Code_ The Hidden Language of Computer Hardware and Software - Charles Petzold [139]
MS-DOS version 2.0, released in March 1983, was enhanced to accommodate hard disk drives, which at the time were small (by today's standards) but which would soon get much larger. The larger a disk drive, of course, the more files it can store. And the more files a disk can store, the more confusing it becomes to find a particular file or to impose any type of organization on the files.
The solution in MS-DOS 2.0 is called a hierarchical file system. This was added to the existing MS-DOS file system with a minimum number of changes. As you'll recall, a disk contains an area called a directory, which is a list of files that includes information about where the files are stored on the disk. In a hierarchical file system, some of these files might themselves be directories—that is, they're files that contain a list of other files. Some of these files might also be directories. The normal directory on the disk is called the root directory. Directories contained in other directories are called subdirectories. The directories (sometimes called folders) become a way to group related files.
The hierarchical file system—and some other features of MS-DOS 2.0—were borrowed from an operating system named UNIX, which was developed in the early 1970s at Bell Telephone Laboratories largely by Ken Thompson (born 1943) and Dennis Ritchie (born 1941). The funny name of the operating system is a play on words: UNIX was originally written as a less hardy version of an earlier operating system named Multics (which stands for Multiplexed Information and Computing Services) that Bell Labs had been codeveloping with MIT and GE.
Among hard-core computer programmers, UNIX is the most beloved operating system of all time. While most operating systems are written for specific computers, UNIX was designed to be portable, which means that it can be adapted to run on a variety of computers.
Bell Labs was, of course, a subsidiary of American Telephone & Telegraph at the time UNIX was developed, and therefore subject to court decrees intended to curb AT&T's monopoly position in the telephone industry. Originally, AT&T was prohibited from marketing UNIX; the company was obliged to license it to others. So beginning in 1973, UNIX was extensively licensed to universities, corporations, and the government. In 1983, AT&T was allowed back into the computer business and released its own version of UNIX.
The result is that there's no single version of UNIX. There are, instead, a variety of different versions known under different names running on different computers sold by different vendors. Lots of people have put their fingers into UNIX and left their fingerprints behind. Still, however, a prevalent "UNIX philosophy" seems to guide people as they add pieces to UNIX. Part of that philosophy is using text files as a common denominator. Many UNIX utilities read text files, do something with them, and then write another text file. UNIX utilities can be strung together in chains that do different types of processing on these text files.
UNIX was originally written for computers that were too large and too expensive for just one person to use. Such computers allow multiple users to interact with them simultaneously through a technique known as time-sharing. The computer is connected to multiple displays and keyboards called terminals. By quickly switching attention among all the terminals, an operating system can make it seem as if the computer is servicing everyone at the same time.
An operating system that runs multiple programs concurrently is known as a multitasking operating system, and obviously such an operating system is more complex than single-tasking operating systems such as CP/M and MS-DOS. Multitasking complicates the file system because multiple users might try to use the same files at the same time. It also affects how the computer allocates memory to