Introduction to the Processor

Introduction to the Processor

Chapter Topics:

• Machine operations and machine language
• Example of machine language
• Different types of processor chips
• High level programming languages
• Language translators (compilers)
• Language interpreters

From previous chapters you know that the major hardware components of a computer system are:

• Processor
• Main memory
• Secondary memory devices
• Input/output devices

This chapter examines the "brain" of the computer system — the processor. Then it shows how the programs you write are translated into instructions for the processor.

Source: Bradley Kjell, http://programmedlessons.org/Java9/chap04/ch04_01.html
This work is licensed under a Creative Commons Attribution-NonCommercial 3.0 License.

Electronic Operations of a Processor

Electronic Operations of a Processor

When a program is running on a computer the processor is constantly performing very many tiny electronic operations.

For example, one such operation reads one byte of data from main memory into the processor. Another operation tests if one of the bits in a byte is a 1 bit or a 0 bit. Most processors are able to perform several thousand types of tiny operations like these, and can perform billions of them per second.

Those are the only things that a processor can do. It has a set of tiny electronic operations that it can perform, and that is all. These tiny electronic operations are performed one at a time. But billions of them are performed per second, and billions of small operations can add up to a large and useful action.

Everything that a processor does is built out of these tiny operations! Luckily, you don't need to know the details of these operations to write programs in Java.The purpose of a high-level language like Java is to organize the tiny electronic operations into large, useful units represented by program statements.

Machine Instructions

Machine Instructions

Users and programmers of computers usually don't think about the billions of tiny electronic operations that go on each second. The situation is (very roughly) similar to when you are driving your car. You think about the "big operations" it can perform, such as "accelerate", "turn left", "brake", and so on. You don't think about tiny operations, such as the valves in your engine opening and closing 24,000 times per minute or the crankshaft spinning at 3000 revolutions per minute.

Each tiny electronic operation that a processor core can perform is called a machine operation. A processor (a "machine") performs these one at a time, but billions of them in a second.

A machine instruction consists of several bytes in memory that tell the processor to perform one machine operation. The processor looks at machine instructions in main memory one after another, and performs one machine operation for each machine instruction. A collection of machine instructions in main memory is called a machine language program or (more commonly) an executable program.

Don't panic if the above seems incomprehensible. It takes some getting used to. (And to really understand it all takes several courses.)

Example of Machine Language

Example of Machine Language

Machine Language Program

Machine Language Program

Executing Instructions

Executing Instructions

The word "execute" is often used to mean "perform the machine operation that an instruction asks for."So you can say that "execute the instruction 00000000 to stop the processor," or "billions of instructions execute per second." "Execute" is also used for an entire program or part of a program: "to execute the program, turn the switch to on."

Most programs have groups of instructions that are executed again and again. The light bulb program does this with an instruction that causes the processor to return to the beginning of the program and so to repeat what it just did.

A group of machine instructions that executes repeatedly is called a loop.

You will in the near future write programs in Java that run continuously. Underneath your program, way down at the machine level, there will be machine operations performing a loop.

A modern processor executes billions of instructions per second. A program without loops would execute for only a few seconds even if it had billions of instructions.

A typical processor is made up of millions of transistors on a small wafer of silicon called an integrated circuit (also called a chip.) The light bulb's processor could probably be built with just a few hundred transistors. Integrated circuits are used for other electronic parts of a computer. For example, main memory is implemented with memory chips.

Most modern processor chips have two or more processors on them. Each of the processors on the chip is called a "processor core" or often just called a core. Each core has the same set of tiny electronic operations and runs programs independently of the other cores. When a program is running, it runs on a single core, which performs the tiny electronic operations of the program one at a time.

Different Processors

Different Processors

There are many types of processors used in computer systems. You probably know something about the processors used in many desktop computers, Intel Corporation's Core processors. But there are other types of processors, such as the processors used in cell phones and game machines. A computer system is designed around its processor. The electronics of a computer system are designed for a particular type of processor.

