1.1 Terminology of Change

Computer power has increased exponentially for nearly fifty years [73] in many areas including processor, memory and mass-storage capacity, circuit density, hardware reliability and I/O bandwidth. The growth has continued in the past decade , along with sophisticated instruction pipelines on single CPUs, placement of multiple CPUs on the desktop and an explosion in network connectivity.

The dramatic increases in communication and computing power have triggered fundamental changes in commercial software.

• Large database and other business applications, which formerly executed on a mainframe connected to terminals, are now distributed over smaller, less expensive machines.

• Terminals have given way to desktop workstations with graphical user interfaces and multimedia capabilities.

• At the other end of the spectrum, standalone personal computer applications have evolved to use network communication. For example, a spreadsheet application is no longer an isolated program supporting a single user because an update of the spreadsheet may cause an automatic update of other linked applications. These could graph the data or perform sales projections.

• Applications such as cooperative editing, conferencing and common whiteboards facilitate group work and interactions.

• Computing applications are evolving through sophisticated data sharing, realtime interaction, intelligent graphical user interfaces and complex data streams that include audio and video as well as text.

These developments in technology rely on communication, concurrency and asynchronous operation within software applications.

Asynchronous operation occurs because many computer system events happen at unpredictable times and in an unpredictable order. For example, a programmer cannot predict the exact time at which a printer attached to a system needs data or other attention. Similarly, a program cannot anticipate the exact time that the user presses a key for input or interrupts the program. As a result, a program must work correctly for all possible timings in order to be correct. Unfortunately, timing errors are often hard to repeat and may only occur once every million executions of a program.

Concurrency is the sharing of resources in the same time frame. When two programs execute on the same system so that their execution is interleaved in time, they share processor resources. Programs can also share data, code and devices. The concurrent entities can be threads of execution within a single program or other abstract objects. Concurrency can occur in a system with a single CPU, multiple CPUs sharing the same memory, or independent systems running over a network. A major job of a modern operating system is to manage the concurrent operations of a computer system and its running applications. However, concurrency control has also become an integral part of applications. Concurrent and asynchronous operations share the same problems ”they cause bugs that are often hard to reproduce and create unexpected side effects.

Communication is the conveying of information by one entity to another. Because of the World Wide Web and the dominance of network applications, many programs must deal with I/O over the network as well as from local devices such as disks. Network communication introduces a myriad of new problems resulting from unpredictable timings and the possibility of undetected remote failures.

The remainder of this chapter describes simplified examples of asynchronous operation, concurrency and communication. The buffer overflow problem illustrates how careless programming and lack of error checking can cause serious problems and security breaches. This chapter also provides a brief overview of how operating systems work and summarizes the operating system standards that are used in the book.