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Linux assemblers: A comparison of GAS and NASM
A side-by-side look at GNU Assembler (GAS) and Netwide Assembler (NASM)
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By Ram Narayan
Published October 17, 2007

Introduction
Unlike other languages, assembly programming involves understanding the 
processor architecture of the machine that is being programmed. Assembly 
programs are not at all portable and are often cumbersome to maintain and 
understand, and can often contain a large number of lines of code. But with 
these limitations comes the advantage of speed and size of the runtime binary 
that executes on that machine.

Though much information is already available on assembly level programming on 
Linux, this article aims to more specifically show the differences between 
syntaxes in a way that will help you more easily convert from one flavor of 
assembly to the another. The article evolved from my own quest to improve at 
this conversion.

This article uses a series of program examples. Each program illustrates some 
feature and is followed by a discussion and comparison of the syntaxes. 
Although it’s not possible to cover every difference that exists between 
NASM and GAS, I do try to cover the main points and provide a foundation for 
further investigation. And for those already familiar with both NASM and GAS, 
you might still find something useful here, such as macros.

This article assumes you have at least a basic understanding of assembly 
terminology and have programmed with an assembler using Intel® syntax, 
perhaps using NASM on Linux or Windows. This article does not teach how to 
type code into an editor or how to assemble and link. You should be familiar 
with the Linux operating system (any Linux distribution will do; I used Red 
Hat and Slackware) and basic GNU tools such as gcc and ld, and you should be 
programming on an x86 machine.

Now I’ll describe what this article does and does not cover.

Building the examples

Assembling:
GAS:
as –o program.o program.s

NASM:
nasm –f elf –o program.o program.asm

Linking (common to both kinds of assembler):
ld –o program program.o

Linking when an external C library is to be used:
ld –-dynamic-linker /lib/ld-linux.so.2 –lc –o program program.o

This article covers:

Basic syntactical differences between NASM and GAS
Common assembly level constructs such as variables, loops, labels, and macros
A bit about calling external C routines and using functions
Assembly mnemonic differences and usage
Memory addressing methods
This article does not cover:

The processor instruction set
Various forms of macros and other constructs particular to an assembler
Assembler directives peculiar to either NASM or GAS
Features that are not commonly used or are found only in one assembler but not 
in the other
For more information, refer to the official assembler manuals (see resources 
section in the right for links), as those are the most complete sources of 
information.

Basic structure
Listing 1 shows a very simple program that simply exits with an exit code of 
2. This little program describes the basic structure of an assembly program 
for both GAS and NASM.

Line	NASM	GAS
001 002 003 004 005 006 007 008 009 010 011 012 013 014 015 016	; Text segment 
begins section .text global _start ; Program entry point _start: ; Put the 
code number for system call mov eax, 1 ; Return value mov ebx, 2 ; Call the OS 
int 80h	# Text segment begins .section .text .globl _start # Program entry 
point _start: # Put the code number for system call movl $1, %eax /* Return 
value */ movl $2, %ebx # Call the OS int $0x80
Listing 1. A program that exits with an exit code of 2

Now for a bit of explanation.

One of the biggest differences between NASM and GAS is the syntax. GAS uses 
the AT&T syntax, a relatively archaic syntax that is specific to GAS and some 
older assemblers, whereas NASM uses the Intel syntax, supported by a majority 
of assemblers such as TASM and MASM. (Modern versions of GAS do support a 
directive called .intel_syntax, which allows the use of Intel syntax with GAS.)