Different types of processors have different machine operations and different machine languages. A machine language program for a typical Intel system would make no sense to a computer built around a different processor type.

However, the machine operations available with all types of processors can be used to build the same things. All processor types have enough power in their little machine operations to create the same large applications. Anything one processor can do with its machine language program another processor can do with a program written in its machine language. For example, cell phones are built around a variety of processor types, but all cell phones can do the same things.

The architecture of a processor is the choices that have been made for its machine operations, how they have been organized and implemented, and how they interact with main memory and other components. Architecture is concerned with the general plan and functions of a processor; it is not much concerned with electronic details. A course in computer architecture is part of most computer science departments.

High Level Programming Languages

High Level Programming Languages

int sum = 0;

Source Programs

Source Programs

Programmers create programs by writing commands in a high level language. A high level language program consists of lines of text that have been created with a text editor and are kept in a file on the hard disk. For example, here is a complete program in C (Java will be discussed later):

#include <stdio.h>
main()
{
 int sum = 0;
 sum = 2 + 2;
 printf( "%d\n", sum );
}

This program could be saved on the hard disk in a file called addup.c. Like all files, it consists of a sequence of bytes. Since it is a text file, these bytes represent character data. You can edit the file with a text editor and print the file on a printer. It does not contain machine instructions. If the bytes are copied into main memory, they cannot run as a program without some extra work being done.

A source program is a text file that contains instructions written in a high level language. It can not be executed (made to run) by a processor without some additional steps.

A source program is also called a source file, source code, or sometimes, just source.

Usually a source program is translated into a machine language program. An application program called a translator takes a source program as input and produces a machine language program as output.

A machine language program is also called an executable program, executable file, or sometimes, just executable.

For example, the C program addup.c could be translated into an executable program. The executable program might be called addup.exe and can be saved on the hard disk. Now the executable version of the program can be copied into main memory and executed.

The word compile means the same thing as translate. So one can say that a source program is compiled into an executable program.

Program Translation

Program Translation

The picture shows what happens with programs written in C (Java is different; it will be discussed in the next chapter.) Here is an overview:

1. The source program is created using a text editor.
   • It contains instructions in a high level language.
   • It contains bytes that represent characters.
   • The source program is kept on the hard disk.
   • The source program can not be run by the processor.
2. A translator (compiler) program translates the source program into an executable program.
   • The source program remains unchanged; a new executable program is created.
   • The executable program contains machine instructions.
   • A translator translates from a specific high level language (like C) into machine instructions for a specific processor type (like Pentium).
   • The executable program is also kept on hard disk.
3. The machine language program is copied from disk into main memory.
4. The processor directly executes these machine language instructions.

Program Execution

Program Execution

Once the source program has been translated into machine code (the executable program), the machine code can be directly executed by the processor. This is like the light bulb's processor directly executing its machine code. The source program is only a means to create the machine code.

For commercial software like games and word processors, the machine code is the product that is sold to the user. The user does not get a copy of the source program.

A student learning programming, or a programmer developing an application, creates source programs and translates them (with a compiler) into executable programs.

The above is what goes on with languages like Ada, Pascal, C, C++, FORTRAN and others. Java adds a few more steps, which will be discussed in the next chapter.

Portability

Portability

Ideally, only one program needs to be written in the high level language. That source file can then be translated into several executable files, each containing the correct machine instructions for its intended processor. This is how the same game can be made for desktop computers and game machines.

The idea of using one source file for executable programs that run on different processors is called software portability. You would like to write a program just once (in a high level language) and then to run it on any computer system by translating it into that system's machine language.

Usually, unfortunately, things do not work out that nicely. There are enough little problems so that it takes a substantial amount of human effort to get a source program running on  several different systems. Sometimes it is months before a game program that has been released for PCs is released for game consoles.

One of the big advantages of Java is that it is automatically portable between computer systems that have Java support. No human effort is involved at all.

Interpreter

Interpreter

Virtual Machine

Virtual Machine

Speed

Speed

End of the Chapter

End of the Chapter