An improved disk-directory structure that included information about a file's attributes (such as whether it was a system or a hidden file), its exact size in bytes, and the date that the file was created or last modifiedIn only seven years, MS-DOS has evolved from a simple program loader into a sophisticated, stable operating system for personal computers that are based on the Intel 8086 family of microprocessors (Figure 1-1). MS-DOS supports networking, graphical user interfaces, and storage devices of every description; it serves as the platform for thousands of application programs; and it has over 10 million licensed users——dwarfing the combined user bases of all of its competitors.
The progenitor of MS-DOS was an operating system called 86-DOS, which was written by Tim Paterson for Seattle Computer Products in mid-1980. At that time, Digital Research's CP/M-80 was the operating system most commonly used on microcomputers based on the Intel 8080 and Zilog Z-80 microprocessors, and a wide range of application software (word processors, database managers, and so forth) was available for use with CP/M-80.
To ease the process of porting 8-bit CP/M-80 applications into the new 16-bit environment, 86-DOS was originally designed to mimic CP/M-80 in both available functions and style of operation. Consequently, the structures of 86-DOS's file control blocks, program segment prefixes, and executable files were nearly identical to those of CP/M-80. Existing CP/M-80 programs could be converted mechanically (by processing their source-code files through a special translator program) and, after conversion, would run under 86-DOS either immediately or with very little hand editing.
Because 86-DOS was marketed as a proprietary operating system for Seattle Computer Products' line of S-100 bus, 8086-based microcomputers, it made very little impact on the microcomputer world in general. Other vendors of 8086-based microcomputers were understandably reluctant to adopt a competitor's operating system and continued to wait impatiently for the release of Digital Research's CP/M-86.
In October 1980, IBM approached the major microcomputer-software houses in search of an operating system for the new line of personal computers it was designing. Microsoft had no operating system of its own to offer (other than a stand-alone version of Microsoft BASIC) but paid a fee to Seattle Computer Products for the right to sell Paterson's 86-DOS. (At that time, Seattle Computer Products received a license to use and sell Microsoft's languages and all 8086 versions of Microsoft's operating system.) In July 1981, Microsoft purchased all rights to 86-DOS, made substantial alterations to it, and renamed it MS-DOS. When the first IBM PC was released in the fall of 1981, IBM offered MS-DOS (referred to as PC-DOS 1.0) as its primary operating system.
IBM also selected Digital Research's CP/M-86 and Softech's P-system as alternative operating systems for the PC. However, they were both very slow to appear at IBM PC dealers and suffered the additional disadvantages of higher prices and lack of available programming languages. IBM threw its considerable weight behind PC-DOS by releasing all the IBM-logo PC application software and development tools to run under it. Consequently, most third-party software developers targeted their products for PC-DOS from the start, and CP/M-86 and P-system never became significant factors in the IBM PC—compatible market.
In spite of some superficial similarities to its ancestor CP/M-80, MS-DOS version 1.0 contained a number of improvements over CP/M-80, including the following:
An improved disk-directory structure that included information about a file's attributes (such as whether it was a system or a hidden file), its exact size in bytes, and the date that the file was created or last modified
A superior disk-space allocation and management method, allowing extremely fast sequential or random record access and program loading
An expanded set of operating-system services, including hardware-independent function calls to set or read the date and time, a filename parser, multiple-block record I/O, and variable record sizes
An AUTOEXEC.BAT batch file to perform a user-defined series of commands when the system was started or reset
IBM was the only major computer manufacturer (sometimes referred to as OEM, for original equipment manufacturer) to ship MS-DOS version 1.0 (as PC-DOS 1.0) with its products. MS-DOS version 1.25 (equivalent to IBM PC-DOS 1.1) was released in June 1982 to fix a number of bugs and also to support double-sided disks and improved hardware independence in the DOS kernel. This version was shipped by several vendors besides IBM, including Texas Instruments, COMPAQ, and Columbia, who all entered the personal computer market early. Due to rapid decreases in the prices of RAM and fixed disks, MS-DOS version 1 is no longer in common use.
