Date: Tue, 21 Mar 2000 02:20:10 -0500 (EST) From: tms2@mail.ptd.net To: FreeBSD-gnats-submit@freebsd.org Subject: docs/17521: Proposed FAQ on assembly programming Message-ID: <200003210720.CAA01053@beowulf.bsd.home>
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>Number: 17521
>Category: docs
>Synopsis: Proposed FAQ on assembly programming
>Confidential: no
>Severity: non-critical
>Priority: medium
>Responsible: freebsd-doc
>State: open
>Quarter:
>Keywords:
>Date-Required:
>Class: change-request
>Submitter-Id: current-users
>Arrival-Date: Mon Mar 20 23:30:00 PST 2000
>Closed-Date:
>Last-Modified:
>Originator: Thomas M. Sommers
>Release: FreeBSD 3.2-RELEASE i386
>Organization:
>Environment:
The FAQ applies only to i386 machines.
>Description:
Information on assembly programming is not readily available. In
particular, the correct way to make syscalls is not at all obvious. The
proposed FAQ shows how to write a "Hello, world." program in assembly.
>How-To-Repeat:
>Fix:
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
<html>
<head>
<title>Hello, World in FreeBSD Assembler</title>
</head>
<body bgcolor=white>
<h1>How do I write "Hello, world" in FreeBSD assembler?</h1>
<p>This program prints "Hello, world." on the standard output, and then
exits with an exit status of 0. It is written for Intel
machines, to be assembled by the GNU
assembler, <code>as</code>. The syntax used by <code>as</code> is
different from Intel's, but is common in the Unix world. See
<code>man as</code> or
<code>info as</code> for details. This syntax is known as AT&T
syntax. The most important difference for
present purposes is that the order of operands is reversed: the
source operand comes first, then the destination.
<p>The program works by first calling write(2) to write the message, and
then calling exit(2) to exit.
<pre>
1: .data # Data section
2:
3: msg: .string "Hello, world.\n" # The string to print.
4: len = . - msg - 1 # The length of the string.
5:
5: .text # Code section.
6: .global _start
7:
8: _start: # Entry point.
10: pushl $len # Arg 3 to write: length of string.
11: pushl $msg # Arg 2: pointer to string.
12: pushl $1 # Arg 1: file descriptor.
13: movl $4, %eax # Write.
14: call do_syscall
15: addl $12, %esp # Clean stack.
16:
17: pushl $0 # Exit status.
18: movl $1, %eax # Exit.
19: call do_syscall
20:
21: do_syscall:
22: int $0x80 # Call kernel.
23: ret
</pre>
<p><code>_start</code> (line 8) is the default name for a program's entry
point.
<p>Arguments to system calls are placed on the stack from right to
left, just as in C. Lines 10 through 12 push the arguments for
write(2) on the stack, and line 17 pushes the argument for
exit(2). Note that the caller is responsible for cleaning up the
stack after control has returned from the <code>call</code>.
<p>System calls are made by putting the call's index in <code>EAX</code>
(lines 13 and 18), and then invoking <code>int $0x80</code> (line
22). <strong>Important:</strong> A <code>call</code> must be
made after
the arguments are placed on the stack and before the <code>int
$0x80</code>. If you replace <code>call do_syscall</code> (lines
14 and 19) with <code>int $0x80</code>, the program will not work.
<p>The kernel puts the system call's return value in <code>EAX</code>.
This program ignores the value returned by write(2).
<p>Assemble the program with (assuming you saved it as <code>hello.s</code>)
<pre> as -o hello.o hello.s</pre>
and link it with
<pre> ld -o hello hello.o</pre>
<p>It is also possible to invoke system calls using libc instead of
doing it directly through <code>int $0x80</code>.
<pre>
1: .data
2:
3: msg: .string "Hello, world.\n"
4: len = . - msg - 1
5:
6: .text
7: .extern write
8: .extern exit
9: .global main
10:
11: main:
12: pushl $len
13: pushl $msg
14: pushl $1
15: call write
16: addl $12, %esp
17:
18: pushl $0
19: call exit
</pre>
<p>Since we are linking with libc, we must also use the C startup code,
which means that the entry point to our program is now
<code>main</code> (line 11) instead of <code>_start</code>.
<p>The easiest way to assemble and
link this program is through <code>cc</code>, which will take care
of linking in the proper startup modules in the correct order:
<pre> cc -o hello hello.s</pre>
<h3>Resources</h3>
<p>There is a lot of information available about assembly programming on
Intel machines, but little if any of it applies to FreeBSD
specifically (hence this FAQ). All of the books I have seen, and
most of the Web sites, are
about programming in an MS-DOS environment. These books can be
useful for a FreeBSD programmer to the extent that they discuss
general principles or the Intel instruction set, but of course
nothing specific to MS-DOS or the PC BIOS will work under
FreeBSD. There is also some material on the Web concerning
assembly programming under Linux, but the same caveat applies.
<p>Here are some Web links that you might find useful:
<dl>
<dt><a href="http://developer.intel.com/design/litcentr/index.htm">;
Intel Manuals</a>
<dd>Reference manuals for Intel processors can be found here.
<dt><a href="http://webster.cs.ucr.edu/">Art of Assembly
Language</a>
<dd>A well-regarded and very long (~1500 page) online textbook for
assembly programming in MS-DOS.
<dt><a href="http://linuxassembly.org/">Linux Assembly</a>
<dd>Assembly programming under Linux. Some useful information for
FreeBSD programmers, but be wary of the differences between
FreeBSD and Linux.
<dt><a href="http://www.web-sites.co.uk/nasm/">NASM</a>;
<dd>If you prefer Intel syntax in your assembler, try NASM. It is in
the FreeBSD ports system.
<dt><a href="news:comp.lang.asm.x86">comp.lang.asm.x86</a>
<a href="http://www.geocities.com/SiliconValley/Peaks/8600/">Host page</a>
<dd>Contains links to other Intel assembly resources on the Web.
<dt><a href="http://www.unix.digital.com/faqs/publications/base_doc/DOCUMENTATION/HTML/AA-PS31D-TET1_html/TITLE.html">;
Alpha Assembly Language Programmer's Guide</a>
<dd>Probably useful if you are running FreeBSD on an Alpha.
</dl>
</body>
</html>
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