The following are some of the major differences summarized from the GAS manual:

AT&T and Intel syntax use the opposite order for source and destination 
operands. For example:

Intel: mov eax, 4
AT&T: movl $4, %eax
In AT&T syntax, immediate operands are preceded by $; in Intel syntax, 
immediate operands are not. For example:

Intel: push 4
AT&T: pushl $4
In AT&T syntax, register operands are preceded by %; in Intel syntax, they are 
not.
In AT&T syntax, the size of memory operands is determined from the last 
character of the opcode name. Opcode suffixes of b, w, and l specify byte 
(8-bit), word (16-bit), and long (32-bit) memory references. Intel syntax 
accomplishes this by prefixing memory operands (not the opcodes themselves) 
with byte ptr, word ptr, and dword ptr. Thus:

Intel: mov al, byte ptr foo
AT&T: movb foo, %al
Immediate form long jumps and calls are lcall/ljmp $section, $offset in AT&T 
syntax; the Intel syntax is call/jmp far section:offset. The far return 
instruction is lret $stack-adjust in AT&T syntax, whereas Intel uses ret far 
stack-adjust.
In both the assemblers, the names of registers remain the same, but the syntax 
for using them is different as is the syntax for addressing modes. In 
addition, assembler directives in GAS begin with a “.”, but not in NASM.

The .text section is where the processor begins code execution. The global 
(also .globl or .global in GAS) keyword is used to make a symbol visible to 
the linker and available to other linking object modules. On the NASM side of 
Listing 1, global _start marks the symbol _start as a visible identifier so 
the linker knows where to jump into the program and begin execution. As with 
NASM, GAS looks for this _start label as the default entry point of a program. 
A label always ends with a colon in both GAS and NASM.

Interrupts are a way to inform the OS that its services are required. The int 
instruction in line 16 does this job in our program. Both GAS and NASM use the 
same mnemonic for interrupts. GAS uses the 0x prefix to specify a hex number, 
whereas NASM uses the h suffix. Because immediate operands are prefixed with $ 
in GAS, 80 hex is $0x80.

int $0x80 (or 80h in NASM) is used to invoke Linux and request a service. The 
service code is present in the EAX register. A value of 1 (for the Linux exit 
system call) is stored in EAX to request that the program exit. Register EBX 
contains the exit code (2, in our case), a number that is returned to the OS. 
(You can track this number by typing echo $? at the command prompt.)

Finally, a word about comments. GAS supports both C style (/* */), C++ style 
(//), and shell style (#) comments. NASM supports single-line comments that 
begin with the “;” character.

Variables and accessing memory
This section begins with an example program that finds the largest of three 
numbers.

Line	NASM	GAS
001 002 003 004 005 006 007 008 009 010 011 012 013 014 015 016 017 018 019 
020 021 022 023 024 025 026 027 028 029 030 031	; Data section begins section 
.data var1 dd 40 var2 dd 20 var3 dd 30 section .text global _start _start: ; 
Move the contents of variables mov ecx, [var1] cmp ecx, [var2] jg 
check_third_var mov ecx, [var2] check_third_var: cmp ecx, [var3] jg _exit mov 
ecx, [var3] _exit: mov eax, 1 mov ebx, ecx int 80h	// Data section begins 
.section .data var1: .int 40 var2: .int 20 var3: .int 30 .section .text .globl 
_start _start: # move the contents of variables movl (var1), %ecx cmpl (var2), 
%ecx jg check_third_var movl (var2), %ecx check_third_var: cmpl (var3), %ecx 
jg _exit movl (var3), %ecx _exit: movl $1, %eax movl %ecx, %ebx int $0x80
Listing 2. A program that finds the maximum of three numbers

You can see several differences above in the declaration of memory variables. 
NASM uses the dd, dw, and db directives to declare 32-, 16-, and 8-bit 
numbers, respectively, whereas GAS uses the .long, .int, and .byte for the 
same purpose. GAS has other directives too, such as .ascii, .asciz, and 
.string. In GAS, you declare variables just like other labels (using a colon), 
but in NASM you simply type a variable name (without the colon) before the 
memory allocation directive (dd, dw, etc.), followed by the value of the 
variable.

Line 18 in Listing 2 illustrates the memory indirect addressing mode. NASM 
uses square brackets to dereference the value at the address pointed to by a 
memory location: [var1]. GAS uses a circular brace to dereference the same 
value: (var1). The use of other addressing modes is covered later in this 
article.

Using macros
Listing 3 illustrates the concepts of this section; it accepts the user’s 
name as input and returns a greeting.