MS-DOS version 2.0 (equivalent to PC-DOS 2.0) was first released in March 1983. It was, in retrospect, a new operating system (though great care was taken to maintain compatibility with MS-DOS version 1). It contained many significant innovations and enhanced features, including those listed on the following page.
Support for both larger-capacity floppy disks and hard disks
Many UNIX/XENIX-like features, including a hierarchical file structure, file handles, I/O redirection, pipes, and filters
Background printing (print spooling)
Volume labels, plus additional file attributes
Installable device drivers
A user-customizable system-configuration file that controlled the loading of additional device drivers, the number of system disk buffers, and so forth
Maintenance of environment blocks that could be used to pass information between programs
An optional ANSI display driver that allowed programs to position the cursor and control display characteristics in a hardware-independent manner
Support for the dynamic allocation, modification, and release of memory by application programs
Support for customized user command interpreters (shells)
System tables to assist application software in modifying its currency, time, and date formats (known as international support)
MS-DOS version 2.11 was subsequently released to improve international support (table-driven currency symbols, date formats, decimal-point symbols, currency separators, and so forth), to add support for 16-bit Kanji characters throughout, and to fix a few minor bugs. Version 2.11 rapidly became the base version shipped for 8086/8088-based personal computers by every major OEM, including Hewlett-Packard, Wang, Digital Equipment Corporation, Texas Instruments, COMPAQ, and Tandy.
MS-DOS version 2.25, released in October 1985, was distributed in the Far East but was never shipped by OEMs in the United States and Europe. In this version, the international support for Japanese and Korean character sets was extended even further, additional bugs were repaired, and many of the system utilities were made compatible with MS-DOS version 3.0.
MS-DOS version 3.0 was introduced by IBM in August 1984 with the release of the 80286-based PC/AT machines. It represented another major rewrite of the entire operating system and included the important new features listed on the following page.
Direct control of the print spooler by application software
Further expansion of international support for currency formats
Extended error reporting, including a code that suggests a recovery strategy to the application program
Support for file and record locking and sharing
Support for larger fixed disks
MS-DOS version 3.1, which was released in November 1984, added support for the sharing of files and printers across a network. Beginning with version 3.1, a new operating-system module called the redirector intercepts an application program's requests for I/O and filters out the requests that are directed to network devices, passing these requests to another machine for processing.
Since version 3.1, the changes to MS-DOS have been evolutionary rather than revolutionary. Version 3.2, which appeared in 1986, generalized the definition of device drivers so that new media types (such as 3.5-inch floppy disks) could be supported more easily. Version 3.3 was released in 1987, concurrently with the new IBM line of PS/2 personal computers, and drastically expanded MS-DOS's multilanguage support for keyboard mappings, printer character sets, and display fonts. Version 4.0, delivered in 1988, was enhanced with a visual shell as well as support for very large file systems.
While MS-DOS has been evolving, Microsoft has also put intense efforts into the areas of user interfaces and multitasking operating systems. Microsoft Windows, first shipped in 1985, provides a multitasking, graphical user "desktop" for MS-DOS systems. Windows has won widespread support among developers of complex graphics applications such as desktop publishing and computer-aided design because it allows their programs to take full advantage of whatever output devices are available without introducing any hardware dependence.
Microsoft Operating System/2 (MS OS/2), released in 1987, represents a new standard for application developers: a protected-mode, multitasking, virtual-memory system specifically designed for applications requiring high-performance graphics, networking, and interprocess communications. Although MS OS/2 is a new product and is not a derivative of MS-DOS, its user interface and file system are compatible with MS-DOS and Microsoft Windows, and it offers the ability to run one real-mode (MS-DOS) application alongside MS OS/2 protected-mode applications. This compatibility allows users to move between the MS-DOS and OS/2 environments with a minimum of difficulty.
Figure 1-1. The evolution of MS-DOS.
Please refer to the printed book for this figure.
What does the future hold for MS-DOS? Only the long-range planning teams at Microsoft and IBM know for sure. But it seems safe to assume that MS-DOS, with its relatively small memory requirements, adaptability to diverse hardware configurations, and enormous base of users, will remain important to programmers and software publishers for years to come.