The Netwide Assembler (NASM) is an
assembler and
disassembler for the
Intel x86 architecture. It can be used to write
16-bit
16-bit microcomputers are microcomputers that use 16-bit microprocessors.
A 16-bit register can store 216 different values. The range of integer values that can be stored in 16 bits depends on the integer representation used. With the two mos ...
,
32-bit
In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in 32-bit units. Compared to smaller bit widths, 32-bit computers can perform large calculation ...
(
IA-32) and
64-bit (
x86-64) programs. It is considered one of the most popular assemblers for
Linux.
It was originally written by
Simon Tatham with assistance from Julian Hall. , it is maintained by a small team led by
H. Peter Anvin.
It is
open-source software released under the terms of a simplified (2-clause)
BSD license.
Features
NASM can output several binary formats, including
COFF
The Common Object File Format (COFF) is a format for executable, object code, and shared library computer files used on Unix systems. It was introduced in Unix System V, replaced the previously used a.out format, and formed the basis for exte ...
, OMF,
a.out
a.out is a file format used in older versions of Unix-like computer operating systems for executables, object code, and, in later systems, shared libraries. This is an abbreviated form of "assembler output", the filename of the output of Ken Th ...
,
Executable and Linkable Format (ELF),
Mach-O
Mach-O, short for Mach object file format, is a file format for executables, object code, shared libraries, dynamically-loaded code, and core dumps. It was developed to replace the a.out format.
Mach-O is used by some systems based on the M ...
and
binary file (.bin, binary disk image, used to compile
operating systems), though
position-independent code is supported only for ELF
object files. It also has its own binary format called RDOFF.
The variety of output formats allows
retargeting programs to virtually any x86 operating system (OS). It can also create
flat binary file
A binary file is a computer file that is not a text file. The term "binary file" is often used as a term meaning "non-text file". Many binary file formats contain parts that can be interpreted as text; for example, some computer document fil ...
s, usable to write
boot loaders
In computing, booting is the process of starting a computer as initiated via hardware such as a button or by a software command. After it is switched on, a computer's central processing unit (CPU) has no software in its main memory, so some ...
,
read-only memory (ROM) images, and in various facets of OS development.
It can run on non-x86 platforms as a
cross assembler, such as
PowerPC
PowerPC (with the backronym Performance Optimization With Enhanced RISC – Performance Computing, sometimes abbreviated as PPC) is a reduced instruction set computer (RISC) instruction set architecture (ISA) created by the 1991 Apple Inc., App ...
and
SPARC, though it cannot generate programs usable by those machines.
NASM uses a variant of
Intel assembly syntax
x86 assembly language is the name for the family of assembly languages which provide some level of backward compatibility with CPUs back to the Intel 8008 microprocessor, which was launched in April 1972. It is used to produce object code for ...
instead of
AT&T syntax.
It also avoids features such as automatic generation of
segment overrides (and the related ASSUME directive) used by
MASM
The Microsoft Macro Assembler (MASM) is an x86 assembler that uses the Intel syntax for MS-DOS and Microsoft Windows. Beginning with MASM 8.0, there are two versions of the assembler: One for 16-bit & 32-bit assembly sources, and another (ML64) ...
and compatible assemblers.
Sample programs
A
"Hello, world!" program
A "Hello, World!" program is generally a computer program that ignores any input and outputs or displays a message similar to "Hello, World!". A small piece of code in most general-purpose programming languages, this program is used to illustra ...
for the
DOS operating system:
section .text
org 0x100
mov ah, 0x9
mov dx, hello
int 0x21
mov ax, 0x4c00
int 0x21
section .data
hello: db 'Hello, world!', 13, 10, '$'
An equivalent program for
Linux:
global _start
section .text
_start:
mov eax, 4 ; write
mov ebx, 1 ; stdout
mov ecx, msg
mov edx, msg.len
int 0x80 ; write(stdout, msg, strlen(msg));
xor eax, msg.len ; invert return value from write()
xchg eax, ebx ; value for exit()
mov eax, 1 ; exit
int 0x80 ; exit(...)
section .data
msg: db "Hello, world!", 10
.len: equ $ - msg
An example of a similar program for
Microsoft Windows
Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...
