26. How does the Net work? 1308

The secret of the Net is a network protocol called TCP/IP (transmission control protocol / Internet protocol). It’s, a kind of coding system that lets computers electronically describe data to each other over the network.

The term actually refers to two separate parts: the transmission control protocol (TCP) and the Internet protocol (IP). Together they form the Esperanto of the Internet. Every computer that hooks to the Internet understands these two protocols and uses them to send and receive data from any computer along the network.

TCP/IP creates a packet-switched network, a kind of network intended to minimize the chance of loosing any data that is sent over the wires.

First, TCP breaks down every piece of data – such as an e-mail message – into small chunks called packets, each of which is wrapped in an electronic envelope with Web addresses for both the sender and the recipient. The IP protocol then figures out how the data is supposed to get from point A to point B by passing through a series of routers.

Each router examines the destination addresses of the packets it receives and then passes the packets on to another router as they make their way to their final destination. If your e-mail was broken into ten packets, then each of those may have traveled a completely separate route.

27. How does e-mail work? 1488

E-mail is not that different from regular mail, actually: you have a message, an address, and a carrier that figures out to get it from here to there.

The difference is that e-mail messages--and any attachments--are broken down into small chunks of data called packets, which travel independently, weaving their way along with innumerable other packets traveling to different destinations. It's as if each page of a letter was mailed separately.

Along the way, the packets are passed from one server to the other until they reach their final destination. Any given message's packets and attached file may travel by several different routes, so the components often arrive out of order and at different times. Once all the packets have arrived, they are recombined into their original form.

This makes sending the message faster, because it doesn't require transmitting one big, bandwidth-hogging piece of data. But it also means that an entire message can be held up if one little piece is missing. Usually, however, this entire process, traveling 3,000 miles or more, takes less than a minute to complete.

Every time you send an e-mail or download a file, you're contributing to the load. Web pages are overloaded down with graphics and multimedia.

If you don't know a person's e-mail address it's actually pretty easy to track someone down on the Net. Web sites such as Who Where, Four11, and Bigfoot list individual and business e-mail addresses; all you have to do is type in the name.

28. Stored Program Architecture. 1406

Unlike other machines, computers can be programmed. That means that a list of instructions (the program) can be loaded into the computer and it will store them and carry them out some day.

In most cases, computer instructions are simple: add one number to another, move some data from one location to another, send a message to some external device, etc. These instructions are read from the computer's memory and are generally executed in the order they were given.

However, there are usually specialized instructions to tell the computer to jump ahead or backwards to some other place in the program and to carry on executing from there. These are called "jump" instructions (or branches).

Jump instructions may be made to happen conditionally so that different sequences of instructions may be used depending on the result of some previous calculation or some external event.

Program execution might be likened to reading a book. While a person will normally read each word and line in sequence, they may at times jump back to an earlier place in the text or skip sections that are not of interest.

Similarly, a computer may sometimes go back and repeat the instructions in some section of the program over and over again until some internal condition is met. This is called the flow of control within the program and it is what allows the computer to perform tasks repeatedly without human intervention.

29. Electronic Computer Memory.

Part 1. 1376

A computer is the most important innovation of our age. This multipurpose device is capable of handling enormous amounts of information at tremendous speeds. It’s possible by means of stored program.

A computer can store any kind of information in memory as long as it can be somehow represented in numerical form. Modern computers have billions or even trillions of bytes of memory.

Computer memory is a list of cells into which numbers can be placed or read. Each cell has a numbered "address" and can store a single number. These memory cells are called registers. There are typically between two and one hundred registers depending on the type of CPU.

Registers are used for the most frequently used data items to avoid having to access the main memory every time data is needed.

In almost all modern computers, each memory cell is set up to store binary numbers in groups of eight bits (called a byte). Each byte is able to represent 256 different numbers from 0 to 255.

The information stored in memory may represent practically anything. Letters, numbers, even computer instructions can be placed into memory with equal ease.

Electronic magnetic core memory was used in computers through the 1960s until it was completely replaced by semiconductor memory.

magnetic core memory схема памяти с магнитным сердечником

semiconductor memory. полупроводниковое устройство памяти