Development

The earliest computers were started with rudimentary “loader” program that could be used to configure the system to run the main application program. Gradually a more sophisticated way to schedule and load programs, link programs together, and assign system resources to them was developed.

As systems were developed that could run more than one program at a time, the duties of the operating systems became more complex. Programs had to be assigned individual portions of memory and prevented from accidentally overwriting another program’s memory area. A technique called virtual memory was developed to enable a disk drive to be treated as an extension of the main memory, with data “swapped ” to and from the disk as necessary. This enabled the computer to run more and/or larger applications. The operating system, too, became larger, amounting to millions of bytes worth of code.

During the 1960s, time sharing became popular particularly on new smaller machines such as the DEC PDP series, allowing multiple users to run programs and otherwise interact with the same computer. Operating systems such as MULTICS and its highly successful offshoot UNIX developed ways to assign security levels to files access levels to users. The UNIX architecture featured a relatively small kernel that provides essential process control, memory management, and file system services, while drivers performed the necessary low-level control of devices and a shell provided user control.

Starting in the late 1970s, the development of personal computers recapitulated in many ways the earlier evolution of operating systems in the mainframe world. Early microcomputers had a program loader in read-only memory (ROM) and often rudimentary facilities for entering, running, and debugging assembly language programs.

During the 1980s, more complete operating systems appeared in the form of Apple DOS, CP/M, and MS-DOS for IBM PCs. These operating systems provided such facilities as a file system for floppy or hard disk and a command-line interface for running programs or system utilities. These systems could run only one program at a time (although exploiting a little-known feature of MS-DOS allowed additional small programs to be tucked away in memory).

As PC memory increased from 640 KB to multiple megabytes, operating systems became more powerful. Apple Macintosh operating system and Microsoft Windows could manage multiple tasks. Today PC operating systems are comparable in sophistication and capability to those used on mainframes.

An interesting development that began in the mid-1980s is the growth of networks of personal computers running network operating systems and distributed operating systems. Network operating systems are not fundamentally different from single-processor operating systems. They obviously need a network interface controller and some low-level software to drive it, as well as programs to achieve remote login and remote file access. But these additions do not change the essential structure of the operating system.

A distributed operating system, in contrast, is one that appears to its users as a traditional uniprocessor system, even though it is actually composed of multiple processors. Distributed systems often allow applications to run on several processors at the same time, thus requiring more complex processor scheduling algorithms in order to optimize the amount of parallelism.

Notes:

DEC PDP - Digital Equipment Corporation Programmed (Personal) Data Processor - торговая марка корпорации Digital Equipment для выпускающегося ею семейства недо­рогих миникомпьютеров.

MULTICS - Multiplexed Information and Computing Service - одна из первых операционных систем с разделением вре­мени исполнения программ

Apple DOS - Apple Disk Operating System - Дисковая опера­ционная система

CP/M - Control Program for Microprocessors-операционная систем CP/M - популярная в 1980-х гг. ОС для 8-разрядных ПК

MS/DOS - Microsoft Disk Operating System - дисковая опе­рационная система для компьютеров на базе архитектуры х86 (80-е годы - сер. 90-х годов)