:
global _main
extern _MessageBoxA@16
extern _ExitProcess@4
section code use32 class=code
_main:
push dword 0 ; UINT uType = MB_OK
push dword title ; LPCSTR lpCaption
push dword banner ; LPCSTR lpText
push dword 0 ; HWND hWnd = NULL
call _MessageBoxA@16
push dword 0 ; UINT uExitCode
call _ExitProcess@4
section data use32 class=data
banner: db 'Hello, world!', 0
title: db 'Hello', 0
A 64-bit program for Apple
OS X
macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and lapt ...
that inputs a keystroke and shows it on the screen:
global _start
section .data
query_string: db "Enter a character: "
query_string_len: equ $ - query_string
out_string: db "You have input: "
out_string_len: equ $ - out_string
section .bss
in_char: resw 4
section .text
_start:
mov rax, 0x2000004 ; put the write-system-call-code into register rax
mov rdi, 1 ; tell kernel to use stdout
mov rsi, query_string ; rsi is where the kernel expects to find the address of the message
mov rdx, query_string_len ; and rdx is where the kernel expects to find the length of the message
syscall
; read in the character
mov rax, 0x2000003 ; read system call
mov rdi, 0 ; stdin
mov rsi, in_char ; address for storage, declared in section .bss
mov rdx, 2 ; get 2 bytes from the kernel's buffer (one for the carriage return)
syscall
; show user the output
mov rax, 0x2000004 ; write system call
mov rdi, 1 ; stdout
mov rsi, out_string
mov rdx, out_string_len
syscall
mov rax, 0x2000004 ; write system call
mov rdi, 1 ; stdout
mov rsi, in_char
mov rdx, 2 ; the second byte is to apply the carriage return expected in the string
syscall
; exit system call
mov rax, 0x2000001 ; exit system call
xor rdi, rdi
syscall
Linking
NASM principally outputs
object files, which are generally not executable by themselves. The only exception to this are flat binaries (e.g.,
.COM
The domain name .com is a top-level domain (TLD) in the Domain Name System (DNS) of the Internet. Added at the beginning of 1985, its name is derived from the word ''commercial'', indicating its original intended purpose for domains registere ...
)
which are inherently limited in modern use. To translate the object files into executable programs, an appropriate
linker must be used, such as the
Visual Studio "LINK" utility for Windows or
ld for Unix-like systems.
Development
NASM version 0.90 was released in October 1996.
Version 2.00 was released on 28 November 2007, adding support for
x86-64 extensions.
The development versions are not uploaded to
SourceForge.net
SourceForge is a web service that offers software consumers a centralized online location to control and manage open-source software projects and research business software. It provides source code repository hosting, bug tracking, mirroring ...
, but are checked into
GitHub with binary snapshots available from the project web page.
A search engine for NASM documentation is also available.
In July 2009, as of version 2.07, NASM was released under the Simplified (2-clause) BSD license. Previously, because it was licensed under
LGPL
The GNU Lesser General Public License (LGPL) is a free-software license published by the Free Software Foundation (FSF). The license allows developers and companies to use and integrate a software component released under the LGPL into their own ...
, it led to development of Yasm, a complete rewrite of under the
New BSD License. Yasm offered support for x86-64 earlier than NASM. It also added support for
GNU Assembler
The GNU Assembler, commonly known as gas or as, is the assembler developed by the GNU Project. It is the default back-end of GCC. It is used to assemble the GNU operating system and the Linux kernel, and various other software. It is a part of ...
syntax.
RDOFF
Relocatable Dynamic Object File Format (RDOFF) is used by developers to test the integrity of NASM's object file output abilities. It is based heavily on the internal structure of NASM,
essentially consisting of a header containing a
serialization
In computing, serialization (or serialisation) is the process of translating a data structure or object state into a format that can be stored (e.g. files in secondary storage devices, data buffers in primary storage devices) or transmitted (e ...
of the output driver function calls followed by an array of sections containing executable code or data. Tools for using the format, including a linker and loader, are included in the NASM distribution.
Until version 0.90 was released in October 1996, NASM supported output of only flat-format executable files (e.g.,
DOS COM file
A COM file is a type of simple executable file. On the Digital Equipment Corporation (DEC) VAX operating systems of the 1970s, .COM was used as a filename extension for text files containing commands to be issued to the operating system (simi ...
s). In version 0.90,
Simon Tatham added support for an object-file output interface, and for DOS .OBJ files for 16-bit code only.
NASM thus lacked a 32-bit object format. To address this lack, and as an exercise to learn the object-file interface, developer Julian Hall put together the first version of RDOFF, which was released in NASM version 0.91.
Since this initial version, there has been one major update to the RDOFF format, which added a record-length indicator on each header record,
allowing programs to skip over records whose format they do not recognise, and support for multiple segments; RDOFF1 only supported three segments: ''text'', ''data'' and ''bss'' (containing uninitialized data).
The RDOFF format is strongly deprecated and has been disabled starting in NASM 2.15.04.
See also
*
Assembly language
In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence be ...
*
Comparison of assemblers
This is an incomplete list of assemblers: computer programs that translate assembly language source code into binary programs. Some assemblers are components of a compiler system for a high level language and may have limited or no usable functio ...
References
Further reading
*
External links
*
*
Special edition for Win32 and BeOS.at
IBM
* : a converter between the source format of the assemblers NASM and
GAS
{{X86 assembly topics
1996 software
Assemblers
Disassemblers
DOS software
Free compilers and interpreters
Linux programming tools
MacOS
MacOS programming tools
Programming tools for Windows
Software using the BSD license