Line	NASM	GAS
001 002 003 004 005 006 007 008 009 010 011 012 013 014 015 016 017 018 019 
020 021 022 023 024 025 026 027 028 029 030 031 032 033 034 035 036 037 038 
039 040 041 042 043 044 045 046 047 048 049 050 051 052 053 054 055 056 057 
058 059 060 061 062	section .data prompt_str db 'Enter your name: ' ; $ is 
the location counter STR_SIZE equ $ - prompt_str greet_str db 'Hello ' 
GSTR_SIZE equ $ - greet_str section .bss ; Reserve 32 bytes of memory buff 
resb 32 ; A macro with two parameters ; Implements the write system call 
%macro write 2 mov eax, 4 mov ebx, 1 mov ecx, %1 mov edx, %2 int 80h %endmacro 
; Implements the read system call %macro read 2 mov eax, 3 mov ebx, 0 mov ecx, 
%1 mov edx, %2 int 80h %endmacro section .text global _start _start: write 
prompt_str, STR_SIZE read buff, 32 ; Read returns the length in eax push eax ; 
Print the hello text write greet_str, GSTR_SIZE pop edx ; edx = length 
returned by read write buff, edx _exit: mov eax, 1 mov ebx, 0 int 80h	
.section .data prompt_str: .ascii "Enter Your Name: " pstr_end: .set STR_SIZE, 
pstr_end - prompt_str greet_str: .ascii "Hello " gstr_end: .set GSTR_SIZE, 
gstr_end - greet_str .section .bss // Reserve 32 bytes of memory .lcomm buff, 
32 // A macro with two parameters // implements the write system call .macro 
write str, str_size movl $4, %eax movl $1, %ebx movl \str, %ecx movl 
\str_size, %edx int $0x80 .endm // Implements the read system call .macro read 
buff, buff_size movl $3, %eax movl $0, %ebx movl \buff, %ecx movl \buff_size, 
%edx int $0x80 .endm .section .text .globl _start _start: write $prompt_str, 
$STR_SIZE read $buff, $32 // Read returns the length in eax pushl %eax // 
Print the hello text write $greet_str, $GSTR_SIZE popl %edx // edx = length 
returned by read write $buff, %edx _exit: movl $1, %eax movl $0, %ebx int $0x80
Listing 3. A program to read a string and display a greeting to the user

The heading for this section promises a discussion of macros, and both NASM 
and GAS certainly support them. But before we get into macros, a few other 
features are worth comparing.

Listing 3 illustrates the concept of uninitialized memory, defined using the 
.bss section directive (line 14). BSS stands for “block storage segment” 
(originally, “block started by symbol”), and the memory reserved in the 
BSS section is initialized to zero during the start of the program. Objects in 
the BSS section have only a name and a size, and no value. Variables declared 
in the BSS section don’t actually take space, unlike in the data segment.

NASM uses the resb, resw, and resd keywords to allocated byte, word, and dword 
space in the BSS section. GAS, on the other hand, uses the .lcomm keyword to 
allocate byte-level space. Notice the way the variable name is declared in 
both versions of the program. In NASM the variable name precedes the resb (or 
resw or resd) keyword, followed by the amount of space to be reserved, whereas 
in GAS the variable name follows the .lcomm keyword, which is then followed by 
a comma and then the amount of space to be reserved. This shows the difference:

NASM: varname resb size

GAS: .lcomm varname, size

Listing 2 also introduces the concept of a location counter (line 6). NASM 
provides a special variable (the $ and $$ variables) to manipulate the 
location counter. In GAS, there is no method to manipulate the location 
counter and you have to use labels to calculate the next storage location 
(data, instruction, etc.).

For example, to calculate the length of a string, you would use the following 
idiom in NASM:

prompt_str db 'Enter your name: ' STR_SIZE equ $ - prompt_str ; $ is the 
location counter

The $ gives the current value of the location counter, and subtracting the 
value of the label (all variable names are labels) from this location counter 
gives the number of bytes present between the declaration of the label and the 
current location. The equ directive is used to set the value of the variable 
STR_SIZE to the expression following it. A similar idiom in GAS looks like 
this:

prompt_str: .ascii "Enter Your Name: " pstr_end: .set STR_SIZE, pstr_end - 
prompt_str

The end label (pstr_end) gives the next location address, and subtracting the 
starting label address gives the size. Also note the use of .set to initialize 
the value of the variable STR_SIZE to the expression following the comma. A 
corresponding .equ can also be used. There is no alternative to GAS’s set 
directive in NASM.

