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1.3 Why Study Assembly Language?

The convenience and greater portability of high-level languages raise the very real question of why anyone should study assembly language. Is it some arcane rite of initiation for the truly computer-savvy? Or is the primary motivation to see how a computer really works?

In a purely intellectual sense, an in-depth appreciation of at least one computer architecture is incredibly helpful in trying to comprehend its most basic strengths and weaknesses. Moreover, in a pragmatic sense, assembly language may make it possible to accomplish the following:

the fastest attainable execution speed;
the least memory usage;
very specialized data manipulation, thereby compensating for features lacking in a particular high-level programming language; and
specialized device control, such as a device driver not regularly furnished with the operating system.

Hardware improvements over time certainly bring ever-faster program execution speeds. While the ever-greater densities of memory technology would seem to reduce most concerns about program size, occasions do still arise where the tightest, fastest code has significant value. Good software engineering teams know when to utilize assembly language, even when they primarily use 3GL and 4GL tools. Many compilers for the C programming language explicitly provide a means to embed brief sequences of assembly language into a program using the asm keyword or _Asm intrinsic functions.

To be sure, assembly language is relatively difficult to code, debug, and maintain. Most severely, assembly language lacks portability from one architecture to another and thus, in long- term value, suffers significantly in comparison to high-level languages. Hyde has discussed these and other objections to assembly language, but has offered further reasons why learning assembly language deserves the attention of aspiring and practicing computer scientists. In particular, good assembly language programmers make better high-level language programmers "because they understand the limitations of the compiler and they know what it's doing with their code." We will return to this concept in later chapters.

1.4 Prefixes for Binary Multiples

For certain topics, especially architecture, computer science is a quantitative science. We shall introduce appropriate discussions of number systems and related matters as they are needed. The first such discussion here treats the matter of scale factors and abbreviations for "big" numbers .

Treating 2 ¹⁰ (1024) and 10 ³ (1000) as synonymous introduced a sloppy situation for nomenclature in the fields of computer science, data processing, and data transmission. People were using the prefix kilo , from the International System of Units (SI), not only for its proper meaning of precisely 1000, but also to denote the binary multiple 1024. Doing so introduces an uncertainty of 2.4% in magnitude. The uncertainty compounds to more than 4.8% for the SI prefix mega, and the discrepancy just gets worse and worse for larger quantities . Moreover, kilo is typically abbreviated as a capital K in computer-related contexts, even though the proper SI abbreviation is a small k for kilo (which is an exception to the capitalization of most other abbreviated SI prefixes that denote multiples).

In 1998, the International Electrotechnical Commission (IEC) approved a new standard of names and symbols for the prefixes for binary multiples. The new names have mnemonic analogies to corresponding decimal multiples, as shown in Table 1-2.

Table 1-2. Prefixes for Binary Multiples

Factor	Name	Symbol	Origin	Analogy
2 ¹⁰	kibi	Ki	kilobinary: (2 ¹⁰ ) ¹	kilo: (10 ³ ) ¹
2 ²⁰	mebi	Mi	megabinary: (2 ¹⁰ ) ²	mega: (10 ³ ) ²
2 ³⁰	gibi	Gi	gigabinary: (2 ¹⁰ ) ³	giga: (10 ³ ) ³
2 ⁴⁰	tebi	Ti	terabinary: (2 ¹⁰ ) ⁴	tera: (10 ³ ) ⁴
2 ⁵⁰	pebi	Pi	petabinary: (2 ¹⁰ ) ⁵	peta: (10 ³ ) ⁵
2 ⁶⁰	exbi	Ei	exabinary: (2 ¹⁰ ) ⁶	exa: (10 ³ ) ⁶

The IEC-approved prefixes are not part of the International System of Units (SI), but they have been supported by the International Committee for Weights and Measures (CIPM) and the Institute of Electrical and Electronics Engineers (IEEE ).

We use these new prefixes in this book. For instance, in the next three tables of this chapter, you will see KiB and MiB denoting kibibytes and mebibytes, where you might have expected KB and MB denoting kilobytes and megabytes, respectively.

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Evans J. S., Trimper G. L.

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1.3 Why Study Assembly Language?