Design of OS/2 by Michael S. Kogan, Harvey M. Deitel Outline 1.1 Introduction 1.2 DOS History 1.3 DOS Limitations 1.3.1 Memory Management 1.3.2 I/O Management 1.3.3 Multitasking 1.3.4 System Extendibility 1.3.5 Graphical User Interface OS/2 History 1.4.1 IBM–Microsoft Joint Development 1.4.2 Multisite Development 1.4.3 OS/2 1.0 1.4.4 OS/2 1.1 1.4 The Evolving Market 1.5.1 Microsoft Windows 1.5.2 DOS Expanded Memory 1.5.3 DOS Extended Memory 1.5.4 DOS Extenders 1.5.5 Intel 80386 1.5.6 DOS Multitaskers 1.6 OS/2 1.2 1.7 OS/2 1.3 1.8 Windows 3.0 1.9 OS/2 2.0 1.10 The 1990s Summary 1.1 INTRODUCTION This chapter reviews the history of personal computers and operating systems. It traces the evolution of personal computer hardware from the original IBM Personal Computer to the latest IBM PS/2, and examines how this process has affected the content and design of DOS and OS/2. Before 1980 most computers were mainframes and minicomputers, large computing resources that were mainly job- and transaction-processing systems. Operating system technology had evolved from its early simplistic control program stages to sophisticated multiprogrammed virtual memory systems such as VM, MVS, and later versions of UNIX. In the era of large, centralized computing resources, computer time was expensive, learning was time consuming, assistance was difficult to obtain, and computing resources were scarce. Users rarely had opportunities to interact privately with a local computing resource. The advent of the microprocessor and of inexpensive, off-the-shelf computer components enabled the creation of the first microcomputer systems. The Altair, a primitive computer kit based on the Intel 8080, was one of the most popular early systems. The Altair was surpassed by the Apple I and II computers created by Steve Jobs and Steve Wozniak. The Apples used the MOS Technology 6502 chip and included a keyboard and display. Also gaining acceptance were microcomputer systems configured with the Intel 8080 and Zilog Z80 processors. Besides the Apple, which had its own proprietary operating system, the Intel and Zilog systems principally ran the CP/M operating system. CP/M was primarily designed for 8-bit single-user microcomputers that had floppy-disk drives. At this point, during the late 1970s, IBM decided to spin off an independent business unit (IBU) to investigate the potential of an IBM microcomputer system. In the late 1970s, IBM used IBUs to respond rapidly to new opportunities, and granted them considerable freedom within IBM’s business processes. An IBU is similar to a venture capital operation that attempts to exploit evolving technologies. The IBU ultimately became the current Entry Systems Division (ESD) of the IBM Corporation, which is responsible for personal computer hardware and operating system development. 1.2 DOS HISTORY The DOS era of microcomputer operating system technology began when the first IBM Personal Computer (PC) was designed. The first IBM PC went beyond the current 8-bit technology available and used the then-new 16-bit Intel 8088 processor. This choice was made because the current 8-bit systems were being eclipsed by the newer 16-bit systems, and the 16-bit system architecture provided a base for more robust software. The 8088 and 8086 processors are functionally identical, but the 8088 was used in the IBM PC since it was cheaper to configure in hardware. The 8088 processor could address up to 1MB of memory; few designers could envision using all that memory in a desktop personal computer in 1979. IBM also chose the 8088 microprocessor because porting software from existing 8080-based systems to the 8088 would be relatively straightforward. With the hardware for the first PC under development, IBM sought to adapt existing software for the system. Developing a new operating system and software tools would have taken too long. IBM contracted Microsoft, at the time a new company, to provide a BASIC interpreter, assembler, and link editor for the machine. IBM chose Microsoft because of Bill Gates’s experience in writing the most popular BASIC interpreter to date for the Altair systems. Since many CP/M-based programs were available, IBM initially attempted to interest Digital Research, Inc. (DRI) in providing a 16-bit version of the CP/M operating system for the IBM PC. However, DRI did not foresee the success of the 8088 microprocessor and declined to participate in the venture. IBM then approached Microsoft and, after explaining the requirements, asked Microsoft whether it was interested in providing the operating system software as well as the tools. The main concern of both IBM and Microsoft at the time was whether Microsoft had