SECTION 1

INTRODUCTION

MANUAL CONVENTIONS

Part 2 of the Introduction to Multics continues the discussion of Multics command language which was begun in Part 1. Its purpose is to explain some of the detailed uses possible with the basic components of the language - commands and arguments - and to equip the user with a wide range of features and conventions that make command language extremely flexible and easy to use. This is accomplished through sections that illustrate detailed command applications, explain the use of active functions, and present important command language features such as the star convention and exec_com. Also included is a section on important computer concepts and their specialized application in Multics. Finally, there is a list of additional glossary items that supplement those in Part 1 and that will be valuable as you become more fully acquainted with computers and, in particular, Multics.

The conventions and special symbols used in this manual are the same as those in Part 1.

Technical or other unfamiliar terms are underlined when used the first time and are included in the glossary (Appendix A).

When a command is referred to for the first time, its short name is shown in parentheses immediately following the long name. For example, print (pr).

Examples of lines printed on a terminal use exclamation points to indicate lines that the user types. These characters will not be typed by the system as a prompt to you, and they are not to be typed by you.

The ready message used in examples is the regular message printed by the system:

r 13:02 1.423 77

The first set of numbers tells the time of day on the 24-hour clock. The 13:02 indicates it is two minutes after one o'clock in the afternoon. The second set of numbers shows the amount of CPU time you've used since the last ready message, and the third number (77 here) indicates the number of pages of information brought into main memory from secondary storage since the last ready message. (See below for a discussion of CPU, memory and storage.)

But before we embark on this further discussion of Multics software, it may be interesting to you to understand the fundamental operation of computer hardware. Hardware is a term used to refer to the physical units that make up a computer system, the apparatus as opposed to the programs. Multics itself is not a computer; it is a software system, a system of sophisticated programs. As we shall see in the following discussion, there are a number of machines that comprise the hardware system that the Multics software runs on.

PERIPHERALS

Peripherals are machines that can be operated under computer control but do not perform the control and computational functions of the central computer. Terminals are peripheral devices with which you are probably already familiar. Others include keypunch machines and card readers, line printers, and storage devices such as magnetic tapes and disks.

Terminals

There are basically two types of terminals: printing or hardcopy terminals and video terminals, commonly called CRTs (cathode ray tube). Both have keyboards that resemble those on typewriters, and both accept output as well as input. The video screen shows your input and output on a television-like screen whereas hardcopy terminals print input and output on paper as you work and thereby provide immediately a printed record of your terminal session.

Card Readers

Card readers transfer programs and data punched on computer cards to the central computer. A necessary accompanying device is the keypunch, a typewriter-like machine with which you type characters onto computer cards.

Storage Devices

On most systems, storage consists of two parts - main memory and secondary storage. When information in secondary storage is to be processed, it is "brought into main memory where it can be manipulated more rapidly. On the Multics system, however, all information can be processed at the same high speed, so there is no essential difference between main memory and secondary storage. This is the special Multics feature called virtual memory, which is discussed in Section 7.

The principal device used for storage in the Multics system is the disk; all information stored in the virtual memory system is on disks. Information can be stored on tapes and cards in the Multics system, but when it is, it is not part of the virtual memory system. A special connection must be made in order to process that information.

Line Printers

Line printers are so called because they print out results from a computer one line at a time. Unlike terminals, line printers are strictly output devices; they do not accept input. Line printers do, however, print output at very high speeds, much faster than terminals.

CENTRAL PROCESSING UNIT (CPU)

The central processing unit is the nerve center of the computer system; it coordinates and controls the activities of all the other units and performs all the arithmetic and logical processes to be applied to data. We can consider it as being divided into three functional parts:

1.   an appending unit (APU)
2.   a control unit
3.   computation units

The appending unit locates information in memory and checks the access that the particular user has to the information. Multics software uses the APU to provide the unique level of security that protects information kept on the system.

The control unit acts as a synchronizer. It interprets instructions sent to it by the APU and issues the appropriate commands to the computation units.

There are two units in the CPU that perform computations: the operations unit and the decimal unit. Between them these two units perform basic computations such as addition, subtraction, division and comparison.

INPUT/OUTPUT MULTIPLEXER (IOM)

The input/output multiplexer processes information coming from terminals, card readers, and some parts of storage and returning to terminals or going to line printers. On some systems, a similar mechanism is called the input/output processor and sometimes it even resides within the CPU.

FRONT-END NETWORK PROCESSOR (FNP)

The front-end network processor is the piece of hardware incorporated into some systems to process information coming into and going out of the system "by way of communication channels (e.g., telephone lines). Thus all input from terminals, and output directed to terminals, goes through the FNP because the terminals are all connected to the computer by one type of communication channel or another- Card readers, on the other hand, do not necessarily send their input through the FNP; they do so only if they are connected to the computer by way of communication channels, as is the case when they are at a different site than the computer. The FNP must be at the same site as the central computer.

With the description of major hardware items now complete, we can construct an illustration of how the hardware used by the Multics system is interconnected:

Figure 1. Components of Multics Hardware

COMPUTER LANGUAGES

The CPU and front-end processor operate according to a coded language that is very intricate and hard for humans to use efficiently. When we put information into the computer, we use coded languages that are easy for us to understand. These languages are usually called higher level languages. Some that you've probably heard of already are FORTRAN, PL/1, and COBOL. But these languages must be translated into the machine's language before the computer can perform its work. A compiler is the thing that translates a higher level language into machine language.

The compiler is not a hardware item in the computer system; it is a program. It resides in storage and can be called when needed. As a matter of fact, there are usually several compilers with each computer because each higher level language has its own program to translate it into the machine's code. On the Multics system there are compilers for FORTRAN, PL/1, COBOL, BASIC, and APL.

There is also another translating program similar to a compiler called an assembler. It too translates a programming language into machine code. But instead of translating a higher level language like COBOL, it translates a programming language called assembly, a language whose code is a symbolic form of machine language that resembles English much less than higher level languages do.

But the commands, active functions, texts, etc. that you enter at a terminal are not written in either higher level languages or assembly language. Instead, you are typing in a language (much of it specific to Multics) that calls or activates programs that are already written and reside in the system. On Multics, these programs are written in PL/1, and, indeed, these programs are the system, the Multics software system. What you type at a terminal calls programs and supplies data for their running, and it is to this procedure that we will now turn our attention for the remainder of this manual.

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