Database security

Security is another important database issue. Data residing on a computer is under threat of being stolen, destroyed, or modified maliciously. This is true whenever the computer is accessible to multiple users but is particularly significant when the computer is accessible over a network. The first line of defense is to allow access to a computer only to authorized, trusted users and to authenticate those users by a password or similar mechanism. But clever programmers have learned how to evade such mechanisms, designing, for example, so-called computer viruses—programs that replicate themselves and spread among the computers in a network, “infecting” systems and potentially destroying files. Data can be stolen by devices such as “Trojan horses”—programs that carry out some useful task but contain hidden malicious code—or by simply eavesdropping on network communications. The need to protect sensitive data (e.g., for national security) has led to extensive research in cryptography and the development of encryption standards for providing a high level of confidence that the data is safe from decoding by even the most powerful computer attacks. The term computer theft, however, usually refers not to theft of information from a computer but rather to theft by use of a computer, typically by modifying data. If a bank’s records are not adequately secure, for example, someone could set up a false account and transfer money into it from valid accounts for later withdrawal.

БИЛЕТ 10 ВОПРОС 1

Virtual memory

Another area of operating-system research has been the design of virtual memory. Virtual memory is a scheme that gives users the illusion of working with a large block of contiguous memory space (perhaps even larger than real memory), when in actuality most of their work is on auxiliary storage (disk). Fixed-size blocks (pages) or variable-size blocks (segments) of the job are read into main memory as needed. Questions such as how much actual main memory space to allocate to users and which page should be returned to disk (“swapped out”) to make room for an incoming page must be addressed in order for the system to execute jobs efficiently. Some virtual memory issues must be continually reexamined; for example, the optimal page size may change as main memory becomes larger and quicker.

The allocation of system resources to various tasks, known as job scheduling, is a major assignment of the operating system. The system maintains prioritized queues of jobs waiting for CPU time and must decide which job to take from which queue and how much time to allocate to it, so that all jobs are completed in a fair and timely manner.

БИЛЕТ 11 ВОПРОС 1

Early programminglanguages

Programming languages are the languages in which a programmer writes the instructions that the computer will ultimately execute. The earliest programming languages were assembly languages, not far removed from the binary-encoded instructions directly executed by the machine hardware. Users soon (beginning in the mid-1950s) invented more convenient languages.

The early language FORTRAN (Formula Translator) was originally much like assembly language; however, it allowed programmers to write algebraic expressions instead of coded instructions for arithmetic operations. As learning to program computers became increasingly important in the 1960s, a stripped down “basic” version of FORTRAN called BASIC (Beginner’s All-Purpose Symbolic Instruction Code) was written by John G. Kemeny and Thomas E. Kurtz at Dartmouth College, Hanover, New Hampshire, U.S., to teach novices simple programming skills. BASIC quickly spread to other academic institutions, and, beginning about 1980, versions of BASIC for personal computers allowed even students at elementary schools to learn the fundamentals of programming.

At roughly the same time as FORTRAN was created, COBOL (Common Business-Oriented Language) was developed to handle records and files and the operations necessary for simple business applications. The trend since then has been toward developing increasingly abstract languages, allowing the programmer to think and communicate with the machine at a level ever more remote from machine code.

БИЛЕТ 12 ВОПРОС 1