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Computer System Software

Software is the coded instructions or programs, created by programmers and users to tell the computer what to do. Software can be characterize as either:

System Software - used to manage and support computing resources

Operating System - manages other programs' access to the ALU (arithmetic logic unit), main memory, and other hardware.

Communications Control Program - such as modem interface programs

System Support Programs - utilities such as virus detection programs, disk compression, disk fix

System Development - languages such as Fortran, Basic, C++, etc., Database Management Languages or CASE Tools

Middleware - a client/server class of software whose purpose is to simplify the complex problem of developing and using applications on different platforms of computers, connected over different types of networks.

Application Software - used to direct the performance of a particular use or application to meet end-user needs

This general discussion is directed toward desktop operating systems designed to use Intel x86 series CPUs used in most desktop computers -- other CPU designs are also roughly discussed.

Operating System Classification

There are several ways one may classify operating systems. In classifying operating systems, one may describe how the end-user interface is accomplished, or one may describe how processes or specified tasks are accomplished and how many simultaneous end-users may be using the system.

End-User Interface

From an end-user interface standpoint, operating system may be either

Command Driven where commands are types at a system

prompt (or executed through a program like script) or

Graphical User Interface (GUI) where the end-user uses a mouse or other pointing device to select objects that are assigned specific instructions to perform when activated -- uses Object Oriented Programming concepts.

Users / Tasks

In a processing mode, some computer systems may process only one series of tasks or instructions concurrently. Other types of systems may achieve multiprocessing where several tasks or instructions are processed concurrently. In a Complex Instruction Set Computing (CISC) architecture, the CPU is executing only one task, but within a larger time span, the CPU alternates between many tasks by interleaving their execution so that it appears that multi-tasking is being performed.

A somewhat related concept to multi-tasking is the number of end-users that the system may simultaneously support. In a multi-user operating system, it appears to multiple end-users that they have exclusive use of the system. Each end-user may be wishing the CPU to be executing several simultaneous tasks, with the status of each task needs to be stored in main memory (RAM). The number of users depends on a combination of the amount of main memory and how the operating system partitions main memory for use among the end-users. Multi-user operating systems have security features (ex. passwords) to protect the files and RAM space from other users of the system.

**Operating System Types**
Single-User Single-Tasking	MS-DOS (command-driven)
Single-User Multi-Tasking	Windows 95/98 (GUI) IBM - OS/2 (GUI) MAC-OS, System 7.5 (GUI)
Multi-User Multi-tasking	UNIX family (AIX, Xenix, Linix, etc.) (command driven) Windows NT, 2003 (GUI)

Elements of an Operating System

The operating system is a group of software programs that makes your applications programs and computer hardware work together. In general, the primary components of an operating system include:

Application Program Interface (API)

-- the point where application programs interact with the rest of the system; the set of commands that an application can issue to the computer system so that (for example) a file can be read, or data can be displayed, etc. Operating systems reside on one section of a system's RAM (random access memory) and application programs reside in another section of RAM. An application interfaces with the operating system through a system of well-defined interfaces called Application Program Interfaces (API). In Microsoft's case, Windows95/98 contains API for both DOS 3.1 (a 16 bit operating system) and for Win32 (the 32 bit kernel for Windows 95 and Windows NT). However Windows NT does not contain 16 bit APIs.

Kernel

-- the overall manager of the system that keeps track of multitasking, memory management, and device allocation (see Unix Kernel)

Device Drivers

-- the point where hardware devices interact with the rest of the system; programs that let the kernel take generic requests such as display this letter on the screen, and make it specific to a particular hardware device. The advantage to using device drivers is that as hardware is changed and upgraded, the entire operating system does not need to be modified, just the hardware's specific device driver.

Categories of Operating Systems

In addition to the characterizations of operating systems discussed in the Background section of this document, one may look at operating systems being used in three areas:

sales of Network Operating Systems

Workstation / Server / Network

UNIX

Windows NT & Windows NT Server

	UNIX? NT Time to Decide
	NT Terminal Server based on the Citrix Terminal Server, the basis for the Ultra-Thin Client (NC-Server) architecture

OS/2 Warp Server from IBM

Stepwise Server from Apple Computer

Open VMS from Digital Equipment Corporation

NetWare network operating system from Novell

Desktop

	Windows95/98 and Windows NT Workstation from Microsoft
	OS/2 Warp from IBM
	MacOS from Apple Computer
	Java and the Network Computer

Hybrid

VMWare - allows users to run multiple operating systems concurrently on a single PCwithout repartitioning or rebooting

Handheld / PCA

	Windows CE with ActiveX Controls
	GEOS from Geoworks
	Magic Cap from General Magic
	Network Computer Inc.'s operating system based on JAVA.

Operating system interfaces with the Internet, using tools such as

Internet Explorer and ActiveX (and OLE) controls

Netscape Communicator and

Sun Microsystem's Network computer using JAVA (and CORBA) controls.

The WinTel Model

Windows / InTel

An overview of the structure of Microsoft's Windows series of operating systems is available.

DOS vs. Win32

To understand the significance of APIs, look at the differences between DOS as an operating system and its Windows operating system successors (Win32 series). DOS was basically designed as a single-user single-tasking operating system. Although DOS performs typical operating system functions such as memory management and other low-level functions, programmers typically did not use DOS APIs. Instead, they wrote programs to go either directly into DOS device drivers, or into the hardware itself. The problem with this was that DOS applications tended to hog hardware access and memory regions, limiting its ability to provide multi-tasking operations. Additionally, if an application crashed, the entire system needed re-booting.

APIs & Win32

When Microsoft introduced the Windows applications (which ran on top of DOS) APIs were also introduced. These commands were eventually called Win 16 APIs because DOS was a 16 bit operating system. Windows 95 is a 32 bit operating system, requiring a new set of APIs. To take avoid scrapping the application programs written for the DOS/Windows environment, Windows 95 contains both the new Win 32 API and the older Win 16 APIs. Windows NT only contains Win32 APIs. Windows 95 in real-mode operates similar to the DOS/Windows systems. Windows 95 in protected mode uses the Win32 multi-tasking features discussed below.

Virtual Machine Manager

To multitask DOS programs, most current operating systems (OS/2, 95, NT) establish multiple (or virtual) PCs that each behave as if it were a separate computer. These virtual machines take advantage of the page mapping feature Intel designed into their 386 series processors. Where a DOS program may demanded the first 1MB of memory (i.e. address location 7000), with page mapping the virtual machine manager through its virtual memory manager can, for example, designate memory addresses from 4MB to 5MB to the DOS program as the virtual machines first 1MB of memory. Where a DOS program may lock onto a specific hardware device (i.e. I/O address 60 for a keyboard), the virtual machine manager through its virtual device driver would be an interface type of application that would allow many applications to think they each had exclusive control over the hardware device.

Virtual Memory

Virtual memory is a concept that allows an operating system to promise currently running applications more memory than is actually available by using disk space on your computer as if it were memory. Memory on 386 or later style computers is partitioned into 4K sections called pages. A virtual memory manager knows which pages are assigned to which applications, and where the pages are located (which ones are sitting in RAM, which are on a hard drive and may need to be moved into RAM, and which are promises.)

Intel Memory Management

The x86 processor is designed to accept privilege levels for execution of computer code. These privilege levels are designated by ring levels with ring 0 being the highest level and level 3 being the lowest level. Ring 0 programs can protect memory areas and detect when another program attempts to invade the space; a General Protection Fault. Ring 1 programs can protect memory areas so long as they do not override a command of ring 0, etc

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