As I mentioned, Listing 3 uses macros (line 21). Different macro techniques 
exist in NASM and GAS, including single-line macros and macro overloading, but 
I only deal with the basic type here. A common use of macros in assembly is 
clarity. Instead of typing the same piece of code again and again, you can 
create reusable macros that both avoid this repetition and enhance the look 
and readability of the code by reducing clutter.

NASM users might be familiar with declaring macros using the %beginmacro 
directive and ending them with an %endmacro directive. A %beginmacro directive 
is followed by the macro name. After the macro name comes a count, the number 
of macro arguments the macro is supposed to have. In NASM, macro arguments are 
numbered sequentially starting with 1. That is, the first argument to a macro 
is %1, the second is %2, the third is %3, and so on. For example:

%beginmacro macroname 2 mov eax, %1 mov ebx, %2 %endmacro

This creates a macro with two arguments, the first being %1 and the second 
being %2. Thus, a call to the above macro would look something like this:

macroname 5, 6

Macros can also be created without arguments, in which case they don’t 
specify any number.

Now let’s take a look at how GAS uses macros. GAS provides the .macro and 
.endm directives to create macros. A .macro directive is followed by a macro 
name, which may or may not have arguments. In GAS, macro arguments are given 
by name. For example:

.macro macroname arg1, arg2 movl \arg1, %eax movl \arg2, %ebx .endm

A backslash precedes the name of each argument of the macro when the name is 
actually used inside a macro. If this is not done, the linker would treat the 
names as labels rather then as arguments and will report an error.

Functions, external routines, and the stack
The example program for this section implements a selection sort on an array 
of integers.

