An operating system is the fundamental software layer that bridges computer hardware and user applications. In 1969, Ken Thompson and Dennis Ritchie at Bell Labs created UNIX, introducing hierarchical filesystems, pipes, and portable C architecture that influenced all subsequent systems.
By 1981, IBM selected Tim Paterson's QDOS (licensed and refined by Microsoft as MS-DOS) for the IBM Personal Computer (Model 5150), setting off fifteen years of command-line computing across millions of homes and businesses worldwide.
| OS Name | Release Year | Base Architecture | Min RAM Required | Key Landmark Feature |
|---|---|---|---|---|
| UNIX Version 1 | 1969 / 1971 | PDP-11 Assembly & C | 16 KB Core | Hierarchical filesystem, shell redirection pipes, multi-user capability. |
| CP/M-80 | 1974 | 8-bit Intel 8080 | 16 KB RAM | BIOS hardware abstraction layer separating OS from specific machine circuitry. |
| MS-DOS 3.3 / 6.22 | 1981 / 1994 | 16-bit x86 Real Mode | 256 KB - 640 KB | Standard FAT filesystem, batch scripting, and TSR background drivers. |
| Macintosh System 7 | 1991 | Motorola 680x0 | 2 MB RAM | Cooperative multitasking, Color QuickDraw GUI, and TrueType fonts. |
| Windows 95 | 1995 | 32-bit Protected Mode | 4 MB RAM (8 MB rec.) | Start Menu, Taskbar, Win32 API, Preemptive Multitasking, and Plug & Play. |
Maximizing conventional memory below the 640 KB barrier was an essential art for running 90s DOS games like DOOM and flight simulators: