDC who invented Mace, a late 60's replacement for the original CDC operating system, COS. Mansfield is mainly notable for: writing almost all of the operating system himself; the invention of internal code documentation; and the software engineering principle that code should be readable by human beings. His style was much imitated throughout the CDC programming community due its clarity of expression. His code withstood the test of time, and parts of his final Mace code, dating to around 1970, continued to exist in the final cyber-mode emulations of the early 1990's. He was regarded as a genius and hero among CDC customers.