Line	NASM	GAS
001 002 003 004 005 006 007 008 009 010 011 012 013 014 015 016 017 018 019 
020 021 022 023 024 025 026 027 028 029 030 031 032 033 034 035 036 037 038 
039 040 041 042 043 044 045 046 047 048 049 050 051 052 053 054 055 056 057 
058 059 060 061 062 063 064 065 066 067 068 069 070 071 072 073 074 075 076 
077 078 079 080 081 082 083 084 085 086 087 088 089 090 091 092 093 094 095 
096 097 098 099 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 
115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 
134 135 136 137 138 139 140 141 142 143 144 145	section .data array db 89, 10, 
67, 1, 4, 27, 12, 34, 86, 3 ARRAY_SIZE equ $ - array array_fmt db " %d", 0 
usort_str db "unsorted array:", 0 sort_str db "sorted array:", 0 newline db 
10, 0 section .text extern puts global _start _start: push usort_str call puts 
add esp, 4 push ARRAY_SIZE push array push array_fmt call print_array10 add 
esp, 12 push ARRAY_SIZE push array call sort_routine20 ; Adjust the stack 
pointer add esp, 8 push sort_str call puts add esp, 4 push ARRAY_SIZE push 
array push array_fmt call print_array10 add esp, 12 jmp _exit extern printf 
print_array10: push ebp mov ebp, esp sub esp, 4 mov edx, [ebp + 8] mov ebx, 
[ebp + 12] mov ecx, [ebp + 16] mov esi, 0 push_loop: mov [ebp - 4], ecx mov 
edx, [ebp + 8] xor eax, eax mov al, byte [ebx + esi] push eax push edx call 
printf add esp, 8 mov ecx, [ebp - 4] inc esi loop push_loop push newline call 
printf add esp, 4 mov esp, ebp pop ebp ret sort_routine20: push ebp mov ebp, 
esp ; Allocate a word of space in stack sub esp, 4 ; Get the address of the 
array mov ebx, [ebp + 8] ; Store array size mov ecx, [ebp + 12] dec ecx ; 
Prepare for outer loop here xor esi, esi outer_loop: ; This stores the min 
index mov [ebp - 4], esi mov edi, esi inc edi inner_loop: cmp edi, ARRAY_SIZE 
jge swap_vars xor al, al mov edx, [ebp - 4] mov al, byte [ebx + edx] cmp byte 
[ebx + edi], al jge check_next mov [ebp - 4], edi check_next: inc edi jmp 
inner_loop swap_vars: mov edi, [ebp - 4] mov dl, byte [ebx + edi] mov al, byte 
[ebx + esi] mov byte [ebx + esi], dl mov byte [ebx + edi], al inc esi loop 
outer_loop mov esp, ebp pop ebp ret _exit: mov eax, 1 mov ebx, 0 int 
80h	.section .data array: .byte 89, 10, 67, 1, 4, 27, 12, 34, 86, 3 
array_end: .equ ARRAY_SIZE, array_end - array array_fmt: .asciz " %d" 
usort_str: .asciz "unsorted array:" sort_str: .asciz "sorted array:" newline: 
.asciz "\n" .section .text .globl _start _start: pushl $usort_str call puts 
addl $4, %esp pushl $ARRAY_SIZE pushl $array pushl $array_fmt call 
print_array10 addl $12, %esp pushl $ARRAY_SIZE pushl $array call 
sort_routine20 # Adjust the stack pointer addl $8, %esp pushl $sort_str call 
puts addl $4, %esp pushl $ARRAY_SIZE pushl $array pushl $array_fmt call 
print_array10 addl $12, %esp jmp _exit print_array10: pushl %ebp movl %esp, 
%ebp subl $4, %esp movl 8(%ebp), %edx movl 12(%ebp), %ebx movl 16(%ebp), %ecx 
movl $0, %esi push_loop: movl %ecx, -4(%ebp) movl 8(%ebp), %edx xorl %eax, 
%eax movb (%ebx, %esi, 1), %al pushl %eax pushl %edx call printf addl $8, %esp 
movl -4(%ebp), %ecx incl %esi loop push_loop pushl $newline call printf addl 
$4, %esp movl %ebp, %esp popl %ebp ret sort_routine20: pushl %ebp movl %esp, 
%ebp # Allocate a word of space in stack subl $4, %esp # Get the address of 
the array movl 8(%ebp), %ebx # Store array size movl 12(%ebp), %ecx decl %ecx 
# Prepare for outer loop here xorl %esi, %esi outer_loop: # This stores the 
min index movl %esi, -4(%ebp) movl %esi, %edi incl %edi inner_loop: cmpl 
$ARRAY_SIZE, %edi jge swap_vars xorb %al, %al movl -4(%ebp), %edx movb (%ebx, 
%edx, 1), %al cmpb %al, (%ebx, %edi, 1) jge check_next movl %edi, -4(%ebp) 
check_next: incl %edi jmp inner_loop swap_vars: movl -4(%ebp), %edi movb 
(%ebx, %edi, 1), %dl movb (%ebx, %esi, 1), %al movb %dl, (%ebx, %esi, 1) movb 
%al, (%ebx, %edi, 1) incl %esi loop outer_loop movl %ebp, %esp popl %ebp ret 
_exit: movl $1, %eax movl $0, %ebx int $0x80
Listing 4. Implementation of selection sort on an integer array

Listing 4 might look overwhelming at first, but in fact it’s very simple. 
The listing introduces the concept of functions, various memory addressing 
schemes, the stack and the use of a library function. The program sorts an 
array of 10 numbers and uses the external C library functions puts and printf 
to print out the entire contents of the unsorted and sorted array. For 
modularity and to introduce the concept of functions, the sort routine itself 
is implemented as a separate procedure along with the array print routine. 
Let’s deal with them one by one.

After the data declarations, the program execution begins with a call to puts 
(line 31). The puts function displays a string on the console. Its only 
argument is the address of the string to be displayed, which is passed on to 
it by pushing the address of the string in the stack (line 30).

