CPM was the first disk operating system for microcomputers. After installing it on your computer, it allow you to type in preset of commands that controlled your system. It was revolutionary. Everyone building their own computer immediately installed CPM. There wasn't anything like it. It could it organize files on a magnetic storage medium, and to load and run programs stored on a disk.
When IBM announced the PC, you could buy CP/M for $75 or you could buy Microsoft DOS for $20. The problem with MS DOS was no programs run on it while there were hundred of programs running CP/M.
The reason IBM decided to sell CPM was they were afraid that Digital Research would sue them because MS DOS was a blatant copy. They actually delayed the PC announcements until they had a sign contract with Killdall. However they way over changed for it killing it sales.
CPM ERA -> DOS DELETE
CPM SAVE - > DOS COPY
Garry Killdall stated he had no intention of sueing anybody!