"DOS addresses only 1 Megabyte of RAM because we can not imagine any applications needing more." Microsoft on the development of DOS. Development of MS-DOS/PC-DOS began. Microsoft( known mainly for thier programming languages) were commissioned to write the Operating System for the PC, Digital Research failed to get the contract (there is much legend as to the real reson for this.) DR's Operating System, CP/M-86 was later shipped but it was actually easier to adapter programs to DOS rather than CP/M-86 cost $ 495. As Microsoft didn't have an operating system to sell they bought Seattle Computer Product's 86-DOS which had been written by Tim Paterson ealier than year(86-dos was also know as Q-DOS, Quick & Dirty Operating System, it was a more-or-less 16 bit version of CP/M). The rights were actually bought in July 1981. It is reputed that IBM found over 300 bugs in the code when they subjected the operating system and re-wrote much of the code. Tim Paterson's DOS 1.0 was 4000 lines of the code.