In NASM, any label that is not part of our program and needs to be resolved 
during link time must be predefined, which is the function of the extern 
keyword (line 24). GAS doesn’t have such requirements. After this, the 
address of the string usort_str is pushed onto the stack (line 30). In NASM, a 
memory variable such as usort_str represents the address of the memory 
location itself, and thus a call such as push usort_str actually pushes the 
address on top of the stack. In GAS, on the other hand, the variable usort_str 
must be prefixed with $, so that it is treated as an immediate address. If 
it’s not prefixed with $, the actual bytes represented by the memory 
variable are pushed onto the stack instead of the address.

Since pushing a variable essentially moves the stack pointer by a dword, the 
stack pointer is adjusted by adding 4 (the size of a dword) to it (line 32).

Three arguments are now pushed onto the stack, and the print_array10 function 
is called (line 37). Functions are declared the same way in both NASM and GAS. 
They are nothing but labels, which are invoked using the call instruction.

After a function call, ESP represents the top of the stack. A value of esp + 4 
represents the return address, and a value of esp + 8 represents the first 
argument to the function. All subsequent arguments are accessed by adding the 
size of a dword variable to the stack pointer (that is, esp + 12, esp + 16, 
and so on).

Once inside a function, a local stack frame is created by copying esp to ebp 
(line 62). You can also allocate space for local variables as is done in the 
program (line 63). You do this by subtracting the number of bytes required 
from esp. A value of esp – 4 represents a space of 4 bytes allocated for a 
local variable, and this can continue as long as there is enough space in the 
stack to accommodate your local variables.

Listing 4 illustrates the base indirect addressing mode (line 64), so called 
because you start with a base address and add an offset to it to arrive at a 
final address. On the NASM side of the listing, [ebp + 8] is one such example, 
as is [ebp – 4] (line 71). In GAS, the addressing is a bit more terse: 
4(%ebp) and -4(%ebp), respectively.

In the print_array10 routine, you can see another kind of addressing mode 
being used after the push_loop label (line 74). The line is represented in 
NASM and GAS, respectively, like so:

NASM: mov al, byte [ebx + esi]

GAS: movb (%ebx, %esi, 1), %al

This addressing mode is the base indexed addressing mode. Here, there are 
three entities: one is the base address, the second is the index register, and 
the third is the multiplier. Because it’s not possible to determine the 
number of bytes to be accessed from a memory location, a method is needed to 
find out the amount of memory addressed. NASM uses the byte operator to tell 
the assembler that a byte of data is to be moved. In GAS the same problem is 
solved by using a multiplier as well as using the b, w, or l suffix in the 
mnemonic (for example, movb). The syntax of GAS can seem somewhat complex when 
first encountered.

The general form of base indexed addressing in GAS is as follows:

%segment:ADDRESS (, index, multiplier)

or

%segment:(offset, index, multiplier)

or

%segment:ADDRESS(base, index, multiplier)

The final address is calculated using this formula:

ADDRESS or offset + base + index * multiplier.

Thus, to access a byte, a multiplier of 1 is used, for a word, 2, and for a 
dword, 4. Of course, NASM uses a simpler syntax. Thus, the above in NASM would 
be represented like so:

Segment:[ADDRESS or offset + index * multiplier]

A prefix of byte, word, or dword is used before this memory address to access 
1, 2, or 4 bytes of memory, respectively.

