Linux assemblers: A comparison of GAS and NASM
A side-by-side look at GNU Assembler (GAS) and Netwide Assembler (NASM)
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 (but see the sidebar for a quick refresher). 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.
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 Related topics for links), as those are the most complete sources of information.
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.
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
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
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:
mov eax, 4
movl $4, %eax
- In AT&T syntax, immediate operands are preceded by
$; in Intel syntax, immediate operands are not. For example:
- 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
lspecify 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
word ptr, and
dword ptr. Thus:
mov al, byte ptr foo
movb foo, %al
- Immediate form long jumps and calls are
lcall/ljmp $section, $offsetin AT&T syntax; the Intel syntax is
call/jmp far section:offset. The far return instruction is
lret $stack-adjustin 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.
.text section is where the processor begins code
.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
_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
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.
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
You can see several differences above in the declaration of memory
variables. NASM uses the
db directives to declare 32-, 16-, and 8-bit numbers,
respectively, whereas GAS uses the
.byte for the same purpose. GAS has other directives too,
.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 (
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.
Listing 3 illustrates the concepts of this section; it accepts the user's name as input and returns a greeting.
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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
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
.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
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
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:
varname resb size
.lcomm varname, size
Listing 2 also introduces the concept of a location counter (line 6). NASM
provides a special variable (the
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
$ 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
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:
.ascii "Enter Your Name: "
.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
.set to initialize the value of the variable STR_SIZE to
the expression following the comma. A corresponding
also be used. There is no alternative to GAS's
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
This creates a macro with two arguments, the first being
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
.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
.macro macroname arg1, arg2
movl \arg1, %eax
movl \arg2, %ebx
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.
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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 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
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
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
usort_str must be prefixed with
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
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
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
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:
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:
mov al, byte [ebx + esi]
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
l suffix in the mnemonic
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)
%segment:(offset, index, multiplier)
%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
used before this memory address to access 1, 2, or 4 bytes of memory,
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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 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
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:
movl $2, %eax
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
%rep directive and ends with
%rep directive is followed by an expression (unlike in
GAS where the
.rept directive is followed by a count):
There is also an alternative in NASM, the
%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
mov eax, 2
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
%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
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.
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.
- Consult the NASM and GAS manuals for complete introductions to these two assemblers:
- Read an explanation of selection sort on Wikipedia.