To and From Your Terminal

Even before you log on, you can begin a "dialog" with the computer. That might seem an odd way to describe what is going on. The computer is not a person, and its vocabulary is severely limited compared to yours. Yet, an exchange of information is taking place.

Before you log on, the computer displays a log on prompt:

   MPE/iX:

That is the computer's message: "I am waiting for an instruction." In this case, it is waiting for you to log on using the HELLO command.

After you log on, the computer displays a system prompt:

This message says "I am the operating system. What do you want to do?" Now you have at your disposal all of the built-in commands and whatever programs may be on your system that fall within your capabilities as defined by your user name, account name, and group name.

An exchange of information goes on every time that you press a key on your keyboard. Every key sends (as output) a number code (as input) to the computer.

Pressing the A key is input to the computer. If you press A and do nothing else, the key code for "A" is sent to the computer. The computer examines the code, determines that it is a letter, and acknowledges your input by sending back (echoing) a letter "A" to your video terminal. This happens so quickly that it appears instantaneous.

Your part of this dialog consists of typing (input) "A".

The computer's part of this dialog is: "I received something from you. Here is what I understood you to say."

Until you press Return, nothing beyond this simple input and output happens.




	TIP: Return says "Please interpret and carry out my instruction if you can."

If you now press Return, something happens: the computer attempts to find the significance of whatever you have entered.

If you are at the system prompt ( : ), the computer might--or might not--recognize what you typed. There is no built-in command called A. Unless someone has instructed the computer to accept "A" as being somehow significant, the computer will display an error message on your video terminal.

In general, an error message--there are many kinds--says: "I received something from you. I cannot understand it, or I cannot carry out your instruction. Here is what puzzled me."




	Worth Knowing: If your keyboard does not have a key labeled Return, the same function is performed by the Enter key. If this is the case, be sure to think "Enter" when you see Return in this guide. If your keyboard has both Enter and Return, the Return key is probably the key that tells the computer to accept what you type at your terminal. Computer keyboards do differ. If you are in doubt, look in the documentation that describes the operation of your terminal and find which key sends information to your computer.

Pressing A alone produces a simple response by the computer. Following A with Return, however, triggers a series of processes by the computer. The computer examines everything that preceded Return and attempts to determine its significance.

To do that, the computer examines several areas of its own memory to find the meaning of "A". If it cannot find any meaning, it displays an error message on your video screen.

You could, if you like, create a command file called A that would have significance for the computer. Once you did that, AReturn would be a command to the computer to execute the instructions in the command file that you created and called A. Command files are discussed in Chapter 8 “Command Files and Jobs”.

A is not very informative to human beings. Unless you are certain of remembering what you put into A, it would be better to call it something else, perhaps MYFIRST--something that would jog your memory about what the command file does.

What computers "know"




	TIP: Computers "know" only what they have been told. They are able to do only what they have been instructed to do.

When this guide speaks of the computer "knowing" or "understanding," it means only that someone has programmed the computer to react in a useful and predictable way to a given set of circumstances. Whoever has done that programming has anticipated those circumstances and has created rules by which the computer is able to determine its course of action. When the computer encounters a set of circumstances for which it has no rules to follow, it is likely to "hang" (to become temporarily paralyzed) or confused, and it may display a message indicating its confusion about your intentions.

There are experimental computers that do "learn." Among them are neural network computers that, in their design, attempt to mimic the activity of the human brain. Using artificial intelligence (AI) programs, they are able to add to or modify the rules (programs) by which they are governed and, consequently, are able to alter their own behavior over time. They are not electronic brains--although their design is modeled on the human brain--nor are they intelligent in any human sense of the word. Where such developments might lead is open to speculation.

Most computer programs can answer only "Yes" or "No" to questions that involve comparison: Does X equal Y? (Yes or No)--Is X bigger than Y? (Yes or No)--Is X smaller than Y? (Yes or No). They thrive on precision.

Artificial intelligence programs attempt to instruct the computer in the sort of vague comparisons that human beings make all of the time: almost, not quite, more or less, nearly, a little, many, few, big small, probably, maybe. Human beings use these sorts of comparisons when we make decisions that are based not on precise information but on estimations, guesses, or hunches.

Understanding your command

The scanner and parser are programs (to be accurate, they are parts of a program). Like any program, they are recorded instructions that the computer has been told to follow in well-defined circumstances.

Together the scanner and parser and the command interpreter (CI) examine what you have entered. For the computer, HELLO BUDGET,JOHN.FINANCE,REPORTSReturn constitutes a complete sentence of instruction. Return terminates the sentence. Spaces, commas, periods, slash marks (/), and other symbols are the punctuation of the sentence.




	Worth Remembering: You can examine a sentence on a page and make sense of it, even if all the punctuation is removed. It may take a moment to sort it out, but you can do it. However, exact punctuation is indispensable for the computer. The computer does not have your ability to make an educated guess to determine the meaning of an instruction.

The punctuation tells the parser where each word in the sentence begins and ends. The computer then examines each word to determine its meaning and significance.

The computer will look in its own memory to determine whether HELLO is a command or some other instruction that it can execute. In contrast, HELLOW is a misspelling. More significantly, the computer would not find in its memory any command called HELLOW and would conclude that you had made an error of some kind.

The computer will accept BUDGET as session name.
It will look in its own memory to determine whether a user called JOHN is on its list of users to accept. At the same time, it will make note of the capabilities attached to JOHN.
It will look to determine whether an account called FINANCE exists and whether the user JOHN is attached to the FINANCE account. At the same time, it will make note of the capabilities attached to FINANCE.
It will look to determine whether a group called REPORTS exists. At the same time, it will make note of the capabilities attached to REPORTS.

If the computer determines that all of the names (and any passwords) are correct, it will accept your logon command.

Acting on your command

Now what?

The command interpreter does not by itself cause your instructions to be executed. Its purpose is to make sense of what you enter at the keyboard and then to send that information to the operating system, of which the CI is part. The operating system is the central and indispensable program in any computer. All other programs that you might use are dependent upon--and, in a sense, are subordinate to--the operating system.

At the heart of a computer is the processor. In MPE/iX computers, it is called the system processing unit (SPU). In some computers, it may be called the central processing unit. It is hardware, a computer chip, or a collection of computer chips. The processor counts, performs all arithmetic functions, and changes information from one form into another form.

To and From Your Terminal

Technical documentation

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What computers "know"

Understanding your command

Acting on your command