Assembly Language
汇编语言
Introducing Assembly Language
Assembly language unlocks the secrets of your computer's hardware and software[1]. It teaches you about the way the computer's hardware and operating system work together and how application programs communicate with the operating system.
To understand a computer and it.s operating system fully, one needs to study software at various levels. One is the application program level, where such programs interact with DOS. Another is the high-level language level, where powerful statements are expanded into many machine instructions recognized directly by CPU (Central Processing Unit), as well as the way programs communicate with DOS.
What Is Assembly Language？
Assembly language is a programming language with a one-to-one correspondence between its statements and a computer's machine language. There is no single assembly language because there is no single type of computer CPU. Each assembly language is directly influenced by a computer's machine instruction set and hardware architecture.
Strictly speaking, IBM-PC assembly language refers to instructions recognized by the intel 8086-80486(CPU) microprocessor family. But there is such close interaction between the CPU, computer peripherals, the DOS operating system, and the macro assembler itself that our discussions will often include all these topics.
What Is An Assembler？
An assembler is a program that converts source-code programs into machine language. In this passage, we will refer to an assembler that generates machine instructions for IBM- compatible microcomputers. All such computers use the Intel family of microprocessors, beginning with the Intel 8088, through the Intel 80486 (and beyond). Our programs will run under the PC-DOS/MS-DOS operating system, version 3. 0 or later. The two best- known assemblers for the IBM-PC are MASM (Microsoft Assembler) and TASM (Borland Turbo Assembler).
Assembly language is a specific set of instructions for a particular computer system. It provides a direct correspondence between symbolic statements and machine language. An assembler is a program that translates a program written in assembly language into machine language, which may in turn be executed by the computer. Each type of computer has a different assembly language, because the computer's design influences the instructions it can execute.
Assembly language is called a low-level language because it is close to machine language in structure and function. We can say that each assembly language instruction corresponds to one machine instruction. In contrast, high-level languages such as Pascal[2], BASIC[3], FORTRAN[4], and COBOL[5]contain powerful statements that are translated into many machine instructions by a compiler.
Why Learn Assembly Language？
People learn assembly language for various reasons. The most obvious one may be to learn about the computer's architecture and operating system. You may want to learn more about the computer you work with and about the way languages generate machine code. Because of assembly language's close relationship to machine language, it is closely tied to the computer's hardware and software.
You may also want to learn assembly language for its utility. Certain types of programming are difficult or impossible to do in high-level language. For example, direct communication with the computer's operating system may be necessary. A high-speed color graphics program may have to be written using a minimum of memory space. A special program may be needed to interface a printer to a computer. Perhaps you will need to write a telecommunications program for the IBM-PＣ. Clearly, the list of assembly language applications is endless.
Often there is a need to remove restrictions. High-Ievel languages, out of necessity, impose rules about what is allowed in a program. For example, Pascal does not allow a character value to be assigned to an integer variable.[6] This makes good sense unless there is a specific need to do just that. An experienced programmer will find a way around this restriction or rule; nearly everything is left to the discretion of the programmer. The price for such freedom is the need to handle many details that would otherwise be taken care of by the programming language itself.
Assembly language's usefulness as a learning tool should not be underestimateＤ. By having such intimate contact with the operating system, assembly language programmers come to know instinctively is how the operating system works. This knowledge, coupled with knowledge of hardware and data storage, gives them a tremendous advantage when tackling unusual programming problems. They have a different viewpoint than programmers who know only high-level language.
Assembly Language Applications
At first, the assembly language programs presented later will seem almost trivial. Those new to assembly language often cannot believe the amount of work required to perform relatively simple tasks. The language requires a great deal of attention to detail. Most programmers don't write large application programs in assembly language. Instead, they write short, specific routines.
Often we write subroutines in assembly language and call them from high-level language programs. You can take advantage of the strengths of the high-level languages by using them to write applications. Then you can write assembly language subroutines to handle operations that are not available in the high-level language.
Suppose you are writing a business application program in COBOL for the IBM-PＣ. You then discover that you need to check the free space on the disk, create a subdirectory, write a file, and create overlapping windows, all from within the program. Assuming that your COBOL compiler does not do all this, you can then write assembly language subroutines to handle these tasks.
Let's use another example: you might have written a word processing program in C[7]or Pascal, but it performs slowly when.updating the screen display. If you knew how, you could write routines in assembly language to speed up critical parts of the application and allow the program to perform up to professional standards.
Large application programs written purely in assembly language, however, are beyond the scope of the person who has just finished this book. There are many people who write complete assembly language application program for the IBM-PＣ. The few programmers in this group are familiar with several machine architectures and assemblers, and have been programming professionally for at least several years. These fortunate individuals still had to start with a basic foundation, and this book is intended to help you acquire just that.
Above all, assembly language programmers must know their date, for without a detailed understanding of how each date type is stored(at the bit level) , one might make serious mistakes. High-Ievel programming languages intentionally shield programmers from implementation-specific details, in the name of convenience and source-code portability. Assembly language, in contrast, is highly machine-specific and imposes few, if any, restrictions.
Machine Language
Before we embark on a rather long and detailed study of assembly language, let's put it into perspective. A computer doesn't actually understand assembly language-it understands machine language. Machine language is a language made up of numbers, which can be interpreted by a computer's CPU. A CPU usually has a small program embedded directly in the chip, called microcode. The microcode interpreter translates machine instructions directly into hardware signals.
Machine language makes it possible for the CPU to perform ordinary tasks, such as moving numbers or performing arithmetiＣ. Each CPU has its own machine language; or, in the case of IBM-compatible computers, all CPUs that belong to the intel family (8088, 8086,80186,80286,80386,80486) share a common machine language. This is an example of a machine language instruction that moves 5 into the AL register: 10110000 00000101. The number is written in binary, a number system made up of only the digits 1 and 0. The first 8 bits are the operation code (op code), which identifies it as the instruction that moves an 8-bit number to the AL register. The second 8 bits are the operand[8]. The complete instruction moves the number 5 to a register called AL. Registers are high-speed storage locations inside the CPU. They are identified by two-letter names, such as AH, AL, or AX.
A CPU's instructions set is the set of machine instruciions that the CPU is able to execute. For the Intel CPU family, the instruction set is downward-compatible, meaning that an instruction that works on a lower-level processor will always work on a higher-level processor. For example, the MOV instruction[9]works on the 8088, and therefore must work on the 80286. But there are many advanced instructions for the 80286 that do not work on the 8088.
At one time, all programs were written in machine language. But it's easy to see that machine instructions are difficult for humans to read and write. This is why assemblers and compilers were created, which would convert more readable instructions, created by a text editor, into machine language. Instead of writing the machine instruction shown earlier, we would write the following in assembly language:
M o v ah, 5
Notes
[1]unlocks the secrets：揭开了……奥秘。
[2]Pascal(Philips Automatic Sequence Calculator)：菲利浦自动顺序计算机语言。
[3]Basic(Beginner's All-purpose Symbolic Instruction Code)：初学者通用符号指令码。
[4]Fortran(Formula Translator)：公式翻译程序设计语言，FORTRAN语言。
[5]COBOL(Common Business-Oriented Language)：面向商业的通用语言，COBOL语言。
[6]For example，Pascal does not allow a character value to be assigned to an integer variable．例如，Pascal语言就不允许给一个整变量赋予字符值。
[7]C：C语言，一种高级程序设计语言，由贝尔实验室开发成功。
[8]Operand：操作数；运算数。
[9]Mov instruction：数据传输指令。
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