the resources to develop both the software tools and the operating system in the time required. Realizing that writing a new system was not feasible due to the schedules, Microsoft acquired from Seattle Computing Products a CP/M clone called SCP- DOS. With the SCP-DOS technology as a base, Microsoft predicted that it could complete the operating system, and the original operating system agreement between IBM and Microsoft was established. In 1981, the first version of the DOS operating system, 1.0, was shipped for IBM PCs. The system supported PCs with up to 256KB RAM, two 180KB floppy-disk drives, and included a Basic Input Output System (BIOS) built into the system ROM. DOS 1.0 was similar to CP/M in the way it managed the diskette devices and files, and it provided the base platform for the first 8088 DOS applications. Since the primary data structure used by the DOS file system to map file blocks to diskette addresses was the file allocation table (FAT), the DOS file system became known as the FAT file system. Figure 1.1 illustrates the DOS system structure. In 1982, IBM began shipping PCs with 360KB floppy disk drives. Since the new diskette medium had a format different from that of the 180KB diskettes, DOS had to be updated. IBM shipped DOS 1.1 when the new diskette drives became available in 1982. IBM next enhanced its PC line in 1983 with the addition of the IBM PC/XT. The PC/XT had a hard disk that could store far more data than could traditional floppy diskettes. This development illustrates how mainframe technology was becoming less expensive and more widely available in the PC market. The IBM PC/XT also included a new system board that allowed 640KB of memory to be installed. At this point, a trend emerged that continues to this day in the computer industry: The hardware drives the software. The addition of a hard disk to the PC was a problem for DOS 1.1, since the FAT file system was written for floppy-disk systems in which a single 360KB diskette could contain a maximum of 64 files. This limitation had to be removed; even the smallest hard disks could hold 10MB of data. Responding to this requirement, the DOS team at Microsoft explored different hard-disk and file- allocation strategies to select one that would enable DOS software to exploit future improvements in storage technology. Merely extending the limit of 64 files per disk would yield too many files to manage in a single file space, so the Microsoft team chose a hierarchical file-management approach similar to the one found in UNIX. They implemented this approach to support both diskettes and hard drives. Another requirement for the next version of DOS was an architecture for extending the system to support different peripheral devices. This support took the form of device drivers, user-installable program modules that interface the DOS system and applications to devices. The version of DOS that included the hierarchical FAT file system, support for hard disks, and a device-driver model for extending the system was shipped as DOS 2.0 in 1983. IBM’s next PC enhancements involved providing faster systems with larger hard disks. The Intel 80286 chip was selected for the next-generation IBM PC, the PC/AT. The 80286 has two modes of operation called real mode and protected mode. In real mode, the 80286 functions as a fast 8088. In protected mode, the 80286 allows up to 16MB of memory to be addressed and provides features that support a protected multitasking environment. These protection features allow an operating system to separate the memory spaces associated with different programs. However, since the 80286 was not designed to allow existing DOS applications to run in protected mode, they could neither be executed concurrently, nor use more than 1MB of memory. Therefore, DOS applications used the 80286 as a fast 8088. Other operating systems—such as Intel’s RMX or Microsoft’s XENIX—used the protected mode of the 80286, but neither of these systems was considered to be a mainstream desktop system due to a lack of applications compared to the number of DOS applications. The PC/AT was also the first PC to use 1.2MB 5.25-inch diskette drives. Since several modifications to DOS 2.0 were necessary for the PC/AT hardware, DOS 3.0 was not released until August 1984. DOS 3.1 was released in 1985 to provide support for PC local-area networks (LANs). Another update to the system (DOS 3.2) was made for supporting 3.5-inch diskettes in 1986. Table 1.1 shows the evolution of the DOS operating system. 1.3 DOS LIMITATIONS Between 1983 and 1985, IBM, Microsoft, and most application developers began to be aware of certain limitations of DOS and the 8088 environment.