INTRODUCTION OS/2 was initially designed to extend the capabilities of DOS by IBM and Microsoft Corporations. To create a single industry-standard operating system. Initially the OS/2 was developed from the MT-DOS and DOS 3.2 source code bases and did not include the Presentation Manager (PM) GUI. The later versions included the PM Designed to support radical changes in hardware, from older 8- or 16-bit microprocessors to new high-powered 32-bit processors. CS 550-Main Campus Fall 2001

REQUIREMENTS TO DEVELOP OS/2 Break the 640KB physical memory barrier and support up to 16MB of physical memory  Make use of Virtual Memory to extend the Physical Memory resource of a system  To provide a protected Multitasking environment  Provide an extendible, flexible system Application Program Interface (API)  Provide a Graphical User Interface (GUI)  Support DOS application binary compatibility to encourage migration from DOS to OS/2. CS 550-Main Campus Fall 2001

SYSTEM STRUCTURE 1 - Hardware 2 - Device Drivers 3 - OS/2 Kernel   Figure 1   SYSTEM STRUCTURE 6     7   8  Figure 1 5   1   - Hardware 2    - Device Drivers 3    - OS/2 Kernel 4    - Dynamic Link Libraries 5    - OS/2 API 6    - User Shell / Desktop Manager 7    - Application Programs 8    - Utilities 4 3 2   1   CS 550-Main Campus Fall 2001

SYSTEM STRUCTURE - contd   Figure 1   SYSTEM STRUCTURE - contd The OS/2 system is composed of the kernel, device drivers, dynamic link libraries, and applications programs.   As in DOS, the OS/2 kernel uses device drivers to access the system’s hardware resources. The most critical portions of the operating system, such as multitasking, memory management, inter-process communication, DOS compatibility, and I/O reside in the kernel. Many of the system’s API is located in the kernel, but some of them are located in the dynamic link libraries – shared libraries that can be used to extend the functionality of the system. CS 550-Main Campus Fall 2001

  Figure 1   MULTITASKING The OS/2 multitasking architecture provides the capability to execute programs concurrently in a protected environment. The model consists of a hierarchy of multitasking objects called sessions, processes, and threads. OS/2 is a preemptive, priority based, multitasking system. The scheduler implements a multilevel priority scheme with dynamic priority variation and round-robin scheduling within a priority level. OS/2 is interrupt driven to allow the processor to be used while I/O is occurring. The 32-bit OS/2 system supports up to 4095 processes and threads CS 550-Main Campus Fall 2001

  Figure 1   MEMORY MANAGEMENT OS/2’s virtual memory model provides 4-gigabyte addressing The large 32-bit memory model frees the application developer from the memory constraints of the 16-bit segmented model. The 32-bit paging model achieves better utilization of memory and higher performance. The flat model is easily portable to most processor architectures Two forms of protection - Protection is among the processes in the system and isolation of the system from the user processes. The memory management component of the system is responsible for allocating process virtual address spaces and for setting up the required hardware structures to enable processes to be protected from one another and from the system. OS/2 provides functions for object allocation, deallocation, and sharing. CS 550-Main Campus Fall 2001

  Figure 1   DYNAMIC LINKING Dynamic linking allows the binding of code and data references to be delayed until the program is actually loaded or until the program specifically requests the operating system to link dynamically to a dynamic link library (DLL). Two types of dynamic linking - load-time dynamic linking, and run-time dynamic linking. There are two executable modes in OS/2 environment: EXE modules for programs and DLL modules containing shared libraries. Dynamic linking is a powerful mechanism for providing linkages to shared code and data objects in a multitasking virtual memory environment. CS 550-Main Campus Fall 2001

  Figure 1   I/O OS/2 can be categorized in two categories: system I/O and user I/O. Processes use system I/O to perform file level; I/O to secondary storage devices managed by the file systems. Processes utilize user I/O to interact with the user by employing keyboard, display, and mouse. OS/2 supports the FAT file system used by DOS, and consequently can read and write DOS files. OS/2 provides an alternative file system, called the High Performance File System (HPFS) to support many concurrent I/O requests from different processes on large block devices. OS/2 has an installable file system (IFS) to provide an architecture in which programs are transparent to the type of file system. Device drivers run in the most privileged execution state, privilege level 0 CS 550-Main Campus Fall 2001

  Figure 1   PAGING The paging feature is not only used to support the flat model and multiple DOS address spaces, but also to allow OS/2 to provide memory overcommitment. The storage can be virtualized on fixed disk media at a much lower I/O cost, because the size of the page is not variable. It makes use of demand paged, virtual memory system and is designed so that the system will run acceptably in nominally overcommited situations. CS 550-Main Campus Fall 2001

  Figure 1   OS/2 API OS/2 provides the dynamically linked 32-bit API to allow flat-model applications to use the OS/2 system services The 32-bit API has been designed so that applications and subsystems that use and provide 32-bit APIs will be portable to any future OS/2 platform. The multitasking API provides better thread management in the areas of creation and termination, and the system supports up to 4095 processes and 4095 threads. The 32-bit API uses the Presentation Manager (PM) for managing the user I/O. CS 550-Main Campus Fall 2001

INTERPROCESS COMMUNICATION   Figure 1   INTERPROCESS COMMUNICATION Inter-process communication primitives provide the basic features for data sharing and thread synchronization. The IPC facilities of the OS/2 system are organized into a tiered hierarchy based on the complexity of the IPC mechanism. The simplest IPC mechanisms are shared memory, semaphores, signals, and exceptions. Queues and named pipes are examples of higher-level abstractions that allow processes to exchange data and to synchronize their execution. The highest-level abstraction is the API call. CS 550-Main Campus Fall 2001

PRESENTATION MANAGEMENT   Figure 1   PRESENTATION MANAGEMENT Sessions, or screen groups, are managed by the session manager, a component of OS/2. The user shell allows the users to start or stop applications, and to select the foreground session The Presentation Manager is the graphical user interface of OS/2. It extends the functionality of the base user I/O services to include a windowed user interface and device-independent graphic presentations. The PM is divided into two functional groups: the windows API and the graphics API CS 550-Main Campus Fall 2001

  Figure 1   DOS COMPATIBILILTY OS/2 provides DOS compatibility using the virtual mode OS/2 uses paging to provide more than one DOS compatibility environment. DOS applications can be run full screen, windowed, or iconized in the background. Since DOS environments are swappable, starting many DOS sessions does not drive up the system memory requirements. CS 550-Main Campus Fall 2001

  Figure 1   CONCLUSION The OS/2 is an advanced single-user, multitasking personal computer operating system that exploits advanced hardware platforms. It provides a demand-paged system with a 32-bit programming model that is portable to other 32-bit processor architectures OS/2 is compelling because it offers the best system for running 16-bit DOS and Windows applications while moving into the more flexible and powerful 32-bit world. OS/2's object-oriented interface and free technical support are compelling factors OS/2 is smallest of the three major operating system (Linux, Windows NT, and OS/2) when using a graphical interface. OS/2 is the strongest of the three for backward compatibility with DOS and Windows. It is a high performance, highly reliable, information accessible, and integrated operating system CS 550-Main Campus Fall 2001