COMPUTER SOFTWARE Computer Languages

Machine Language

The actual binary instructions that are executed by the CPU are calledmachine lan-guage.There are disadvantages to writing a program in machine language. The pro-grammer must have a very detailed knowledge of the computer's construction to write such a program. Also, machine-language programs are very difficult to read and understand. For these reasons, programs are almost never written in machine language.

High-Level Languages

To make programming easier and to make programs more readable, high-levellan-guages, such as Fortran, Basic, Pascal, C, and Java, have been developed. These languages enable computer programs to be written using statements similar to simple English, resulting in programs that are much easier to read than those in machine language. Also, much less knowledge of the particular computer is required to write a program in a high-level language. Using a program written in a high-level language requires a program called acompiler or an interpreter to con-vert the statements written in the high-level language into machine-language instructions that the CPU can execute. Figure 4-8 shows a simple program in the language Basic.

10 1=1 20 PRINT I 301=1+1

40 IF 1<11THEN GOTO 20

50 END ^FIGURE 4-8. A simple program in the language Basic. The program prints the integers from 1 to 10 on the video monitor.

Applications Programs

An applications program is a program that performs a specific function, such as word processing, accounting, scheduling, or image processing. It may be written in machine language or a high-level language. A good applications program is said to beuser-friendly, meaning that only a minimal knowledge of the computer is needed to use the program.

Even when a computer is not executing an assigned program and seems to be idle, there is a fundamental program, called the operating system, running. When the user instructs a computer to run a particular program, the operating system copies the program into memory from a disk, transfers control to it, and regains control of the computer when the program has finished. The operating system also handles most of the complex aspects of input/output operations and controls the sharing of resoutces on multiuser computers. Common operating systems include Windows, Windows NT, and Linux for IBM-compatible PCs; the Mac OS on Apple com-puters; and Unix for workstations.

Most operating systems store information on mass storage devices asfiles. A sin-gle file may contain a program, one or more images, patient data, or a manuscript.

Each mass storage device has adirectorythat lists all files on the device and describes the location of each.

The operating system can be used to perform a number of utility functions.

These utility fu'nctions include listing the files on a mass storage device, determin-ing how much unused space remains on a mass storage device, deletdetermin-ing files, or making a copy of a file on another device. To perform one of these functions, the user types a command or designates a symbol on the computer screen using a point-ing device, and the operatpoint-ing system carries it out.

The goals of computer security are to deny unauthorized persons access to confi-dential information, such as patient data, and to protect software and data from accidental or deliberate modification or loss. A major danger in using computers is that the operating system, applications programs, and important data are often stored on a single disk; an accident could cause all of them to be lost. The pri-mary threats to data and software on computers are mechanical or electrical

mal-function, such as a disk head crash; human error, such as the accidental deletion of a file or the accidental formatting of a disk (causing the loss of all information on the disk); and malicious damage. To reduce the risk of data loss, important files should be copied ("backed up") onto floppy disks, optical disks, or magnetic tape at regularly scheduled times.

Programs written with malicious intent are a threat to computers. The most prevalent of these is the computer virus. A virus is a string of instructions hid-den in a program. If a program containing a virus is loaded into a computer and executed, the virus copies itself into other programs stored on mass storage devices. If a copy of an infected program is sent to another computer and exe-cuted, that computer becomes infected. Although a virus need not do harm, some deliberately cause damage, such as the deletion of all files on the disk on Friday the 13th. Viruses that are not intended to cause damage may interfere with the operation of the computer or cause damage because they are poorly written. There are three types of viruses-executable file infectors, boot-sector viruses, and macroviruses. Executable file infectors infect files containing exe-cutable instructions. An infected file must be imported and executed to trigger one of these. Boot-sector viruses infect the boot sectors of floppy disks. A boot sector is a small portion of a disk that contains instructions to be executed by the computer when power is first turned on. Turning power on to an IBM-compat-ible personal computer, if an infected floppy is in the floppy drive, will cause an infection. Macroviruses infect macros, programs attached to some word process-ing and other files. A computer cannot be infected with a virus by the importa-tion of data alone or by the user reading an e-mail message. However, a com-puter can become infected if an infected file is attached to an e-mail message and if that file is executed.

There are other types of malicious programs. These include Trojan horses, grams that appear to serve one function, but have a hidden purpose; worms, pro-grams that automatically spread over computer networks; and password grabbers, programs that store the passwords of persons logging onto computers for use by an unauthorized person. The primary way to reduce the chance of a virus infection or other problem from malicious software is to establish a policy forbidding the load-ing of storage media and software from untrustworthy sources. Commercial virus-protection software, which searches files on storage devices and files received over a network for known viruses and removes them, should be used. The final line of defense, however, is the saving of backup copies. Once a computer is infected, it may be necessary to reformat all disks and reload all software and data from the backup copies.

More sophisticated computer operating systems, such as Unix, provide security features including password protection and the ability to grant individual users dif-ferent levels of access to stored files. Measures should be taken to deny unauthorized users access to your system. Passwords should be used to deny access, directly and over a network or a modem. Computers in nonsecure areas should be "logged off"

when not in use. Each user should be granted only the privileges required to accom-plish needed tasks. For example, technologists who acquire and process studies and interpreting physicians should not be granted the ability to delete or modifY system software files or patient studies, whereas the system manager must be granted full privileges to all files on the system.

4.6 STORAGE, PROCESSING, AND DISPLAY OF DIGITAL