Netwide Assembler

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Netwide Assembler
NASM logo
Original author(s) Simon Tatham, Julian Hall
Developer(s) H. Peter Anvin, et al.
Stable release 2.11.06 / October 20, 2014 (2014-10-20)
Operating system Microsoft Windows, Unix-like, OS/2, OS X, DOS
Available in English
Type x86 assembler
License 2-clause BSD
Website www.nasm.us

The Netwide Assembler (NASM) is an assembler and disassembler for the Intel x86 architecture. It can be used to write 16-bit, 32-bit (IA-32) and 64-bit (x86-64) programs. NASM is considered to be one of the most popular assemblers for Linux.[1]

NASM was originally written by Simon Tatham with assistance from Julian Hall and is currently maintained by a small team led by H. Peter Anvin.[2] It is available as free software under the terms of the simplified (2-clause) BSD license.[3]

Features[edit]

NASM can output several binary formats including COFF, Portable Executable, a.out, ELF, Mach-O and .bin (binary disk image, used for compiling operating systems), though position-independent code is only supported for ELF object files. NASM also has its own binary format called RDOFF.[4]

The variety of output formats allows programs to be retargeted to virtually any x86 operating system. In addition, NASM can create flat binary files, usable in writing boot loaders, ROM images, and in various facets of OS development.[4] NASM can run on non-x86 platforms, such as SPARC and PowerPC, though it cannot generate programs usable by those machines.

NASM uses a variation of Intel assembly syntax instead of AT&T syntax.[5] It also avoids features such as automatic generation of segment overrides (and the related ASSUME directive) used by MASM and compatible assemblers.[4]

Examples of programs for various operating systems[edit]

This is a "Hello, world!" program 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 example of a similar program for Microsoft Windows:

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

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));
 
	mov	eax, 1 ; exit
	mov	ebx, 0
	int	0x80   ; exit(0)
 
section .data
msg:	db	"Hello, world!", 10
.len:	equ	$ - msg

Below is a 64-bit program for Apple OS X 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
	mov	rdi, 0
	syscall

Linking[edit]

NASM principally outputs object files, which are generally not executable in and of themselves. The only exception to this are flat binaries (e.g., .COM)[4] 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[edit]

On 28 November 2007, version 2.00 was released, adding support for x86-64 extensions.[2] The development versions are not uploaded to SourceForge.net; instead, they are checked in to the project's own Git repository with binary snapshots available from the project web page.

A search engine for NASM docs is also available.[6]

As of version 2.07, NASM is now under the Simplified (2-clause) BSD license.

RDOFF[edit]

RDOFF
Developed by Julian Hall
Type of format Object file format
Container for Object code

RDOFF (Relocatable Dynamic Object File Format) is used by developers to test the integrity of NASM's object file output capabilities. It is based heavily on the internal structure of NASM,[7] essentially consisting of a header containing a serialization 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 of NASM was released in October 1996, NASM only supported output of flat-format executable files (e.g. MS-DOS COM files). In version 0.90, Simon Tatham added support for an object-file output interface, along with support for DOS .OBJ files for 16-bit code only.[8]

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.[8]

Since this initial version, there has been one major update to the RDOFF format, which added a record-length indicator on each header record,[9] 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).[7]

See also[edit]

References[edit]

  1. ^ Ram Narayan. "Linux assemblers: A comparison of GAS and NASM". "two of the most popular assemblers for Linux, GNU Assembler (GAS) and Netwide Assembler (NASM)" 
  2. ^ a b "The Netwide Assembler". Retrieved 2008-06-27. 
  3. ^ "NASM Version History". Retrieved 2009-07-19. 
  4. ^ a b c d "NASM Manual". Retrieved 2009-08-15. 
  5. ^ Randall Hyde. "NASM: The Netwide Assembler". Retrieved 2008-06-27. 
  6. ^ "NASM Doc Search Engine". Retrieved 2009-09-14. 
  7. ^ a b "NASM Manual Ch. 6". Retrieved 2008-06-27. 
  8. ^ a b "NASM CVS". 2008-06-08. Retrieved 2008-06-27. 
  9. ^ "V1-V2.txt". 2002-12-04. Retrieved 2008-06-27. 

Further reading[edit]

  • Jeff Duntemann (2000). Assembly Language Step by Step. J Wiley and Sons. ISBN 0-471-37523-3. 

External links[edit]