Journal of Computer and Communications, 2016, 4, 1-38 http://www.scirp.org/journal/jcc ISSN Online: 2327-5227 ISSN Print: 2327-5219 DOI: 10.4236/jcc.2016.412001 October 18, 2016 Source Code Comparison of DOS and CP/M Bob Zeidman Zeidman Consulting, Cupertino, CA, USA Abstract In a previous paper [1], I compared DOS from Microsoft and CP/M from Digital Research Inc. (DRI) to determine whether the original DOS source code had been copied from CP/M source code as had been rumored for many years [2] [3]. At the time, the source code for CP/M was publicly available but the source code for DOS was not. My comparison was limited to the comparison of the DOS 1.11 binary code and the source code for CP/M 2.0 from 1981. Since that time, the Computer History Museum in Mountain View, California received the source code for DOS 2.0 from Microsoft and was given permission to make it public. The museum also received the source code for DOS 1.1 from Tim Paterson, the developer who was originally con- tracted by Microsoft to write DOS. In this paper, I perform a further analysis using the newly accessible source code and determine that no code was copied. I further conclude that the commands were not copied but that a substantial number of the system calls were copied. Keywords Copyright Infringement, CP/M, Digital Research, DOS, Intellectual Property, Microsoft, Operating Systems, Software Forensics 1. CP/M Oddities The DOS files were written in standard Intel assembly language syntax, but some CP/M files used a variation I call DRI assembler that was created at DRI while other files were written in the PL/M programming language developed at DRI. In particular, I found that an exclamation point could be used to separate multiple instructions on a single line. I eventually found an assembler user’s guide from DRI [4] that confirmed this syntax. 1.1. Cleaning the Code For CP/M version 1.3, the code consisted of low-resolution PDF scans of dot matrix printouts of source code. I performed a number of processes to recover the source code How to cite this paper: Zeidman, B. (2016) Source Code Comparison of DOS and CP/ M. Journal of Computer and Communica- tions, 4, 1-38. http://dx.doi.org/10.4236/jcc.2016.412001 Received: August 20, 2016 Accepted: October 15, 2016 Published: October 18, 2016 Copyright © 2016 by author and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access