Chapter 12 x86 Assembly Language Programming

Table of Contents
12.1 Synopsis
12.2 The Tools
12.3 System Calls
12.4 Return Values
12.5 Creating Portable Code
12.6 Our First Program
12.7 Writing UNIX® Filters
12.8 Buffered Input and Output
12.9 Command Line Arguments
12.10 UNIX® Environment
12.11 Working with Files
12.12 One-Pointed Mind
12.13 Using the FPU
12.14 Caveats
12.15 Acknowledgements

This chapter was written by G. Adam Stanislav .

12.1 Synopsis

Assembly language programming under UNIX® is highly undocumented. It is generally assumed that no one would ever want to use it because various UNIX systems run on different microprocessors, so everything should be written in C for portability.

In reality, C portability is quite a myth. Even C programs need to be modified when ported from one UNIX to another, regardless of what processor each runs on. Typically, such a program is full of conditional statements depending on the system it is compiled for.

Even if we believe that all of UNIX software should be written in C, or some other high-level language, we still need assembly language programmers: Who else would write the section of C library that accesses the kernel?

In this chapter I will attempt to show you how you can use assembly language writing UNIX programs, specifically under FreeBSD.

This chapter does not explain the basics of assembly language. There are enough resources about that (for a complete online course in assembly language, see Randall Hyde's Art of Assembly Language; or if you prefer a printed book, take a look at Jeff Duntemann's Assembly Language Step-by-Step). However, once the chapter is finished, any assembly language programmer will be able to write programs for FreeBSD quickly and efficiently.

Copyright © 2000-2001 G. Adam Stanislav. All rights reserved.

This, and other documents, can be downloaded from

For questions about FreeBSD, read the documentation before contacting <>.
For questions about this documentation, e-mail <>.