Leftovers
Line	NASM	GAS
001 002 003 004 005 006 007 008 009 010 011 012 013 014 015 016 017 018 019 
020 021 022 023 024 025 026 027 028 029 030 031 032 033 034 035 036 037 038 
039 040 041 042 043 044 045 046 047 048 049 050 051 052 053 054 055 056 057 
058 059 060 061	section .data ; Command table to store at most ; 10 command 
line arguments cmd_tbl: %rep 10 dd 0 %endrep section .text global _start 
_start: ; Set up the stack frame mov ebp, esp ; Top of stack contains the ; 
number of command line arguments. ; The default value is 1 mov ecx, [ebp] ; 
Exit if arguments are more than 10 cmp ecx, 10 jg _exit mov esi, 1 mov edi, 0 
; Store the command line arguments ; in the command table store_loop: mov eax, 
[ebp + esi * 4] mov [cmd_tbl + edi * 4], eax inc esi inc edi loop store_loop 
mov ecx, edi mov esi, 0 extern puts print_loop: ; Make some local space sub 
esp, 4 ; puts function corrupts ecx mov [ebp - 4], ecx mov eax, [cmd_tbl + esi 
* 4] push eax call puts add esp, 4 mov ecx, [ebp - 4] inc esi loop print_loop 
jmp _exit _exit: mov eax, 1 mov ebx, 0 int 80h	.section .data // Command 
table to store at most // 10 command line arguments cmd_tbl: .rept 10 .long 0 
.endr .section .text .globl _start _start: // Set up the stack frame movl 
%esp, %ebp // Top of stack contains the // number of command line arguments. 
// The default value is 1 movl (%ebp), %ecx // Exit if arguments are more than 
10 cmpl $10, %ecx jg _exit movl $1, %esi movl $0, %edi // Store the command 
line arguments // in the command table store_loop: movl (%ebp, %esi, 4), %eax 
movl %eax, cmd_tbl( , %edi, 4) incl %esi incl %edi loop store_loop movl %edi, 
%ecx movl $0, %esi print_loop: // Make some local space subl $4, %esp // puts 
functions corrupts ecx movl %ecx, -4(%ebp) movl cmd_tbl( , %esi, 4), %eax 
pushl %eax call puts addl $4, %esp movl -4(%ebp), %ecx incl %esi loop 
print_loop jmp _exit _exit: movl $1, %eax movl $0, %ebx int $0x80
Listing 5. A program that reads command line arguments, stores them in memory, 
and prints them

Listing 5 shows a construct that repeats instructions in assembly. Naturally 
enough, it’s called the repeat construct. In GAS, the repeat construct is 
started using the .rept directive (line 6). This directive has to be closed 
using an .endr directive (line 8). .rept is followed by a count in GAS that 
specifies the number of times the expression enclosed inside the .rept/.endr 
construct is to be repeated. Any instruction placed inside this construct is 
equivalent to writing that instruction count number of times, each on a 
separate line.

For example, for a count of 3:

.rept 3 movl $2, %eax .endr

This is equivalent to:

movl $2, %eax movl $2, %eax movl $2, %eax

In NASM, a similar construct is used at the preprocessor level. It begins with 
the %rep directive and ends with %endrep. The %rep directive is followed by an 
expression (unlike in GAS where the .rept directive is followed by a count):

%rep <expression> nop %endrep

There is also an alternative in NASM, the times directive. Similar to %rep, it 
works at the assembler level, and it, too, is followed by an expression. For 
example, the above %rep construct is equivalent to this:

times <expression> nop

And this:

%rep 3 mov eax, 2 %endrep

is equivalent to this:

times 3 mov eax, 2

and both are equivalent to this:

mov eax, 2 mov eax, 2 mov eax, 2

In Listing 5, the .rept (or %rep) directive is used to create a memory data 
area for 10 double words. The command line arguments are then accessed one by 
one from the stack and stored in the memory area until the command table gets 
full.

As for command line arguments, they are accessed similarly with both 
assemblers. ESP or the top of the stack stores the number of command line 
arguments supplied to a program, which is 1 by default (for no command line 
arguments). esp + 4 stores the first command line argument, which is always 
the name of the program that was invoked from the command line. esp + 8, esp + 
12, and so on store subsequent command line arguments.

Also watch the way the memory command table is being accessed on both sides in 
Listing 5. Here, memory indirect addressing mode (line 33) is used to access 
the command table along with an offset in ESI (and EDI) and a multiplier. 
Thus, [cmd_tbl + esi * 4] in NASM is equal to cmd_tbl(, %esi, 4) in GAS.

Conclusion
Even though the differences between these two assemblers are substantial, 
it’s not that difficult to convert from one form to another. You might find 
that the AT&T syntax seems at first difficult to understand, but once 
mastered, it’s as simple as the Intel syntax.