CP/M

the crap os CP/M (Control Program for Microcomputers) is an operating system from which MS-DOS was cloned. It was created by Digital Research, Inc. for Intel 8080 computers. The original proprietary version of CP/M from 1978 used up to 64K of RAM on an 8-bit processor. By version 3.1 (1983), it could work on a 16-bit processor with multi-user mode.